I 


I 


ILLUMINATING 
ENGINEERING  PRACTICE 

LECTURES 

ON  ILLUMINATING  ENGINEERING 
DELIVERED  AT  THE 

UNIVERSITY  OF  PENNSYLVANIA 

PHILADELPHIA,  SEPTEMBER  20  TO  28,  1916 

UNDER  THE  JOINT  AUSPICES  OF 
THE  UNIVERSITY  AND  THE 

ILLUMINATING  ENGINEERING 
SOCIETY 


McGRAW-HILL  BOOK  COMPANY,  INC. 
239  WEST  39TH  STREET.    NEW  YORK 


LONDON:  HILL  PUBLISHING  CO.,  Lnx 
6  &  8  BOUVERIE  ST.,  E.  C. 

1917 


VI  PREFACE 

volume,  were  delivered  at  the  University  of  Pennsylvania  between  the 
dates  September  20  and  September  28  inclusive,  by  men  peculiarly 
qualified  by  training  and  experience  to  present  the  most  advanced 
treatment  of  illumination  problems. 

It  is  worthy  of  record  here  that  there  were  180  subscriptions  to 
the  entire  course  and  that  in  addition  59  tickets  to  individual 
lectures  were  sold.  Supplementing  the  lectures  an  exhibit  was 
arranged  which  exemplified  modern  methods  of  illumination  and 
illustrated  modern  lighting  appliances.  An  inspection  tour  was  also 
organized  in  connection  with  the  lectures,  including  visits  to  places 
of  interest  to  lighting  men,  in  Pittsburgh,  Washington,  Philadelphia, 
Atlantic  City,  New  York,  Boston,  Schenectady,  Buffalo,  Cleveland 
and  Chicago. 

EDWARD  P.  HYDE. 


THE  INCEPTION  OF  THE  1916  ILLUMINATING  ENGINEERING  COURSE 

In  considering  special  activities  when  undertaking  the  Presidency 
of  the  Illuminating  Engineering  Society  in  the  summer  of  1915,  I 
conceived  the  idea  of  a  course  of  lectures  on  illuminating  engineering 
which  would  be  supplementary  to  the  course  held  at  The  Johns 
Hopkins  University  in  1910,  and  which  would  emphasize  the  practical 
rather  than  the  theoretical  aspect  of  the  subject.  Later  it  developed 
that  members  of  the  faculty  of  the  University  of  Pennsylvania  had 
discussed  a  like  project.  Happily  these  two  ideas,  of  independent 
origin,  were  brought  together  before  the  Council  of  the  Illuminating 
Engineering  Society,  and  the  lecture  course  was  duly  consummated. 
The  result  has  been  very  gratifying  to  the  Illuminating  Engineering 
Society.  The  value  of  the  course  was  demonstrated  at  the  time  of 
its  presentation.  This  book  is  expected  to  extend  that  value 
materially. 

CHARLES  P.  STEINMETZ. 


OPENING  EXERCISES 

The  lecture  course  followed  immediately  upon  the  adjournment 
of  the  1916  Annual  Convention  of  the  Illuminating  Engineering 
Society,  which  was  held  in  Philadelphia.  On  the  evening  preceding 
the  first  lecture,  and  following  the  closing  session  of  the  Convention, 
a  meeting  was  held  in  the  auditorium  of  the  Museum  of  the  Uni- 
versity of  Pennsylvania,  to  which  meeting  the  public  was  invited. 
The  following  interesting  program  was  carried  out: 

Address — CHARLES  P.  STEINMETZ, 

President  Illuminating  Engineering  Society. 

Address — EDGAR  F.  SMITH, 

Provost  University  of  Pennsylvania. 

Address — EDWARD  P.  HYDE, 

Chairman   1910   and  1916  I.E.S.  Committees   on 
Lectures. 

Popular  Lecture— Subject,  "Controlled  Light" 
WM.  A.  DURGIN, 
Director  Illuminating  Engineering  Society. 

A  large  and  enthusiastic  audience  greeted  the  distinguished 
speakers.  Representations  from  the  faculty  and  undergraduate 
body  of  the  University,  from  the  membership  of  the  Illuminating 
Engineering  Society,  and  from  the  local  lighting  organizations, 
combined  to  make  the  occasion  auspicious. 

Expression  of  Appreciation  Tendered  by  the  Illuminating  Engineering 
Society  to  the  University  of  Pennsylvania 

The  very  able  and  cordial  cooperation  of  the  faculty  and  staff 
of  the  University  of  Pennsylvania  which  contributed  largely  to  the 
success  of  the  Illuminating  Engineering  Lecture  Course  prompted 
the  Council  of  the  Illuminating  Engineering  Society  to  forward 
to  Provost  Smith  of  the  University  an  engrossed  "appreciation" 
couched  in  the  following  terms: 

The  Council  of  the  Illuminating  Engineering  Society  expresses 

vii 


Vlll  OPENING   EXERCISES 

its  appreciation  of  the  courteous  cooperation  of  the  Provost  and 
Faculty  of  the  University  of  Pennsylvania  in  the  joint  organization 
and  conduct  of  the  Illuminating  Engineering  Lecture  Course, 
September  2ist  to  28th,  1916. 

(Signed)  G.  H.  STICKNEY,  (Signed)  WM»  J.  SERRILL, 

General  Secretary.  President. 

December  14,  1916. 


CONTENTS 

PAGE 
PREFACE   v 

COMMITTEE  ON  LECTURES x 

?    Illumination  Units  and  Calculations i 

By  A.  S.  MCALLISTER. 
The  Principles  of  Interior  Illumination,  Parts  I  and  II 37 

Committee:  J.  R.  CRAVATH,  WARD  HARRISON,  R.  ff.  PIERCE. 
The  Principles  of  Exterior  Illumination 77 

By  Louis  BELL. 
Modern  Photometry go. 

By  CLAYTON  H.  SHARP. 
Recent  Developments  in  Electric  Lighting  Appliances 131 

By  G.  H.  STICKXEY. 
Recent  Developments  in  Gas  Lighting  Appliances 165 

By  ROBERT  ff.  PIERCE. 
Modern  Lighting  Accessories 183 

By  W.  F.  LITTLE. 
Light  Projection:  Its  Applications 213 

By  E.  J.  EDWARDS  and  H.  H.  MAGDSICK. 
The  Architectural  and  Decorative  Aspects  of  Lighting 253 

By  GUY  LOWELL. 
Color  in  Lighting 267 

By  M.  LUCKIESH. 
Church  Lighting  Requirements 1 297 

By  E.  G.  PERROT. 
,    The  Lighting  of  Schools,  Libraries  and  Auditoriums 307 

By  F.  A.  VAUGHN. 
The  Lighting  of  Factories,  Mills  and  Workshops 337 

By  C.  E.  CLEWELL. 
The  Lighting  of  Offices,  Stores  and  Shop  Windows 363 

By  NORMAN  MACBETH. 
The  Lighting  of  the  Home 395 

By  H.  W.  JORDAN. 
The  Lighting- of  Streets  (Part  I) 415 

By  PRESTON  S.  MILLAR. 
Street  Lighting  (Part  II) 461 

By  C.  F.  LACOMBE. 
Railway  Car  Lighting 493 

By  GEORGE  H.  HULSE. 
The  Lighting  of  Yards,  Docks  and  Other  Outside  Works 513 

By  J.  L.  MINICK. 
Sign  Lighting 535 

By  L.  G.  SHEPARD. 

ix 


2  ILLUMINATING   ENGINEERING  PRACTICE 

curves  and  can  be  converted  into  " light  curves"  only  after  making 
proper  modifications  in  accordance  with  certain  well-defined  solid 
geometrical  relations.  It  seems  appropriate  to  give  emphasis  to 
this  statement  by  defining  the  solid  geometrical  relations  referred 
to,  which  are  equally  as  simple  as  plane  geometrical  or  trigonomet- 
rical relations. 

SOLID  GEOMETRICAL  RELATIONS 

Of  the  several  space  geometrical  relations  with  which  an  illuminat- 
ing engineer  should  be  familiar,  by  far  the  most  important,  and 
happily  the  simplest,  is  that  existing  between  the  external  area  or 
zonal  area  of  a  sphere  and  its  diameter  or  zonal  width.  This  rela- 
tion is  one  of  direct  proportion.  That  is  to  say,  the  external  area 


Fig.  I. — Spherical  geometrical  relations. 

of  a  zone  of  any  chosen  sphere  varies  directly  with  the  width  of  the 
zone,  and  the  total  external  area  is  that  of  a  zone  having  a  width 
equal  to  the  diameter  of  the  sphere. 

In  almost  all  cases  of  application  to  illumination  problems,  one 
is  interested  in  the  relative  values  rather  than  the  actual  values  of 
the  various  zonal  areas  and  the  above  mentioned  proportion  is  all 
that  he  needs  to  take  into  consideration.  However,  one  can  de- 
termine the  actual  as  well  as  the  relative  values  with  extreme  sim- 
plicity by  means  of  certain  plane  geometrical  or  trigonometrical 
relations  applied  to  the  sphere. 

In  Fig.  i,  which  represents  a  sphere  cut  along  a  vertical  plane 
through  the  center  O,  the  zone  of  infinitesimal  vertical  width*  ED, 
along  the  diameter,  has  an  external  area  represented  by  the  sloping 


MCALLISTER:  ILLUMINATION  UNITS  3 

width  at  C  multiplied  by  the  circumference  of  the  zonal  circle  passing 
horizontally  through  C.  Now  the  circumference  of  the  horizontal 
circle  through  C  bears  to  that  of  the  horizontal  circle  through  B 
(that  is,  the  " great  circle"  of  the  sphere),  the  relation  of  cos  <£  to  i. 
Likewise  the  sloping  width  of  the  zone  at  C  bears  to  the  vertical 
width  ED  the  inverse  ratio,  i  to  cos  <£. 

Since  these  two  ratios,  one  the  inverse  of  the  other,  are  to-  be 
multiplied  together  in  determining  the  zonal  area,  it  is  obvious 
that  the  external  area  of  the  zone  having  a  width  ED  along  the 
diameter  is  equal  to  the  product  of  this  width  by  the  circumference 
of  the  "great  circle."  Similarly  the  total  external  area  of  the  sphere 
is  found  by  multiplying  the  sphere  diameter  (=  total  width  of  all 
zones)  by  the  circumference  of  the  great  circle;  or  is  equal  to 
dXird  =  7rdz  =  4irr2  where  d  is  the  diameter  and  r  the  radius  of 
the  sphere. 

Familiarity  with  the  above  fundamental  spherical  (space)  geomet- 
rical relations  is  absolutely  essential  to  a  proper  understanding  of 
the  significance  of  the  curves  showing  the  space  distribution  of  the 
candle-power  of  light  sources;  to  the  derivation  or  interpretation 
of  diagrams  showing^  the  light  from  sources  whose  candle-power 
curves  are  known,  and  to  the  solution  of  problems  relating  to  plane 
surface  or  extended  surface  sources. 

It  is  noteworthy  in  this  connection  that  the  modern  tendency  is 
away  from  point  sources,  and  point-source  candle-power  methods 
of  calculation,  towards  extended  source  and  lumen-output  calculat- 
ing methods,  so  that  the  importance  of  becoming  familiar  with  space 
geometrical  relations  is  ever  on  the  increase. 

UNIT  SOLID  ANGLE— THE  STERADIAN 

Although  the  illuminating  engineer  is  seldom  called  upon  to  make 
use  of  solid  angular  dimensions  expressed  in  terms  of  any  unit  of 
solid  angular  measurement,  because  almost  all  of  the  calculations 
in  which  he  is  interested  can  be  based  on  ratios  rather  than,  actual 
values  of  solid  angles,  yet  it  may  at  times  be  found  convenient  to 
refer  to  some  solid  angular  measurement  in  terms  of  a  unit  of 
measurement.  Two  distinct  units  have  been  employed  for  this 
purpose,  one  represented  by  the  whole  sphere  and  the  other  by 
a  value  i  -5-  4?r  as  large.  For  the  former  no  special  name  has 
been  standardized,  while  to  the  latter  the  name  "steradian"  is 
applied. 


4  ILLUMINATING   ENGINEERING  PRACTICE 

•v. 

From  its  definition  it  will  be  seen  that  any  zone  on  a  sphere  having 
a  diametrical  width  such  that  W  =  d  -r-  471-,  where  d  is  the  diame- 
ter of  the  sphere,  will  subtend  a  solid  angle  of  one  steradian,  and 
that  4?r  =  12.57  -f  steradians  equal  one  sphere  in  solid  angular 
measurement. 

Since  the  external  surface  of  a  sphere  of  unit  radius  is  equal  to 
47r  units  of  area,  it  follows  that  a  steradian  is  an  angle  having  such 
a  value  as  to  subtend  unit  area  on  the  surface  of  a  sphere  of  unit 
radius,  or  an  area  equal  numerically  to  the  radius  squared  on  a 
sphere  of  any  dimension  whatsoever  expressed  in  any  unit  of  length 
or  area. 

It  is  sometimes  stated  that  the  solid  angle  subtended  by  a  chosen 
area  when  viewed  from  a  chosen  position  can  be  calculated  in 
steradians  by  dividing  the  numerical  value  of  the  area  by  the  square 
of  the  distance  between  the  point  selected  and  the  area.  This 
statement  is  correct  only  when  applied  to  an  area  every  infinitesimal 
element  of  which  occupies  the  same  distance  from  the  point  of 
observation;  that  is,  when  the  area  lies  on  the  circumference  of  a 
sphere  having  its  center  at  the  point  chosen. 

RELATION  BETWEEN  LIGHT  AND  CANDLE-POWER  DISTRIBUTION 

In  order  to  present  most  clearly  the  exact  significance  of  the 
candle-power  curve,  explain  most  readily  the  diagram  for  showing 
the  distribution  and  summation  of  the  light  flux  (lumens)  from  the 
source,  and  to  give  proper  emphasis  to  the  necessary  distinction 
between  candle-power  distribution  and  light  distribution  use  will 
be  made  of  the  curve  of  candle-power  of  a  source  giving  light  in 
only  one  hemisphere. 

In  order  definitely  to  fix  ideas  it  will  be  assumed  that  the  maxi- 
mum candle-power  of  the  source  is  100  and  that  the  candle-power 
decreases  uniformly  according  to  a  cosine  function  of  the  angle  of 
vision  to  zero  at  90  degrees  from  the  position  of  maximum  candle- 
power^  The  curve  showing  the  distribution  of  candle-power  of 
such  a  source  (which  could  be  for  example,  an  infinitesimal  plane 
radiating  in  accordance  with  the  " cosine  law"  of  space  distribution 
of  candle-power)  is  represented  in  Fig.  2. 

Assume  now  that  the  source  is  placed  at  the  center  of  a  hollow 
sphere  of  unit  radius  the  interior  surface  of  which  is  illuminated  by 
the  source,  as  indicated  in  Fig.  2.  The  illumination  on  each  ele- 
mentary area  of  the  surrounding  sphere  will  at  each  point  be  numeric- 


MCALLISTER:  ILLUMINATION  UNITS 


5 


ally  equal  to  the  candle-power  of  the  source  when  observed  from 
that  point — expressed  in  foot-candles  if  the  radius  of  the  sphere  is 
one  foot;  in  meter-candles  if  the  radius  is  one  meter,  etc.  Hence 
to  determine  the  lumens  incident  upon  any  chosen  section  of  the 
surrounding  sphere  it  is  necessary  merely  to  multiply  the  area  of 
that  section  by  the  mean  candle-power  of  the  source  effective  over 
that  section. 

It  is  convenient  not  only  for  present  purposes  but  also  for  purposes 
of  subsequent  comparisons,  to  express  the  area  of  sections  of  the 
surrounding  sphere  in  terms  of  the  zones  cut  off  by  various  angles 
below  (and  above)  the  horizontal. 


Surface 

Source  of 

Light 


Figs.  2  and  3. — Space  distribution  of  candle-power  and  light  flux  from  infinitesimal 
surface  source. 


It  should  here  be  observed  that,  for  sake  of  convenience  in  deriva- 
tion and  explanation,  the  angles  indicated  herein  are  measured 
(in  both  the  plus  and  the  minus  direction)  from  the  horizontal  plane, 
whereas  in  actual  curves  of  candle-power  distribution  the  angles 
of  elevation  are  "counted  positively  from  the  nadir  as  zero  to  the 
zenith  as  180  degrees."  That  is  to  say,  whereas  in  the  curves  herein 
shown  the  vertical  angles  are  measured  through  zero  from  minus 
90  degrees  to  plus  90  degrees,  it  is  the  more  usual  plan  to  make  all 
measurements  in  the  positive  direction  from  zero  plotted  at  the 
bottom  of  the  curve  to  180  degrees  at  the  top. 

The  zonal  areas  measured  from  the  horizontal  plane  are  as 
follows : 


ILLUMINATING   ENGINEERING   PRACTICE 


Zonal  angle  from 
horizontal 

Zonal  width 
sine  of  angle 

Zonal  area 
2v  zonal  width 

Max.  C.  P.  of  zone 

0-15 

0.259 

1.63 

25-9 

0-30 

0.500 

3-14 

50.0 

o-45 

0.7070 

4-44 

70.7 

0-60 

0.866 

5-44 

86.6 

o-75 

0.969 

6.06 

96.6 

0-90 

i  .000 

6.28 

IOO.O 

Candle-Power 

Lumens 

Zone 

Area 

Max. 

Min. 

Mean 

Area  X  CP 

0-30 
30-60 
60-90 

3-14 
2.30 
0.84 

50.0 
86.6 

IOO.O 

00.0 

50.0 
86.6 

25.0 
68.3 
93-3 

78.5 
I57-I 
78.4 

Total 

6  28 

Total..  .  . 

314.0 
3   4 

The  vertical  widths  of  the  separate  zones  are  represented  by  the 
vertical  line  at  the  extreme  right  in  Fig.  3.  Along  this  line  have 
been  erected  certain  perpendiculars  for  representing  the  candle- 
power  values  over  each  part  of  the  zone  width.  The  product  of  the 
candle-power  at  each  point  by  the  zone  area  at  that  point  which 
bears  the  constant  relation  of  2ir  -r-  i  to  the  vertical  width  of  each 
zone,  gives  the  lumens  over  that  zone — to  a  certain  scale.  Obviously 
the  area  of  the  triangular  figure  at  the  right  in  Fig.  3  represents  (to  a 
scale  involving  the  candle-power  scale,  the  distance  scale  and  the 
constant  2ir)  the  total  lumens  radiated  by  the  source.  From  this 
figure,  known  as  the  Rousseau  diagram,  the  lumens  effective  over 
any  chosen  zone  can  be  computed  at  once  from  the  intercepted  area 
on  the  diagram.  This  is  not  an  approximate,  but  an  absolutely 
exact  method  of  calculation.  Any  errors  involved  in  using  the 
method  can  be  attributed  to  inaccuracies  in  measuring  or  plotting 
the  candle-power  or  in  determining  the  areas  from  the  diagram; 
that  is,  to  inexactness  in  carrying  out  the  method  rather  than  to  the 
method  itself. 

By  using  the  Rousseau  diagram  merely  as  an  aid  in  visualizing 
the  problem  and  resorting  to  plane  or  spherical  geometrical  or 
trigonometrical  calculations  for  actual  determinations,  one  can  often 
eliminate  all  inaccuracies  other  than  those  inherent  in  the  photometric 
testing  of  the  lighting  source. 


MCALLISTER:  ILLUMINATION  UNITS  7 

If  the  candle-power  had  been  uniform  throughout  the  lower  hemi- 
sphere at  a  value  equal  to  the  actual  maximum  of  100  the  total 
number  of  lumens  would  have  been  628,  or  just  twice  the  actual 
value.  Similarly,  if  the  uniform  candle-power  of  100  has  been 
active  throughout  both  the  upper  and  the  lower  hemisphere,  the 
lumens  output  from  the  source  would  have  totalled  1256,  or  four 
times  the  actual  value  determined  above  by  slide-rule  computation. 
The  mathematically  exact  result  would  be, 

Area  X  c.p.  =  4^  X  100  =  1256.64  +. 

The  exact  ratio  between  the  total  lumens  produced  by  the  light- 
ing source  having  the  candle-power  distribution  indicated  in  Fig. 
2,  and  the  lumens  that  would  be  produced  by  a  source  giving 
uniform  candle-power  in  all  directions  equal  to  the  maximum  in 
Fig.  2,  is  i  -T-  4.  Obviously  this  ratio,  which  is  called  the  "spherical 
reduction  factor,"  in  any  practical  case  depends  upon  the  shape  of 
the  candle-power  distribution  curve,  becoming  indefinitely  small  in 
the  case  of  a  concentrated  beam  and  reaching  a  maximum  of  i.o 
in  the  case  of  a  source  of  uniform  candle-power  such  as  a  spherical 
surface  source. 

It  may  be  well  at  this  point  to  call  attention  to  the  fact  that  the 
"mean  spherical  candle-power"  of  a  surface  source  of  any  shape 
whatsoever  is  equal  to  one-fourth  of  the  product  of  the  effective 
radiating  area  by  the  maximum  candle-power  of  an  (infinitesimal) 
unit  area  of  the  source,  provided  only  that  each  infinitesimal  area 
radiates  in  space  according  to  the  cosine  law  of  space-distribution 
and  all  infinitesimal  areas  have  the  same  maximum  value  of  candle- 
power.  The  total  effective  candle-power  in  any  chosen  direction 
observed  at  any  chosen  position  from  such  a  source  is  equal  to  the 
product  of  the  candle-power  per  unit  area  by  the  "projected  area" 
of  the  source  as  viewed  from  the  direction  (and  exact  position) 
chosen.  These  facts  will  be  discussed  in  greater  detail  later  in 
connection  with  the  subjects  of  "brightness"  "output"  and 
"appearance." 

On  account  of  the  fact  that  such  curves  as  those  shown  in  Fig.  2 
are  often  loosely  referred  to  as  "light-distribution"  curves,  rather 
than  "candle-power-distribution"  curves,  certain  misconceptions 
have  been  produced  in  the  minds  of  persons  not  familiar  with  the 
exact  physical  significance  of  the  geometrical  representation  of  the 
photometric  relations. 

In  order  to  lay  proper  emphasis  on  the  distinction  that  must  be 


8 


ILLUMINATING    ENGINEERING    PRACTICE 


made  between  " light  distribution"  and  " candle-power  distribution," 
a  comparison  will  be  made  with  the  actual  distribution  of  light  in 
each  vertical  zone  (as  accurately  shown  by  the  Rousseau  diagram 
of  Fig.  3)  and  the  distribution  of  light  which  would  exist  if  the  curve 
of  Fig.  2  were  in  reality  a  "light  distribution"  rather  than  a  "candle- 
power  distribution"  curve.  This  curve  is  reproduced  in  Fig.  4, 
where  it  is  treated  as  representing  "light  distribution,"  and  on  the 
basis  of  this  interpretation  the  Rousseau  diagram  of  Fig.  5,  has  been 
constructed  by  the  methods  already  explained.  A  comparison  of 
the  incorrect  diagram  of  Fig.  5,  with  the  correct  diagram  of  Fig.  3 
will  serve  to  show  the  inaccuracy  in  treating  a  "candle-power 
distribution"  curve  as  a  "light  distribution"  curve. 


Figs.  4  and  5. — Space  distribution  of  light  from  an  assumed  source  and  corresponding  flux 

summation  diagram. 


CANDLE   POWER   DISTRIBUTION   FROM    CYLINDRICAL   AND 
SPHERICAL  SURFACE  SOURCES 

In  Fig.  6,  the  smaller  double  circles  show  the  space  distribution 
of  candle  power  around  an  infinitesimal  cylindrical  surface  source 
having  a  vertical  axis.  In  Fig.  7,  the  elliptical  area  is  the  Rousseau 
diagram  showing  the  light  flux  produced  over  various  zones  of  the 
sphere  surrounding  the  light  source,  as  explained  above. 

In  Fig.  6,  the  large  central  circle  shows  the  candle-power  distribu- 
tion around  a  spherical  surface  source;  the  corresponding  Rousseau 
diagram  is  represented  by  the  rectangular  area  in  Fig.  7.  The 
separate  curves  of  Fig.  6  have  been  so  drawn  that  the  rectangular 
area  of  Fig.  7  is  equal  to  the  elliptical  area  of  the  same  figure.  That 


MCALLISTER:  ILLUMINATION  UNITS  9 

is,  the  light  output  from  the  cylindrical  surface  has  been  made  equal 
to  the  light  output  from  the  spherical  surface  source. 

It  will  be  recalled,  from  well-known  trigonometrical  and  geomet- 
rical relations,  that  the  area  of  an  ellipse  is  equal  to  Tr/4  times  the 
product  of  the  major  and  minor  axes,  whereas  that  of  a  rectangle 
is  equal  to  the  product  of  the  major  and  minor  sides.  It  follows 
therefore  that  the  minor  side  of  the  rectangle  in  Fig.  7  is  equal  to 


-90 


Figs.  6  and  7. — Space  distribution  of  candle-power  from  infinitesimal  cylindrical  and  spher- 
ical sources  and  corresponding  flux  summation  diagrams. 

7T/4  times  the  minor  axis  of  the  ellipse,  and  hence  the  maximum 
(uniform)  candle-power  of  the  spherical  surface  source  is  equal  to 
ir/4  times  the  maximum  (horizontal)  candle-power  of  the  cylindrical 
surface  source  in  Fig.  6.  That  is  to  say,  the  "spherical  reduction 
factor"  of  a  cylindrical  surface  source  is  equal  to  7r/4  =  0.7854. 
This  is  the  value  usually  assigned  to  a  so-called  "line-source," 
which  has  no  existence  in  reality,  its  nearest  approach  in  practice 


10  ILLUMINATING   ENGINEERING   PRACTICE 

being  the  cylindrical  surface  of  a  lamp  filament  having  an  inappreciable 
diameter. 

SPACE  REPRESENTATION  OF  CANDLE-POWER  DISTRIBUTION 

By  means  of  models  representing  solids  of  revolution  of  the 
candle-power  curves  about  the  axis  of  reference  one  can  obtain  a 
better  idea  of  the  real  significance  of  the  space  distribution  of  the 
candle-power  than  can  be  obtained  from  the  flat  candle-power  curve 
which  must  in  any  event  be  interpreted  as  showing  merely  a  cross- 
sectional  view  of  such  a  space-model.  In  interpreting  a  candle- 
power  distribution  model  care  must  be  exercised  in  giving  signifi- 
cance to  the  quantities  represented.  Special  emphasis  must  be 
placed  on  the  fact  that  neither  the  volumetric  content  of  the  model 
nor  the  superficial  area  has  any  immediate  relation  to  the  flux  of 
light  from  the  source  giving  the  candle-power  indicated  by  the 
model.  A  striking  illustration  of  this  fact  is  afforded  by  a  com- 
parison of  the  centrally  located  candle-power  circle  in  Fig.  6  with 
the  completely  displaced  candle-power  circle  in  Fig.  2. 

As  already  shown  by  means  of  the  Rousseau  diagrams  of  Fig.  7 
and  Fig.  3,  the  flux  produced  by  the  source  giving  the  circular  candle- 
power  curve  of  Fig.  6  is  exactly  equal  to  that  produced  by  the 
source  giving  the  circular  candle-power  curve  of  Fig.  2,  and  hence 
the  solid  of  revolution  of  Fig.  6  represents  exactly  the  same  amount 
of  flux  as  does  the  solid  of  revolution  of  Fig.  2. 

The  diameter  of  the  circle  in  Fig.  2  is  exactly  twice  as  great 
as  that  in  Fig.  6;  the  superficial  area  of  the  solid  of  revolution  of 
Fig.  2  is  four  times  that  of  Fig.  6,  and  its  volumetric  content  is 
eight  times  as  large. 

A  certain  percentage  of  the  volumetric  content  or  superficial  area 
of  any  chosen  solid  of  revolution  represents  the  same  percentage  of 
the  total  flux  of  light  from  the  source  only  in  the  limiting  cases  of 
uniform  candle-power  in  all  directions  as  shown  by  the  centrally 
located  circle  of  Fig.  6  or  of  a  section  of  the  sphere  cut  vertically 
throughout  the  whole  depth. 

From  the  two  illustrations  chosen  above,  it  will  be  observed  that 
even  when  the  scale  of  candle-power  is  defined,  the  total  flux  repre- 
sented by  a  given  solid  of  revolution  is  known  only  when  the  exact 
location  of  the  light  source  within  the  sphere  is  known.  With  the 
source  at  the  center,  the  sphere  represents  the  maximum  of  light 
flux;  when  the  source  is  at  the  surface  (as  in  Fig.  2)  the  light  flux 


MCALLISTER:  ILLUMINATION  UNITS  n 

has  only  one-half   of   the   maximum   value,   all  other  quantities, 
dimensions  and  scales  remaining  the  same. 


SPHERICAL  SURFACE:  THE  SO-CALLED  "POINT "-SOURCE 

For  many  purposes  it  has  been  found  convenient  to  refer  to  a 
source  of  light  as  though  it  were  a  " point"  (that  is,  without  dimen- 
sions) and  by  certain  mathematical  transformations  certain  equa- 
tions applicable  exclusively  to  surface  sources  have  been  treated 
as  though  they  related  to  true  point-sources.  When  dealing  with 
illumination  effects  at  a  distance,  no  measurable  errors  are  involved 
in  such  assumptions  and  transformations,  but  when  one  attempts 
to  define  the  "brightness"  or  appearance  of  the  source  to  the  eye 
on  the  basis  of  an  assumed  point-source,  the  assumptions  are  found 
to  be  at  conflict  with  the  most  significant  physical  fact,  which  is  that 
the  brightness  is  a  function  of  the  area,  whereas 'points  (even  an  in- 
finite number  of  them)  are  devoid  of  dimensions  or  area. 

By  treating  the  so-called  "point-source,"  not  as  a  true  point  but 
as  an  infinitesimal  surface  having  all  of  the  physical  characteristics 
of  a  surface  source  the  mathematical  difficulties  can  be  overcome, 
but  by  far  the  simplest  and  most  satisfactory  method  is  to  treat  the 
source  initially,  finally  and  all  the  time,  as  a  surface  source  having 
true  surface  source  characteristics. 

Consider,  therefore,  a  spherical  surface  source  of  unit  radius  (i 
cm.)  emitting  100  candle-power  uniformly  in  all  directions.  The 
total  output  from  the  source  will  be  4?r  X  100  =  1257  lumens. 
The  superficial  area  of  the  source  is  4irr2  =  12.57  sq.  cm.,  and 
hence  the  output  is  equal  to  100  lumens  per  square  centimeter. 
At  any  appreciable  distance  from  the  source  the  "projected  area" 
of  the  source  viewed  from  this  distance  is  equal  to  irr2  =  3.14  sq. 
cm.  and  hence  the  "apparent  candle-power  per  unit  of  projected 
area"  is  100  -5-  3.14  =  31.9  a  value  which  in  the  past  has  been 
called  "brightness,"  but  no  name  has  been  adopted  for  designating 
the  unit.  For  the  unit  quantity  "apparent  output"  from  the 
source  expressed  in  "apparent  lumens  per  sq.  cm."  the  term  "lam- 
bert"  has  been  adopted.  This  term  is  applicable  equally  to  the 
"brightness"  (or  appearance  to  the  eye)  of  any  surface  whether 
radiating,  transmitting,  or  reflecting,  and  whether  or  not  it  acts  as 
a  perfectly  diffusing  surface,  but  the  unit  is  defined  by,  and  receives 
its  magnitude  from,  the  appearance  to  the  eye  of  "a  perfectly 
diffusing  surface  radiating  or  reflecting  one  lumen  per  sq.  cm." 


12  ILLUMINATING  ENGINEERING   PRACTICE 

As  is  well  known,  according  to  the  so-called  "  inverse  square 
law"  the  illumination  (or  luminous  flux  density)  on  a  plane  at  any 
chosen  distance  from  a  "  point-source "  varies  inversely  with  the 
distance  from  the  source.  If  it  were  possible  to  obtain  a  true 
point-source,  it  would  be  possible  to  produce  infinite  illumination  by 
bringing  the  plane  within  an  infinitesimal  distance  from  the  source. 
With  a  spherical  surface  source  the  "inverse  square"  law  holds  true 
provided  only  that  the  distance  from  the  source  is  measured  from 
the  center  thereof.  In  this  case  the  minimum  distance  from  the 
source  is  equal  to  the  radius  of  the  sphere.  With  a  spherical  sur- 
face source  i  cm.  in  radius  producing  100  c.p.  uniformly  in  all  direc- 
tions the  maximum  illumination  (at  minimum  distance)  is  equal  to 
100  -f-  r2  =  100  lumens  per  sq.  cm.  This  means  that  the  maximum 
possible  illumination  in  lumens  per  sq.  cm.  is  equal  to  the  "bright- 
ness" of  the  source  expressed  in  "lamberts."  This  relation  holds 
true  for  surface  sources  of  all  kinds  and  shape,  being  absolutely 
fundamental.  Any  assumption  that  would  lead  to  results  contrary 
thereto  can  be  said  not  to  be  in  accord  with  the  physical  fact. 

In  order  always  to  have  before  one  a  correct  mental  picture  of 
the  true  physical  conditions  of  lighting  sources,  it  is  best  always  to 
assume  that  the  so-called  "point- source"  is  in  reality  a  spherical 
surface  source  (having  finite  dimensions),  and  to  base  all  calculations 
on  the  surface  source  rather  than  point-source  conception.  That 
is  to  say,  it  is  not  necessary  to  employ  the  "point-source  conception" 
in  order  to  take  advantage  of  the  "inverse  square  law"  and  similar 
relations  developed  and  employed  on  the  basis  of  the  assumed 
"point-source,"  because  the  same  relations  are  applicable  even 
more  accurately  and  completely  to  the  spherical  surface  source. 
Moreover,  there  are  certain  relations  between  the  output  density 
of  the  surface  sources  and  the  illumination  (flux  density)  produced 
on  surfaces  illuminated  thereby,  which  can  be  utilized  immediately 
when  all  calculations  are  based  on  the  surface  source  conception 
but  which  must  be  ignored  in  effect  when  the  point-source  concep- 
tion is  used.  This  fact  is  becoming  of  increasing  importance  as  the 
indirect  or  semi-indirect  system  of  lighting  is  being  substituted  for 
the  direct. 

FLUX-SUMMATION    ON    MEAN    SPHERICAL    CANDLE-POWER 

DIAGRAM 

Reference  has  already  been  made  to  the  Rousseau  diagram  for 
representing  by  means  of  an  area  the  total  flux  produced  by  a  light 


MCALLISTER:  ILLUMINATION  UNITS  13 

source  of  which  the  candle-power  distribution  curve  is  known.  As 
a  matter  of  actual  practice  in  illumination  calculations  use  may 
be  said  always  to  be  made  for  the  purpose  indicated  of  either  the 
Rousseau  diagram  or  some  one  of  several  modifications  thereof  that 
have  been  developed  for  eliminating  the  necessity  of  a  planimeter 
for  determining  the  area  or  its  equivalent. 

Figs.  8  and  9  have  been  drawn  to  show  one  of  the  methods  em- 
ployed for  representing  the  equivalent  of  an  area  by  means  of  a 
straight  line.  The  irregular  curve  XbeY  of  Fig.  8  is  a  candle-power 


Figs.  8  and  9. — Linear  and  area  representations  of  zonal  flux. 

distribution  curve  of  which  Fig.  9  is  the  corresponding  Rousseau, 
or  flux-summation,  diagram.  Consider  the  small  area  ABCTPS 
of  Fig.  9.  If  such  a  section  be  so  selected  that  its  mean  width 
is  equal  to  PB  then  the  small  area  ABCTPS  is  equal  to  the 
product  of  AC  (the  height)  by  PB  (the  width).  The  problem  is  to 
select  some  one  line  which,  by  geometrical  construction,  is  pro- 
portional to  the  product  of  AC  and  PB.  In  Fig.  8  such  a  line  is 
shown  by  A'C',  which  by  construction,  bears  to  AC  (of  Fig.  9)  the 
direct  ratio  of  Ob  to  OP  (of  Fig.8).  That  is  to  say,  it  is  propor- 
tional directly  to  the  area  ABCTPS,  the  proportionality  constant 


14  ILLUMINATING   ENGINEERING   PRACTICE 

being  dependent  upon  the  linear  candle-power  scale  and  the 
diameter  of  V'  the  circle  of  reference,  or  rather  the  enclosing  sphere. 

The  summation  of  all  the  various  part-areas  of  Fig.  9  as  indicated 
by  A'C,  E'F',  etc.,  of  Fig.  8,  would  produce  a  single  linear  dimen- 
sion directly  proportional  to  the  total  area  of  Fig.  9;  that  is,  directly 
proportional  to  the  total  flux  from  the  source  of  which  the  irregular 
curve  of  Fig.  8  shows  the  space  distribution  of  the  candle-power. 

One  can  easily  define  the  proportionality  constant  by  applying  the 
method  here  outlined  to  the  determination  of  the  total  flux  from  the 
candle-power  curve  of  a  " spherical  surface"  source  producing  equal 
candle-power  in  all  directions.  It  will  be  seen  at  once  that  the 
total  of  the  vertical  lengths  (corresponding  to  A'C  and  E'F',  etc.) 
would  then  equal  twice  the  length  chosen  to  represent  the  uniform 
candle-power  of  the  " spherical  surface"  source;  now  the  total  flux 
is  equal  to  4?r  7  whereas  the  summatio  length  is  2  7  and  hence  the 
proportionality  constant  is  2ir. 

That  is  to  say,  independent  in  every  respect  of  the  irregularities 
of  the  candle-power  curve,  the  linear  summation  method  outlined 
above  gives  at  once  a  value  equal  (if  sufficiently  small  sections  are 
selected  for  summation)  to  twice  the  mean  spherical  candle-power 
of  the  source,  measured  on  the  candle-power  scale,  and  this  value 
multiplied  by  2ir  equals  (with  the  same  degree  of  accuracy)  the  total 
flux  from  the  source  expressed  in  lumens. 

It  will  be  noted  that,  contrary  to  the  relations  involved  in  the 
Rousseau  diagram,  the  diameter  of  the  circle  (or  sphere)  of  reference 
cancels  out  from  the  proportionality  constant  in  the  linear  summa- 
tion of  Fig.  8,  whereas  it  appears  as  a  direct  factor  in  the  area 
summation  of  Fig.  9. 

LINEAR    SUMMATION    BY    GRAPHICAL    CONSTRUCTION 

In  Fig.  1 1  is  reproduced  the  candle-power  curve  of  an  infinitesimal 
cylindrical  surface  source  of  which  the  Rousseau  flux  diagram 
(elliptical)  is  shown  in  Fig.  12,  identical  except  as  to  dimensions  with 
the  elliptical  diagram  in  Fig.  7.  In  Fig.  10  is  shown  a  graphical 
method  for  adding  together  the  vertical  linear  equivalents  of  the 
separate  30  degree  areas  in  the  Rousseau  diagram  of  Fig.  12,  the 
equivalents  in  each  case  being  determined  by  the  geometrical  method 
already  outlined  in  connection  with  Fig.  8.  It  will  be  noted  that 
the  3o-degree,  6o-degree  and  90-degree  angle  lines  have  been  so 
transposed,  while  retaining  their  equivalent  lengths,  that  the  corre- 


MCALLISTER:  ILLUMINATION  UNITS 


spending  vertical  distances  are  directly  added  one  to  the  other  to 
produce  at  once  the  total  length  of  QQ',  which  (according  to  the 
proportionality  constant  derived  above)  is  equivalent  to  twice  the 
mean  spherical  candle-power  represented  by  the  candle-power 
distribution  curve  of  Fig.  n  or  the  Rousseau  diagram  of  Fig.  12. 
The  linear  summation  diagram  briefly  outlined  in  connection  with 
Fig.  10  was  developed  by  Dr.  A.  E.  Kennelly,  past  president  of  the 
Illuminating  Engineering  Society,  and  is  known  as  the  Kennelly 
Diagram. 


Q 

+  90' 
+  60' 

-I- SO' 


-30 


Figs.  10,  ii,  and  12. — Kennelly  linear  summation  diagram;  candle-power  curve  of  cylindrical 
surface  source;  Rousseau  area  summation  diagram. 

ABSORPTION-OF-LIGHT    METHOD    OF    CALCULATION 

One  of  the  most  convenient  and  an  absolutely  reliable  method  of 
calculation  in  illumination  problems  is  that  based  on  the  law  of 
conservation.  According  to  this  law  the  total  flux  (lumens)  of 
light  absorbed  by  the  illuminated  surfaces  within  any  chosen  en- 
closure of  any  size,  shape  or  character  is  exactly  equal  to  the  total 
amount  of  flux  (lumens)  produced  by  the  sources  of  the  lumination. 
This  law  is  fundamental  and  calculations  based  upon  it  give  ab- 
solutely accurate  results  when  the  assumptions  as  to  absorption, 
etc.,  are  correct.  That  is  to  say,  by  adding  together  the  value  of 
the  lumens  separately  absorbed  by  the  various  surfaces  illuminated 
one  obtains  at  once  an  exact  measure  of  the  lumens  produced  by  the 
sources  of  light. 

In  order  to  determine  the  absorbed  flux,  it  is  necessary  to  know 
only  the  value  of  the  incident  flux  and  the  absorption  coefficient; 


i6 


ILLUMINATING   ENGINEERING   PRACTICE 


the  product  of  these  two  represents  accurately  the  lumens  absorbed. 
Any  error  found  in  applying  this  method  is  to  be  attributed  to  the 
inability  to  determine  either  the  value  of  the  incident  flux,  or  the 
absorption  coefficient,  or  both,  but  not  to  the  method  itself. 

For  example,  assume  a  room  25  ft.  wide,  80  ft.  long,  10  ft.  high 
having  a  white  ceiling  with  an  absorption  coefficient  of  0.20;  light 
walls  with  an  absorption  of  0.50;  and  a  dark  floor  with  an  absorption 
of  0.90,  to  be  so  lighted  that  the  incident  illumination  on  the  ceiling 
is  i  foot-candle,  that  on  the  walls  2-foot  candles  and  on  the  floor 
3  foot-candles.  The  following  summation  shows  the  amount  of 
lumens  absorbed: 


Area 

Incident 

Absorption 

Sq.  ft. 

Ft.  C. 

Flux 

Coef. 

Flux 

Ceiling  

2OOO 
2IOO 
2000 

I 
2 

3 

2000 

4200 
6000 

o.  20 
0.50 
0.90 

400 
2IOO 
5400 

Walls     

Floor  

Total  lumens  absorbed  =  7900. 

Total  mean  spherical  candle-power  equals  7900  -7-4^  =  630. 

This  method  is  not  approximate ;  it  is  absolutely  exact.  However, 
it  should  not  be  assumed  that  results  in  practice  can  be  obtained 
with  such  ready  facility  as  here  indicated,  because  the  absorption 
coefficients  of  ceiling,  wall  and  floor  materials  are  not  known  to  a 
high  degree  of  accuracy;  various  surfaces  in  addition  to  those  here 
considered  intercept  and  absorb  much  of  the  light,  and  the  light  is 
not  uniformly  distributed  over  the  various  surfaces.  In  regard  to 
the  last  mentioned  limitation  it  is  worthy  of  note  that  the  mere  lack 
of  uniformity  in  the  distribution  of  light  flux  does  not  affect  the 
accuracy  of  the  absorption  method  provided  only  that  the  true 
mean  effective  values  of  the  incident  illumination  and  of  the 
absorption  coefficient  are  assumed  in  each  case. 

The  actual  distribution  of  the  incident  flux  can  be  approximated 
by  means  of  some  of  the  point-by-point  methods  of  calculations, 
while  the  absorption  coefficient  must  be  based  on  the  results  of 
tests  relating  to  the  materials  composing  the  absorbing  surfaces. 
Values  for  such  coefficients  will  be  given  in  connection  with  other 
lectures  dealing  with  the  practical  application  of  the  methods  of 
calculation  herein  described. 


MCALLISTER:  ILLUMINATION  UNITS 


IXTER-REFLECTIONS  BETWEEN  WALLS,  CEILING  AND  FLOOR 

Some  idea  concerning  the  bearing  of  reflection  upon  illumination 
can  be  gained  readily  from  a  brief  study  of  the  values  derived  from 
the  above  absorption  problem. 

The  total  incident  flux  on  the  ceiling,  walls  and  floor  equal  2000  + 
4200  -f-  6000  =  12,200  lumens,  whereas  the  lighting  units  are  re- 
quired to  produce  only  7900  lumens.  The  "  mean  effective  absorp- 
tion coefficient"  of  the  room  as  a  whole  is,  therefore,  7900  -f-  12,200 
=  0.65.  Of  the  total  of  12,200  lumens  incident  upon  the  surfaces 
only  7900  come  directly  from  the  lamps,  12,200  —  7900  =  4300 
lumens  being  attributable  to  inter-reflection  between  the  surfaces. 

Since  only  2000  lumens  are  directed  toward  the  ceiling  (where 
400  are  absorbed  and  1600  are  reflected),  whereas  6000  are  directed 
toward  the  floor,  it  is  apparent  at  once  that  the  room  selected  is 
lighted  by  lamps  which  produced  considerably  more  light  in  the 
lower  than  in  the  upper  hemisphere;  that  is  to  say  use  is  not  made 
of  the  indirect  system  of  lighting. 

For  sake  of  comparison,  consider  now  the  same  room  with  the  same 
absorption  coefficients  with  the  same  total  amount  of  incident  flux 
upon  the  floor  and  walls  but  with  such  an  amount  directed  toward 
the  ceiling  that  the  reflection  therefrom  equals  the  amount  absorbed 
by  the  floor.  In  other  words  assume  that,  in  effect,  use  is  made  of 
the  " totally  indirect"  system — so  far  as  the  ceiling  and  floor  are 
concerned. 

The  light  flux  reflected  from  the  ceiling  (with  its  0.20  absorption 
=  0.80  reflection)  must  equal  the  5400  lumens  absorbed  by  the  floor. 
Hence,  5400  +  0.80  =  6750  equals  the  flux  incident  upon  the 
ceiling.  The  tabulation  will  then  be  as  follows: 


Area 

Incident 

Absorption 

Sq.  ft. 

Flux 

Ft.   C. 

Coef. 

Flux 

Ceiling  

2OOO 
2IOO 
2000 

6750 
4200 
6000 

3-37 

2.OO 
3.00 

o.  20 
0.50 
0.90 

1350 

2100 
5400 

Walls  
Floor 

Total  lumens  absorbed  =  8850. 

Total  mean  spherical  candle-power  8850  -5-  4*-  =  705. 

The  total  incident  flux  is  equal  to  6750  +  4200  +  6000  =  16,950 
lumens,  as  compared  with  the  former  12,200  lumens.     Thus  with 


1 8  ILLUMINATING   ENGINEERING   PRACTICE 

an  increase  of  1 1 .9  per  cent,  in  the  candle-power  of  the  lighting  units, 
there  is  an  increase  of  16,950  —  12,200  =  4750  or  39  per  cent,  in 
the  total  incident  flux  in  the  room,  with  an  increase  of  1350  — 
400  =  950  or  237  per  cent,  in  the  ceiling  illumination. 

In  referring  above  to  the  change  in  the  system  of  lighting  equip- 
ment use  was  made  of  the  term  "totally  indirect,"  in  order  to  con- 
centrate ideas  on  the  immediate  problem  at  hand  rather  than  to 
describe  the  system  actually  required  to  produce  the  results  indi- 
cated. With  only  6750  lumens  incident  upon  the  ceiling  which 
absorbs  1350  lumens,  and  a  total  of  4200  lumens  incident  upon  the 
walls  which  absorb  2100  lumens,  it  is  evident  that  the  lighting  units 
must  supply  considerable  flux  directly  to  the  walls,  and  hence  a 
"  totally  indirect"  system  of  lighting  would  not  produce  the  results 
required. 

As  already  stated,  in  actual  practice  conditions  are  not  so  readily 
denned  as  assumed  above,  and  the  absorption  method  cannot  be 
applied  practically  with  the  degree  of  simplicity  that  might  be  in- 
ferred from  the  above  examples,  but  it  can  be  looked  upon  as  a  most 
reliable  check  upon  the  more  complicated  methods  of  calculation 
and  as  an  invaluable  aid  in  solving  problems  connected  with  the 
illuminating  of  reflecting  surfaces,  investigating  quantitatively  the 
effect  of  inter-reflection  between  surfaces,  and  ascertaining  the 
limits  in  the  distribution  of  light  flux  between  illuminated  surfaces. 

UTILIZATION  FACTOR 

In  actual  practical  problems  in  illumination  design  it  has  been 
found  quite  convenient  to  make  use  of  the  direct  relations  between 
the  so-called  "total  lumens  utilized"  and  the  lumens  produced  by 
the  lighting  sources,  because  the  former  can  be  considered  to  be  the 
known  quantity  and  the  latter  the  unknown  quantity  in  one  phase 
of  the  practical  illumination  problem.  The  "lumens  utilized"  are 
assumed  to  be  equal  to  the  mean  illumination  (in,  say,  foot-candles) 
over  the  reference  plane  (say  30  in.  above  the  floor)  multiplied  by 
the  area  of  the  floor  (in  square  feet).  The  ratio  between  this 
quantity  of  lumens  to  the  lumens  produced  by  the  source  is  called 
the  "utilization  factor,"  or  "coefficient  of  utilization." 

Referring  to  the  two  examples  given  above  it  will  be  seen  that 
(if  the  illumination  on  the  reference  plane  be  assumed  to  be  equal 
to  that  at  the  floor  level)  the  utilization  factor  in  the  so-called  "  direct 
lighting"  problem  would  be  6000  -f-  7900  =  0.76,  whereas  in  the 
"indirect"  problem  it  would  be  6000  -f-  8850  =  0.68. 


MCALLISTER:  ILLUMINATION  UNITS  19 

A  study  of  the  above  problems  in  the  light  of  the  above  definition 
will  show  that  the  "utilization  factor"  depends  on  not  only  the 
system  of  lighting  and  the  absorption  by  the  ceiling  and  walls  but 
also  on  the  absorption  by  the  floor.  The  fact  of  the  matter  is  that 
with  highly  reflecting  floor,  walls  and  ceiling  the  "utilization  factor" 
would  have  a  value  greater  than  unity.  This  condition  would  seldom 
be  reached  in  practice  but  would  be  closely  approached  in  the  case 
of  a  dining-room  decorated  in  light  colors,  with  a  wide  expanse  of 
table  linen  and  light  floor  covering.  The  value  of  the  utilization 
factor  depends  upon  the  character  of  the  lighting  units,  relative 
dimensions  of  the  room,  color  and  material  of  the  ceiling,  walls  and 
floor.  Utilization  factors,  as  determined  by  actual  tests  under 
service  conditions,  will  be  discussed  fully  in  other  lectures,  and  need 
not  be  dwelt  upon  herein. 

ILLUMINATION  BY  DAYLIGHT 

Mention  has  already  been  made  of  the  simple  solution  of  problems 
that  would  otherwise  prove  quite  complex  by  means  of  certain 
solid  angular  relations.  This  statement  applies  with  particular 
force  to  problems  relating  to  the  illumination  from  either  artificial 
or  natural  sky-light  through  either  ceiling  or  side-wall  windows. 

In  view  of  the  fact  that  as  a  lighting  source  the  sky  is  located  at 
an  indefinite,  if  not  infinite,  distance  from  the  objects  illuminated, 
it  is  obvious  at  once  that  resort  cannot  be  had  to  the  method  of 
calculation  based  upon  the  so-called  "inverse-square  law."  For 
purposes  of  calculation  the  sky  can  best  be  considered  as  an  extended 
surface  source  of  undefined  shape  at  an  indefinite  distance  from  and 
completely  surrounding  the  observer,  being  visible  (except  for  local 
obstructions)  throughout  the  upper  hemisphere  above  the  horizontal 
plane  occupied  by  the  observer.  The  first  and  most  important  step 
is  to  establish  the  relation  between  the  illumination  produced  at 
any  chosen  point  by  such  a  source  and  the  solid  angle  subtended  by 
the  source  when  viewed  from  that  point;  or  rather  first  to  show 
that  the  solid  angular  relations  are  in  strict  agreement  with  the 
''inverse-square  law"  and  that  by  basing  the  calculations  exclusively 
on  the  former  the  latter  may  be  eliminated. 

Referring  to  Fig.  13,  consider  the  perfectly  general  case  of  a  small 
section  (dA)  of  a  surface  lighting  source  of  any  shape  or  inclination 
(a)  situated  at  any  distance  (R)  from  any  chosen  point  (P).  Let 
c  be  the  normal  emitting  density  (here  used  as  "apparent  candle- 


2O 


ILLUMINATING   ENGINEERING   PRACTICE 


power  per  unit  area")  of  this  source.     The  illumination  produced 
at  point  P,  from  the  inverse  square  and  cosine  laws,  is, 


=  c 


(dA)  cos  a 


Consider  now  the  illumination  that  would  be  produced  at  the 
same  point  P,  by  a  surface  source  (da) — at  the  circumference  of  the 
imaginary  enclosing  sphere — subtending  the  same  solid  angle  as 


Surface  of  any  shape 
in  any  position 


Fig.   13.  —  Photometric  relations  based  on  equality  of  solid  angles. 

(dA)  and  having  an  equal  normal  emitting  density  c.     The  illumina- 
tion at  the  central  point,  P,  would  be 


From  simple  geometrical  relations,  the  correctness  of  which  will 
be  appreciated  at  once  from  a  glance  at  Fig.  13,  it  is  seen  that  the 
areas  (da)  and  (dA)  bear  to  each  other  such  a  ratio  that 


(da)  =  ~(dA)  cos  a 


(3) 


Combining  equations  (3)  and  (2)  and  comparing  the  result  with 
equation  (i),  there  is  obtained 


c(dA)  cos  a 


(4) 


MCALLISTER:  ILLUMINATION  UNITS  21 

Equation  (4)  shows  that  when  dealing  with  surface  lighting  sources 
(such  as  the  sky,  artificial  windows,  or  indirect  lighting  systems) 
the  illumination  at  any  chosen  point  is  fully  defined  when  the  emit- 
ting density  of  the  source  and  the  solid  angle  subtended  by  th/e 
source  as  viewed  from  the  point  chosen  are  known.  Upon  this 
relation  can  be  based  some  extremely  simple  graphical  solutions  of 
problems  relating  to  illumination  by  daylight  or  by  surface  lighting 
sources  in  general. 

From  the  relations  derived  above  it  will  be  seen  that  in  calculating 
the  illumination  produced  by  a  surface  source  it  is  unnecessary  to 
know  either  the  candle-power  of  the  source  or  the  distance  of  the 
source  from  the  point  of  observation,  provided  only  that  the  solid 
angle  subtended  by  the  source  and  the  emitting  density  (expressed 
preferably  in  lumens  per  unit  area)  are  known.  It  is  obvious  there- 
fore that,  so  far  as  calculations  are  concerned,  any  surface  source  of 
indefinite  shape,  size  or  location  (such  as  the  exposed  sky  surface) 
can  be  treated  as  equivalent  to  a  definitely  located  source  of  definite 
shape  and  size  provided  only  that  such  values  are  assigned  to  the 
dimensions  and  position  of  the  substituted  surface  source  that  the 
solid  angles  are  the  same  as  before  and  the  assumed  emitting  density 
of  the  substituted  source  is  identical  with  that  of  the  original. 
Hence  in  day-lighting  problems  it  may  be  assumed  that  a  plane  sur- 
face source  of  sky- value  emitting  density  having  the  exact  dimensions 
of  the  exposed  area  of  either  a  ceiling  or  a  side- wall  window  can  safely 
be  substituted  for  the  sky. 

From  all  points  within  a  room  receiving  an  unobstructed  view  of 
the  sky  through  a  window,  the  window  itself  can  be  treated  as  the 
surface  lighting  source  having  an  emitting  density  in  lumens  per 
unit  area  exactly  equal  to  that  of  the  sky.  At  point  where  the  sky 
is  partly  hid  from  view  through  the  window,  the  solid  angle  is  corre- 
spondingly reduced  for  the  full  sky  density,  and  a  lower  density 
must  be  assigned  to  the  remaining  portion  of  the  original  solid  angle 
in  accordance  with  the  relative  reflection  coefficients  of  the  ob- 
structing areas  on  the  side  exposed  to  view  through  the  window. 

CIRCULAR  SKY-WINDOW  SOURCES 

The  above  described  method  of  substituting  a  surface  source  of 
known  dimensions  and  location  for  some  other  source  of  more  com- 
plex dimensions  and  uncertain  location  is  invaluable  in  determining 
the  illumination  produced  by  the  light  received  through  ceiling 


22 


ILLUMINATING   ENGINEERING  PRACTICE 


windows  from  either  natural  or  artificial  sources.  For  this  purpose, 
it  is  most  convenient  to  substitute  for  any  square,  rectangular  or 
irregularly  shaped  window  source  a  circular  or  elliptical  source  of 
equal  emitting  density,  equivalent  in  area  and  in  practical  solid 
angular  relations. 

In  Fig.  14,  let  ACB  represent  an  edgewise  view  of  a  flat  circular 
source,  assumed  to  be  in  the  ceiling  of  a  room,  having  any  chosen 
value  of  uniform  emitting  density  and  any  desired  radius.  If 
through  the  edges  A  and  B  there  be  passed  an  imaginary  sphere 
of  any  chosen  size  whatsoever — such  as  ADB — with  its  center  at 


5R- 


10  11       12      13     14  16 

Pig.   14. — Equilux  spheres  with  illuminating  values  in  per  cent,  for  spheres  passing  through 
points  i,  2,  3,  4,  etc.,  length  units  below  source. 

some  point  on  a  vertical  line  passing  through  the  center  c,  the  inner 
surface  of  this  imaginary  sphere  below  the  lighting  source  will 
receive  an  illumination  which  will  be  uniform  in  intensity  normal 
to  the  surface  of  the  sphere  throughout  the  whole  interior  of  the 
imaginary  sphere. 

The  statement  just  made  is  not  based  on  the  equality  of  the  solid 
angles  subtended  by  the  source  when  viewed  from  various  points 
along  the  interior  of  the  imaginary  sphere;  in  fact,  the  solid  angle  is 
not  constant  but  varies  directly  with  the  cosine  of  the  plane  angular 
deviation  of  the  point  of  observation  from  the  position  directly 
below  the  center  of  the  source.  However,  the  above  statement 


MCALLISTER:  ILLUMINATION  UNITS  23 

applies  not  to  the  illumination  density  on  a  plane  normal  to  the  line 
of  observation  of  the  source  from  the  point  chosen  but  to  the 
density  normal  to  the  imaginary  sphere  at  this  point.  The  ratio 
between  these  two  densities  varies  inversely  with  the  cosine  of  the 
angular  deviation  just  mentioned,  so  that  the  final  product  is  con- 
stant, and  hence  the  density  normal  to  the  imaginary  sphere  is 
constant. 

Evidently  the  exact  value  of  the  density  (in  lumens  per  unit 
area)  of  the  normal  illumination  against  the  inner  surface  of  the 
sphere  will  bear  to  the  emitting  density  (in  lumens  per  unit  area)  of 
the  circular  surface  source  the  inverse  ratio  of  the  interior  area  of  the 
exposed  zone  of  the  sphere  to  the  area  of  the  lighting  source,  since 
the  lumens  produced  must  equal  those  utilized.  From  solid  geo- 
metrical relations  it  will  be  seen  that  this  ratio  equals  the  square  of 
the  radius  AC  to  the  diagonal  AD.  When  the  radius  of  the  circular 
source  is  taken  as  the  unit  of  length  for  the  measurement  of  all  dis- 
tances, and  the  unit  of  illumination  density  (lumens  per  unit  area)  is 
taken  as  the  emitting  density  of  the  source,  then  the  percentage  value 
of  the  illumination  density  on  the  interior  of  the  sphere  is  equal  to 
100  divided  by  the  square  of  the  diagonal  AD. 

For  convenience  any  sphere  passing  through  the  edges  A  and  B,  as 
just  indicated,  can  be  referred  to  as  an  "equilux"  sphere  (the  "lux" 
being  one  of  the  several  units  of  illumination).  Equilux  spheres  of 
the  proper  sizes  being  employed,  one  is  enabled  "to  explore  the 
whole  region"  illuminated  by  the  source,  and  to  ascertain  immedi- 
ately for  any  desired  point  within  the  space  explored  the  exact  value 
of  that  component  of  the  light  flux  which  is  normal  to  the  particular 
equilux  sphere  passing  through  that  point. 

In  Fig.  14  are  indicated  numerous  equilux  spheres  and  the  points 
of  intersection  of  these  spheres  with  horizontal  planes  (floors)  at 
light  distances  of  3.5  units  and  7  units  of  length  (radii)  below  the 
source,  and  with  vertical  planes  (walls)  3  and  5  length  units  distant 
from  the  center  of  the  source. 

Points  of  intersection  of  the  two  assumed  horizontal  planes  with 
the  equilux  spheres  evidently  lie  on  circles  having  as  the  common 
center  the  point  on  the  floor  immediately  below  the  center  of  the 
circular  ceiling  lighting  source. 

It  is  an  interesting  fact  that  at  any  point  on  the  floor  the  compo- 
nent of  the  flux  normal  to  the  floor  is  equal  to  the  component  normal 
to  the  equilux  sphere  at  that  point,  so  that  the  values  of  equilux 
density  are  simultaneously  the  values  of  light  flux  density  normal 


24  ILLUMINATING   ENGINEERING   PRACTICE 

to  the  horizontal  plane  (floor).  Expressed  in  other  words,  the  illu- 
mination along  the  floor  at  any  point  is  known  at  once  when  one  has 
determined  the  value  of  light  flux  density  on  the  equilux  sphere 
passing  through  that  point.  Hence,  the  whole  problem  of  floor 
illumination  density  and  distribution  determination  is  completely 
solved  when  the  equilux  spheres  and  intersecting  lines  have  been 
constructed.  One  could  not  well  wish  for  a  simpler  solution. 

In  Fig.  15  are  shown  results  obtained  directly  from  Fig.  14.  It 
will  be  noted  that  with  a  ceiling  height  equal  to  3.5  times  the  radius 
of  the  lighting  source  the  light  flux  density  at  a  point  on  the  floor 
immediately  below  the  center  of  the  source  reaches  a  value  of  about 


012345678 
Distance  from  Point  below  Center  of  Source 

Fig.   15. — Graphs  of  illuminations  on  floors  with  two  different  ceiling  heights. 

7.55  per  cent,  of  the  emitting  density  of  the  source,  while  the  density 
along  the  floor  decreases  rapidly  with  increase  in  the  distance  from 
the  point  of  maximum  density.  With  a  ceiling  twice  as  high  as 
formerly  the  maximum  light  flux  density  is  reduced  to  2  per  cent., 
but  the  rate  of  decrease  with  increase  of  distance  from  the  point 
below  the  center  of  the  source  is  much  less;  in  fact,  at  distance  greater 
than  5  units  of  length  (radii)  the  light  from  the  high  source  is  greater 
than  that  from  the  low  source.  This  fact  will  be  appreciated  when 
it  is  recalled  that  the  "solid  angle"  subtended  by  the  source  when 
viewed  from  the  floor  at  a  great  distance  from  the  center  is  larger 
with  the  high  ceiling  than  with  the  low  ceiling. 

Even  in  the  case  of  ceiling  sources  it  is  at  times  desirable  to  calcu- 


MCALLISTER:  ILLUMINATION  UNITS  •         25 

late  the  illumination  on  the  side-walls,  and  it  is  well  to  have  avail- 
able some  method  for  this  purpose.  When  it  is  remembered  that 
any  method  developed  for  use  with  ceiling  window  sources  can  be 
supplied  at  once  to  side  window  sources,  it  will  be  appreciated  that 
the  method  of  calculating  the  wall  illumination  with  ceiling  window 
sources  becomes  that  of  calculating  the  floor  illumination  with  side 
window  sources,  and  the  desirability  of  having  a  simple  method 
will  be  apparent.  Such  a  method,  for  convenience  described  in  con- 
nection with  ceiling  sources,  is  as  follows: 

At  any  point  on  any  vertical  plane  as  far  below  the  ceiling  source 
as  this  plane  is  distant  from  the  center  of  the  source  in  the  normal 
(nearest)  direction,  the  illumination  normal  to  the  vertical  plane  at 
that  point  is  equal  to  that  normal  to  the  horizontal  plane  at  this  point. 
At  any  other  point  the  normal  illumination  on  the  vertical  plane  bears 
to  the  normal  illumination  on  the  horizontal  plane  at  this  point,  the 
ratio  of  the  distance  of  the  vertical  to  the  distance  of  the  horizontal 
plane  from  the  center  of  the  source,  each  distance  being  measured 
in  a  direction  normal  (shortest)  to  the  plane  considered.  When 
solving  problems  relating  to  plane  circular  lighting  sources  by  means 
of  the  equilux  spheres  one  can  easily  determine  the  illumination 
normal  to  any  horizontal  plane  and  can  then  calculate  the  illumi- 
nation on  any  vertical  plane  by  direct  proportion. 

By  obvious  modifications  the  above  described  methods  for  deter- 
mining the  floor  and  wall  illumination  produced  by  circular  ceiling 
sources,  can  be  applied  to  similar  problems  relating  to  ceiling  and 
side-wall  sources  of  any  shape  or  size,  and  to  problems  of  all  kinds 
relating  to  daylight  illumination. 

BRIGHTNESS  UNIT— THE  LAMBERT 

Although  it  is  not  unusual  to  refer  to  an  isolated  lighting  unit  of 
the  "point"  type  (that  is,  of  the  type  treated  as  equivalent  to  a 
"  point  source  "  as  distinguished  from  a  "  surface  source  "),  as  possess- 
ing a  certain  candle-power,  yet  it  is  recognized  that  the  lighting 
characteristics  of  the  source  are  not  fully  defined  until  the  whole 
space  distribution  of  the  candle-power  is  so  specified  that  the 
"mean  spherical  candle-power"  or  the  output  in  lumens  can  be 
determined.  Illumination  calculations  have  been  greatly  simplified 
by  the  introduction  of  the  "lumen"  as  a  unit  in  which  to  express 
not  only  the  output  from  the  source  but  also  the  absorption  by  the 
surfaces  illuminated,  the  total  lumens  produced  by  the  source  being 
in  every  case  equal  to  the  total  lumens  absorbed  by  the  surfaces. 


26  ILLUMINATING   ENGINEERING   PRACTICE 

Moreover,  certain  seemingly  complicated  problems  relating  to  sur- 
face sources,  inter-reflecting  walls,  ceilings,  etc.,  permit  of  the  simp- 
lest possible  solution  when  use  is  made  of  the  lumen  rather  than  the 
candle-power  conception  in  expressing  the  output,  the  output  den- 
sity and  the  "appearance"  of  the  source  to  the  eye  when  viewed  from 
various  directions.  The  ratios  involved  in  the  substitution  are 
fundamental  and  do  not  depend  upon  the  character  of  the  source, 
being  the  same  for  " non-mat"  as  for  "mat"  surfaces. 

With  a  perfectly  diffusing  "mat"  surface  source,  multiplying  the 
constant  value  of  "lumens  per  square  foot"  of  the  source  by  the 
total  area  of  the  source  in  square  feet  gives  the  exact  value  of  the 
output  in  lumens  independent  in  every  respect  of  the  shape  or  size 
of  the  source. 

When  use  is  made  of  the  "apparent  candle-power  per  square 
inch"  in  this  connection  multiplying  this  value  by  the  whole  area 
of  the  source  in  square  inches  does  not  give  the  "mean  spherical 
candle-power  of  the  source;  it  gives  a  value  differing  therefrom 
in  the  ratio  of  i  to  4  under  all  conditions.  In  the  single  case  of  a 
perfectly  diffusing  spherical  source — of  which  the  projected  area 
equals  one-fourth  the  total  area — the  total  mean  spherical  candle- 
power  is  equal  to  the  product  of  the  "apparent  candle-power  per 
square  inch"  by  the  projected  area  in  square  inches. 

It  is  evident,  therefore,  that  so  far  as  concerns  the  output  of 
"mat"  surfaces,  the  expression  lumens  per  unit  area  has  a  definite 
significance  and  cannot  be  misinterpreted,  while  the  term  "ap- 
parent candle-power  per  unit  area"  may  or  may  not  be  correctly 
interpreted. 

It  is  frequently  assumed,  tacitly,  that  a  surface  source  is  made 
up  of  an  infinite  number  of  "point  sources."  If  such  were  the  case, 
plain  surface  sources  would  emit  in  all  directions  rather  than  in  one 
hemisphere,  and  the  "cosine  law"  of  emission  would  be  invalid. 
The  fact  is  that  a  surface  source  is  made  up  of  an  infinite  number  of 
infinitesimal  plane  surface  elements,  each  of  which  radiates  in  a 
single  hemisphere  and  does  not  act  like  a  point  source.  When  the 
"cosine  law"  is  applicable  the  "mean  spherical  candle-power"  of 
each  element  of  the  source  is  equal  to  one-fourth  of  the  maximum 
apparent  candle-power  of  the  element,  or  is  equivalent  to  one- 
fourth  of  the  total  area  of  the  element  multiplied  by  the  "apparent 
candle-power"  per  unit  projected  area  viewed  from  any  direction 
within  the  radiating  hemisphere.  Hence  the  universal  i  to  4  ratio 
noted  above  for  "mat"  surfaces. 


MCALLISTER:  ILLUMINATION  UNITS  27 

In  view  of  the  fact  that  as  a  rnatter  of  actual  practice  almost  all 
surface  sources  are  either  of  the  "mat"  type  or  are  treated  as  though 
they  obeyed  the  "cosine  law  of  emission,"  it  would  seem  that  the 
very  great  simplification  in  calculation  brought  about  by  substitut- 
ing the  lumen  conception  for  the  candle-power  conception  would 
fully  justify  the  substitution  even  if  the  results  obtained  were  some- 
what inaccurate  in  the  case  of  "non-mat"  surfaces.  The  fact  is, 
however,  that  the  ratio  involved  in  the  substitution  is  absolutely 
fundamental  and  does  not  depend  upon  the  character  of  the  emitting 
surface. 

As  has  already  been  shown  the  ratio  of  the  "output  in  lumens  per 
square  foot"  to  the  "apparent  candle-power  per  square  foot"  of 
all  "mat"  surface  sources  is  the  constant  IT  -r-  i.  It  is  equally 
true  that  the  "apparent  foot-candles"  of  a  source  of  any  character 
whatsoever  viewed  from  any  chosen  direction  bears  to  the  "ap- 
parent candle-power  per  foot"  of  the  same  source  viewed  from  the 
same  direction  the  identical  ratio  IT  -4-  i  the  "TT  ratio"  not  being 
dependent  in  any  respect  upon  the  "cosine  law  of  emission."  The 
fact  of  the  matter  is  that  the  TT  ratio  is  based  on  solid  geomet- 
rical relations,  and  is  independent  of  the  space  distribution  of  the 
candle-power  in  any  direction  except  that  toward  the  point  under 
consideration. 

That  is  to  say,  there  is  a  definite  numerical  ratio  between  the 
apparent  foot-candle  density  of  a  source  and  its  apparent  candle- 
power  per  square  foot,  which  ratio  is  the  same  under  all  conceivable 
conditions  of  space  distribution  of  the  candle-power. 

It  can,  therefore,  be  stated  that  any  illumination  photometer  of 
the  "pyrometer"  type  calibrated  to.  read  in  "apparent  emitted 
foot-candles"  or  "lamberts"  will  when  pointed  toward  a  bright 
surface  give  an  exact  measure  of  the  "apparent  candle-power  per 
square  inch"  of  the  source  provided  only  that  the  "apparent 
foot-candle"  value  is  divided  by  144^  or  TT  as  the  case  may  be — a 
constant  in  no  way  dependent  upon  the  "cosine  law." 

Hence  in  order  to  determine  the  "apparent  candle-power  per 
square  inch"  of  a  surface  source  in  a  chosen  direction,  it  is  unneces- 
sary to  measure  the  apparent  candle-power  in  this  direction  of  a 
limited  isolated  section  of  known  projected  area;  the  identical 
result  can  be  obtained  much  more  conveniently  by  observing  the 
"apparent  foot-candle  density"  apparent  lumens  per  square  foot)  or 
"lamberts  (lumens  per  sq.  cm.)  of  the  source  when  viewed  from  the 
chosen  direction  and  dividing  this  value  by  the  constant  144^  or  TT. 


28  ILLUMINATING   ENGINEERING   PRACTICE 

It  is  to  be  noted  that,  independent  in  every  respect  of  the  name 
given  to  the  quantity  dealt  with,  the  measurable  value  of  the 
"apparent  foot-candle"  density  of  a  surface  source  differs  from  the 
measurable"  apparent  candle-power  per  square  inch"  of  the  same 
source  viewed  in  the  same  direction  in  a  definite  numerical  ratio, 
without  regard  to  the  character  of  the  surface.  The  ''apparent 
candle-power  per  square  inch"  is  known  as  " brightness"  and  the 
"apparent  foot-candle  density"  observed  from  the  same  direction 
is  also  "brightness." 

The  unit  of  "brightness,"  the  "lambert"  is  equal  to  neither  the 
"apparent  foot-candle"  nor  the  "apparent  candle-power  per  square 
inch."  It  is  identical  with  the  "apparent  lumen  per  square  centi- 
meter," being  929  times  as  bright  as  the  "apparent  lumen  per  square 
foot"  or  its  equivalent  the  "apparent  foot-candle."  Hence  a  per- 
fectly diffusing  surface  emitting  or  reflecting  one  lumen  per  square 
foot  (one  apparent  foot-candle)  will  have  a  brightness  of  1.076 
millilamberts. 

The  mean  effective  value  of  the  output  density  of  a  surface  source 
(and  all  practical  sources  are  surfaces)  can  best  be  found  by  dividing 
the  total  output  in  lumens  by  the  total  area  of  the  source  expressed 
in  some  convenient  unit.  This  numerical  value  is  absolutely 
identical  with  the  mean  effective  value  of  the  appearance  of  the  source 
in  apparent  lumens  per  square  foot  ("apparent  foot-candles")  or 
apparent  lumens  per  sq.  cm.  ("lamberts"),  the  latter  being  the 
standardized  unit  for  expressing  the  "appearance"  or  "brightness" 
of  a  surface  source. 

Only  in  the  case  of  a  perfectly  "mat"  source  is  either  the  "out- 
put density"  or  the  "appearance"  uniform  in  all  directions,  but  no 
error  is  involved  in  the  solution  of  problems  dealing  with  non-uni- 
form sources  when  mean  effective  values  are  substituted  for  the 
variable  space  values,  provided  only  that  the  solution  is  recognized 
as  being  expressed  in  mean  effective  values. 

For  example,  one  can  determine  quickly  by  the  use  of  mean  effect- 
ive values  the  average  illumination  produced  over,  say,  the  whole 
floor  area  of  a  room,  but  when  he  wishes  to  know  the  space  variations 
in  the  illumination  throughout  a  room  he  must  resort  to  some  more 
laborious  point-by-point  method.  In  most  problems  of  today, 
with  lighting  units  giving  widely  distributed  flux  the  prime  essential 
feature  is  no  longer  the  proper  space  distribution  of  the  illumina- 
tion, but  rather  the  production  of  adequate  average  illumination 
without  excessive  brightness  in  the  field  of  view. 


MCALLISTER:  ILLUMINATION  UNITS  29 

PRESENT  DAY  CALCULATING  METHODS 

The  adoption  of  the  method  of  expressing  the  "brightness"  or 
"appearance"  of  a  lighting  source  in  terms  of  physical  reality  based 
on  the  "surface-source"  conception,  rather  than  using  the  mathe- 
matically derived  expression  of  brightness  in  terms  of  tacitly  as- 
sumed point-sources  with  surface-source  characteristics,  represents 
a  step  in  the  progress  of  illumination  calculations  from  the  methods 
of  the  mathematical-physicist  to  those  of  the  engineer  similar 
in  results  accomplished  to  the  adoption  of  the  now  universally  em- 
ployed magnetic  flux  and  flux-density  conceptions  for  the  earlier 
isolated  magnetic  pole  conception  in  the  evolution  of  electrical 
calculations  from  those  of  the  physicist  to  those  of  the  engineer. 

Similarly  the  practical  abandonment  of  the  laborious  point-by- 
point  methods  of  illumination  determination  in  favor  of  the  much 
more  rapid  output-utilization  methods  based  on  the  law  of  conserva- 
tion, converts  the  calculations  of  the  lighting  expert  from  those  of 
the  physicist  to  those  of  the  engineer.  Just  as  the  mathematical 
physicist  will  continue  to  deal  with  fictitious  isolated  magnetic  poles 
and  by  careful  transformation  of  his  equations  will  derive  results 
in  exact  accord  with  physical  facts,  so  will  he  continue  t'o  employ 
point-source  conception  and  the  point-by-point  methods  of  calcula- 
tions and  his  results  will  be  true  to  nature,  but  the  practical  illuminat- 
ing engineer  will  train  his  mind  to  think  in  terms  of  surface  sources 
rather  than  point-sources,  and  will  base  the  few  equations  needed 
by  him  in  his  everyday  work  on  the  law  of  conservation — either 
directly  or  indirectly  recognized.  The  results  obtained  by  him  with 
the  minimum  of  exertion  will  be  absolutely  identical  with  the  results 
derived  much  more  laboriously  by  the  physicist  employing  the  time- 
honored  methods  with  which  he  is  familiar. 

ILLUMINATION  UNITS  AND  NOMENCLATURE 

In  order  to  render  most  serviceable  for  reference  the  book  in  which 
these  lectures  are  reprinted  there  is  here  presented  the  latest  (1916) 
list  of  units,  definitions  and  abbreviations  of  the  Committee  on 
Nomenclature  and  Standards  of  the  Illuminating  Engineering 
Society. 

DEFINITIONS 

1.  Luminous  Flux  is  radiant  power  evaluated  according  to  its  visibility; 
i.e.,  its  capacity  to  produce  the  sensation  of  light. 


30  ILLUMINATING  ENGINEERING   PRACTICE 

2.  The  visibility,  Kx  of  radiation,  of  a  particular  wave-length,  is  the 
ratio  of  the  luminous  flux  to  the  radiant  power  producing  it. 

3.  The  mean  value  of  the  visibility,  K  m,  over  any  range  of  wave-lengths, 
or  for  the  whole  visible  spectrum  of  any  source,  is  the  ratio  of  the  total 
luminous  flux  (in  lumens)  to  the  total  radiant  power  (in  ergs  per  second, 
but  more  commonly  in  watts). 

4.  The  luminous  intensity,  I,  of  a  point  source  of  light  is  the  solid  angular 
density  of  the  luminous  flux  emitted  by  the  source  in  the  direction  con- 
sidered; or  it  is  the  flux  per  unit  solid  angle  from  that  source. 

Defining  equation: 


or,  if  the  intensity  is  uniform, 

[-% 

O) 

where  co  is  the  solid  angle. 

5.  Strictly  speaking  no  point  source  exists,  but  any  source  of  dimensions 
which  are  negligibly  small  by  comparison  with  the  distance  at  which  it  is 
observed  may  be  treated  as  a  point  source. 

6.  Illumination,  on  a  surface,  is  the  luminous  flux-density  on  that  sur- 
face, or  the  flux  per  unit  of  intercepting  area. 

Defining  equation: 


or,  when  uniform, 


where  5  is  the  area  of  the  intercepting  surface. 

7.  Candle  —  the  unit  of  luminous  intensity  maintained  by  the  national 
laboratories  of  France,  Great  Britain,  and  the  United  States.1 

8.  Candlepower  —  luminous  intensity  expressed  in  candles. 

9.  Lumen  —  the  unit  of  luminous  flux,  equal  to  the  flux  emitted  in  a 
unit  solid  angle  (steradian)  by  a  point  source  of  one  candle-power.2 

10.  Lux  —  a  unit  of  illumination  equal  to  one  lumen  per  square  meter. 
The  cgs.  unit  of  illumination  is  one  lumen  per  square  centimeter.     For  this 
unit  Blondel  has  proposed  the  name  "Phot."     One  millilumen  per  square 
centimeter  (milliphot)  is  a  practical  derivative  of  the  cgs.  system.     One 
foot-candle  is  one  lumen  per  square  foot  and  is  equal  to  1.0764  milliphots. 

The  milliphot  is  recommended  for  scientific  records. 

11.  Exposure  —  the  product  of  an  illumination  by  the  time.     Blondel 
has  proposed  the  name  "phot-second"  for  the  unit  of  exposure  in  the  cgs. 
system.     The  microphot  second  (o.oooooi  phot-second)  is  a  convenient 
unit  for  photographic  plate  exposure. 

1  This  unit,  which  is  used  also  by  many  other  countries,  is  frequently  referred  to  as  the 
international  candle. 

1  A  uniform  source  of  one  candle  emits  4  if  lumens. 


MCALLISTER:  ILLUMINATION  UNITS  31 

12.  Specific  luminous  radiation,  E1 — the  luminous  flux-density  emitted 
by  a  surface,  or  the  flux  emitted  per  unit  of  emissive  area.     It  is  expressed 
in  lumens  per  square  centimeter. 

Denning  equation: 

For  surfaces  obeying  Lambert's  cosine  law  of  emission, 

E'  =  T&O. 

13.  Brightness,  b,ot  an  element  of  a  luminous  surface  from  a  given  posi- 
tion, may  be  expressed  in  terms  of  the  luminous  intensity  per  unit  area 
of  the  surface  projected  on  a  plane  perpendicular  to  the  line  of  sight,  and 
including  only  a  surface  of  dimensions  negligibly  small  in  comparison  with 
the  distance  at  which  it  is  observed.    It  is  measured  in  candles  per  square 
centimeter  of  the  projected  area. 

Defining  equation: 

A  dl 

~  dS  cos  0' 

(where  6  is  the  angle  between  the  normal  to  the  surface  and  the  line  of 
sight). 

14.  Normal  brightness,  bQ,  of  an  element  of  a  surface  (sometimes  called 
specific  luminous  intensity)  is  the  brightness  taken  in  a  direction  normal 
to  the  surface.1 

Defining  equation: 

;,        dl 
bo  =  dS' 

or,  when  uniform, 

b°  =  S' 

16.  Brightness  may  also  be  expressed  in  terms  of  the  specific  luminous 
radiation  of  an  ideal  surface  of  perfect  diffusing  qualities,  i.e.,  one  obeying 
Lambert's  cosine  law. 

16.  Lambert — the  cgs.  unit  of  brightness,  the  brightness  of  a  perfectly 
diffusing  surface  radiating  or  reflecting  one  lumen  per  square  centimeter. 
This  is  equivalent  to  the  brightness  of  a  perfectly  diffusing  surface  having 
a  coefficient  of  reflection  equal  to  unity  and  an  illumination  of  one  phot. 
For  most  purposes,  the  millilambert  (o.ooi  lambert)  is  the  preferable 
practical  unit. 

A  perfectly  diffusing  surface  emitting  one  lumen  per  square  foot  will 
have  a  brightness  of  1.076  millilamberts. 

Brightness  expressed  in  candles  per  square  centimeter  may  be  reduced 
to  lamberts  by  multiplying  by  IT  =  3.14. 

Brightness  expressed  in  candles  per  square  inch  may  be  reduced  to  foot- 
candle  brightness  by  multipyling  by  the  factor  144^  =  452. 

1  In  practice,  the  brightness  &  of  a  luminous  surface  or  element  thereof  is  observed  and 
not  the  normal  brightness  60.  For  surfaces  for  which  the  cosine  law  of  emission  holds, 
the  quantities  b  and  &o  are  equal. 


32  ILLUMINATING   ENGINEERING   PRACTICE 

Brightness-expressed  in  candles  per  square  inch  may  be  reduced  to  lam- 
berts  by  multiplying  by  Tr/6.45  =  0.4868. 

In  practice,  no  surface  obeys  exactly  Lambert's  cosine  law  of  emission; 
hence  the  brightness  of  a  surface  in  lamberts  is,  in  general,  not  numerically 
equal  to  its  specific  luminous  radiation  in  lumens  per  square  centimeter. 

Defining  equations: 

/  -dF 

L~dS 
or,  when  uniform, 


17.  Coefficient  of  reflection  —  the   ratio   of   the   total  luminous   flux 
reflected  by  a  surface  to  the  total  luminous  flux  incident  upon  it.     It  is  a 
simple  numeric.     The  reflection  from  a  surface  may  be  regular,  diffuse 
or  mixed.     In  perfect  regular  reflection,  all  of  the  flux  is  reflected  from  the 
surface  at  an  angle  of  reflection  equal  to  the  angle  of  incidence.     In  perfect 
diffuse  reflection  the  flux  is  reflected  from  the  surface  in  all  directions  in 
accordance  with  Lambert's  cosine  law.     In  most  practical  case^  there  is  a 
superposition  of  regular  and  diffuse  reflection. 

18.  Coefficient  of  regular  reflection  is  the  ratio  of  the  luminous  flux 
reflected  regularly  to  the  total  incident  flux. 

19.  Coefficient  of  diffuse  reflection  is  the  ratio  of  the  luminous  flux 
reflected  diffusely  to  the  total  incident  flux. 

Defining  equation: 

Let  m  be  the  coefficient  of  reflection  (regular  or  diffuse). 

Then,  for  any  given  portion  of  the  surface, 

E' 


20.  Lamp  —  a  generic  term  for  an  artificial  source  of  light. 

21.  Primary    luminous    standard  —  a    recognized    standard    luminous 
source  reproducible  from  specifications. 

22.  Representative  luminous  standard  —  a  standard  of  luminous  inten- 
sity adopted  as  the  authoritative  custodian  of  the  accepted  value  of  the 
unit. 

23.  Reference  standard  —  a  standard  calibrated  in  terms  of   the   unit 
from  either  a  primary  or  representative  standard  and  used  for  the  cali- 
bration of  working  standards. 

24.  Working  standard  —  any  standardized  luminous  source  for  daily  use 
in  photometry. 

25.  Comparison  lamp  —  a  lamp  of  constant  but  not  necessarily  known 
candlepower  against  which  a  working  standard  and  test  lamp  are  succes- 
sively compared  in  a  photometer. 

26.  Test  lamp,  in  a  photometer  —  a  lamp  to  be  tested. 


MCALLISTER:  ILLUMINATION  UNITS  33 

27.  Performance  curve — a  curve  representing  the  behavior  of  a  lamp 
in  any  particular  (candlepower,  consumption,  etc.)  at  different  periods  dur- 
ing its  life. 

28.  Characteristic  curve — a  curve  expressing  a  relation  between  two 
variable  properties  of  a  luminous  source,  as  candlepower  and  volts,  candle- 
power  and  rate  of  fuel  consumption,  etc. 

29.  Horizontal   distribution   curve — a   polar   curve   representing   the 
luminous  intensity  of  a  lamp,  or  lighting  unit,  in  a  plane  perpendicular  to 
the  axis  of  the  unit,  and  with  the  unit  at  the  origin. 

30.  Vertical  distribution  curve — a  polar  curve  representing  the  lumin- 
ous intensity  of  a  lamp,  or  lighting  unit,  in  a  plane  passing  through  the 
axis  of  the  unit  and  with  the  unit  at  the  origin.     Unless  otherwise  specified, 
a  vertical  distribution  curve  is  assumed  to  be  an  average  vertical  distri- 
bution curve,  such  as  may  in  many  cases  be  obtained  by  rotating  the  unit 
about  its  axis,  and  measuring  the  average  intensities  at  the  different  eleva- 
tions.    It  is  recommended  that  in  vertical  distribution  curves,  angles  of 
elevation  shall  be  counted  positively  from  the  nadir  as  zero,  to  the  zenith 
as  1 80°.     In  the  case  of  incandescent  lamps,  it  is  assumed  that  the  vertical 
distribution  curve  is  taken  with  the  tip  downward. 

31.  Mean  horizontal  candlepower  of  a  lamp — the  average  candlepower 
in  the  horizontal  plane  passing  through  the  luminous  center  of  the  lamp. 

It  is  here  assumed  that  the  lamp  (or  other  light  source)  is  mounted  in 
the  usual  manner,  or,  as  in  the  case  of  an  incandescent  lamp,  with  its  axis 
of  symmetry  vertical. 

32.  Mean  spherical  candlepower  of  a  lamp — the  average  candle-power 
of  a  lamp  in  all  directions  in  space.     It  is  equal  to  the  total  luminous  flux 
of  the  lamp  in  lumens  divided  by  4?r. 

33.  Mean  hemispherical  candlepower  of  a  lamp  (upper  or  lower) — the 
average  candlepower  of  a  lamp  in  the  hemisphere  considered.     It  is  equal 
to  the  total  luminous  flux  emitted  by  the  lamp  in  that  hemisphere  divided 
by  27r. 

34.  Mean  zonal  candlepower  of  a  lamp — the  average  candlepower  of  a 
lamp  over  the  given  zone.     It  is  equal  to  the  total  luminous  flux  emitted 
by  the  lamp  in  that  zone  divided  by  the  solid  angle  of  the  zone. 

35.  Spherical  reduction  factor  of  a  lamp — the  ratio  of  the  mean  spherical 
to  the  mean  horizontal  candlepower  of  the  lamp.1 

36.  Photometric  tests  in  which  the  results  are  stated  in  candlepower 
should  be  made  at  such  a  distance  from  the  source  of  light  that  the  latter 
may  be  regarded  as  practically  a  point.     Where  tests  are  made  in  the 
measurement  of  lamps  with  reflectors,  or  other  accessories  at  distances 
such  that  the  inverse-square  law  does  not  apply,  the  results  should  always 
be  given  as  "apparent  candlepower"  at  the  distance  employed,  which 
distance  should  always  be  specifically  stated. 

1  In  the  case  of  a  uniform  point-source,  this  factor  would  be  unity,  and  for  a  straight 
cylindrical  filament  obeying  the  cosine  law  it  would  be  ir/4. 
3 


34  ILLUMINATING   ENGINEERING  PRACTICE 

The  output  of  all  illuminants  should  be  expressed  in  lumens. 

37.  Illuminants  should  be  rated  upon  a  lumen  basis  instead  of  a  candle- 
power  basis. 

38.  The  specific  output  of  electric  lamps  should  be  stated  in  terms  of 
lumens  per  watt  and  the  specific  output  of  illuminants  depending  upon 
combustion  should  be  stated  in  lumens  per  British  thermal  unit  per  hour. 
The  use  of  the  term  "efficiency"  in  this  connection  should  be  discouraged. 

When  auxiliary  devices  are  necessarily  employed  in  circuit  with  a  lamp, 
the  input  should  be  taken  to  include  both  that  in  the  lamp  and  that  in  the 
auxiliary  devices.  For  example,  the  watts  lost  in  the  ballast  resistance 
of  an  arc  lamp  are  properly  chargeable  to  the  lamp. 

39.  The  specific  consumption  of  an  electric  lamp  is  its  watt  consump- 
tion per  lumen.     "Watts  per  candle"  is  a  term  used  commercially  in  con- 
nection with  electric  incandescent  lamps,  and  denotes  watts  per  mean 
horizontal  candle. 

40.  Life  tests — Electric  incandescent  lamps  of  a  given  type  may  be 
assumed  to  operate  under  comparable  conditions  only  when  their  lumens 
per  watt  consumed  are  the  same.    Life  test  results,  in  order  to  be  com- 
pared must  'be  either  conducted  under,  or  reduced  to,  comparable  condi- 
tions of  operation. 

41.  In  comparing  different  luminous  sources,  not  only  should  their 
candlepower  be  compared,  but  also  their  relative  form,  brightness,  distri- 
bution of  illumination  and  character  of  light. 

42.  Lamp  Accessories. — A  reflector  is  an  appliance  the  chief  use  of 
which  is  to  redirect  the  luminous  flux  of  a  lamp  in  a  desired  direction  or 
directions. 

43.  A  shade  is  an  appliance  the  chief  use  of  which  is  to  diminish  or  to 
interrupt  the  flux  of  a  lamp  in  certain  directions  where  such  flux  is  not 
desirable.     The  function  of  a  shade  is  commonly  combined  with  that  of  a 
reflector. 

44.  A  globe  is  an  enclosing  appliance  of  clear  or  diffusing  material  the 
chief  use  of  which  is  either  to  protect  the  lamp  or  to  diffuse  its  light. 

45.  Photometric  Units  and  Abbreviations. 

Abbreviation 

Photometric  Name  of  Symbols  and  defin-  for  name 

quantity  unit  ing  equations  of  unit 

1.  Luminous  flux  Lumen  F.^  i 

d¥          d^f 

2.  Luminous  intensity  Candle  I  =  ^,  T  —  -^  cp. 

,  lunation  ^  E  -  f  -  J  cos  „     ph.  f, 

[  Phot-second 

4.  Exposure  i  Micro  phot-          Ei  phs.  /xphs. 

second 


MCALLISTER:  ILLUMINATION  UNITS  35 

Photometric  Name  ok  Symbols  and  defin-  for  name 

quantity  unit  ing  equations  of  unit 


5.  Brightness 


Apparent  candle 

per  sq.cm.  ,  _       dl 

Apparent  -candle  dS  cos  6 

per  sq.  in.  _  rfF 


Lambert  dS 

'XT         i  L  •  i_  f  Candles  per  sq.cm.     ,         dl 

6.  Normal  brightness  {  _  4  .        b0  =  -  _ 

[  Candles  per  sq.  in.  dS 

7.  Specific  luminous  j  Lumens  per  sq.cm.         _  , 

radiation  [Lumens  per  sq.  in.  °' 

•pr 

8.  Coefficient  of  reflection  m  =  — 

E 

9.  Mean  spherical  candlepower  scp. 

10.  Mean  lower  hemispherical  candlepower  Icp. 

11.  Mean  upper  hemispherical  candlepower  ucp. 

12.  Mean  zonal  candlepower  zcp. 

13.  Mean  horizontal  candlepower  mhch. 

14.  1  lumen  is  emitted  by  0.07958  spherical  candlepower. 

15.  i  spherical  candlepower  emits  12.57  lumens.  ' 

1  6.  i  lux  =  i  lumen  incident  per  square  meter  =  o.oooi  phot  =  o.i  milliphot. 
17.  i  phot  =  i  lumen  incident  per  square  centimeter  =  10,000  lux  =  1,000 

milliphots  =  1,000,000  microphots. 
1  8.  i  milliphot  =  o.ooi  phot  =  0.929  foot-candle. 

19.  i  foot-candle  =  i  lumen  incident  per  square  foot  =  1.076  milliphots  = 

10.76  lux. 

20.  i  lambert  =  i  lumen  emitted  per  square  centimeter  of  a  perfectly  diffusing 

surface. 

21.  i  millilambert  =  o.ooi  lambert. 

22.  i  lumen,  emitted,  per  square  foot1  =  1.076  millilamberts. 

23.  i  millilambert  =  0.929  lumen,  emitted,  per  square  foot.1 

24.  i    lambert  =  0.3183    candle   per   square    centimeter  =  2.054   candles   per 

square  inch. 

25.  i  candle  per  square  centimeter  =  3.1416  lamberts. 

26.  i  candle  per  square  inch  =  0.4868  lambert  =  486.8  millilamberts. 

46.  Symbols.  —  In  view  of  the  fact  that  the  symbols  heretofore  pro- 
posed by  this  committee  conflict  in  some  cases  with  symbols  adopted 
for  electric  units  by  the  International  Electrotechnical  Commission,  it 
is  proposed  that  where  the  possibility  of  any  confusion  exists  in  the 
use  of  electrical  and  photometrical  symbols,  an  alternative  system  of 
symbols  for  photometrical  quantities  should  be  employed.  These 
should  be  derived  exclusively  from  the  Greek  alphabet,  for  instance: 

Luminous  intensity  .......................................     T 

Luminous  flux  ...........................................    V 

Illumination  .......................  '.  .....................    0 


1  Perfect  diffusion  assumed. 


PRINCIPLES    OF    INTERIOR   ILLUMINATION 

BY  A  COMMITTEE 

J.   R.  CRAVATH,  CHAIRMAN 

WARD  HARRISON 
ROBERT  ff.   PIERCE 

PART  I.   ELEMENTS  OF  DESIGN 

As  the  subject  of  illumination  units  and  calculations  is  treated  in 
a  separate  lecture  only  those  parts  of  this  subject  of  immediate 
practical  application  to  design  will  be  taken  up  here,  and  no  attempt 
will  be  made  to  explain  the  derivation  of  units,  or  the  terms  or 
diagrams  here  mentioned  in  connection  with  calculations. 

CALCULATIONS 

Measurement  and  Expression  of  Light  Output  from  Sources.— One 
of  the  first  things  necessary  in  illumination  calculations  for  interiors 
is  a  knowledge  of  the  light  output  or  luminous  performance  of  various 
sources  of  light  available  for  lighting  the  interior  in  question.  In 
connection  with  the  light  output  of  a  source  it  is  important  that  we 
should  know:  (a)  how  the  light  is  distributed  from  the  source,  that 
is,  the  candle-power  distribution  or  intensity  in  various  directions; 

(b)  the  flux  of  light  in  lumens  or  mean  spherical  candle-power;  and 

(c)  the  brightness  per  unit  area  of  the  source  of  light. 
Candle-power  Distribution. — The  polar  coordinate  curve,  Fig.  i, 

is  the  common  means  of  expressing  the  intensity  of  candle-power  of 
light  in  various  directions  from  a  source.  Such  a  curve  (in  which 
the  candle-power  is  shown  by  the  distance  of  the  curve  from  the 
reference  point  or  light  source)  gives  at  a  glance  a  good  idea  of  the 
characteristics  of  light  distribution  from  the  source,  provided  the 
distribution  of  light  is  symmetrical  around  a  vertical  axis.  If  it  is 
not  symmetrical,  of  course,  several  curves  plotted  from  candle-power 
readings  in  different  planes  are  necessary. 

The  practising  engineer  should  be  an  industrious  student  and 
collector  of  curves  of  this  kind. 

37 


30  ILLUMINATING   ENGINEERING   PRACTICE 

Light  Flux. — The  total  output  or  flux  of  light  in  lumens  (which 
is  12.57  times  the  mean  spherical  candle-power)  is  sometimes 
graphically  expressed  by  a  Rousseau  diagram  but  more  frequently 
by  numerals  showing  the  lumens  emitted  in  different  zones  together 
with  the  total  lumens. 

The  mathematical  derivation  of  light  flux  from  the  polar  co- 
ordinate curve  is  out  of  the  scope  of  this  lecture  except  that  one 
short-cut  method  of  great  practical  convenience  for  quickly  determin- 
ing the  light  flux  in  any  zone  or  zones  from  a  common  polar  co- 


Fig,  i. — Polar  coordinate  candle-power  curve. 


ordinate  curve  should  be  mentioned.  The  method  is  based  on  the 
principle  that  on  a  polar  coordinate  curve  the  light  flux  in  various 
zones  is  proportional  to  the  length  of  a  perpendicular  line  drawn 
from  the  candle-power  curve  at  the  middle  of  the  zone  to  the  ver- 
tical axis.  If  we  take  the  sum  of  the  perpendicular  distances  for 
lo-deg.  zones  (such  as  AB  plus  CD  plus  EF  etc.,  in  Fig.  i) 
from  the  curve  to  the  vertical  as  measured  from  the  center  of 
each  lo-deg.  zone  (measuring  these  distances  by  the  same  scale  as 
the  candle-power  scale  of  the  curve)  and  add  10  per  cent,  to  this 
sum,  the  result  will  be  the  total  lumens  in  the  zones  under  considera- 


PRINCIPLES   OF   INTERIOR   ILLUMINATION 


39 


tion.  The  quick  way  to  get  this  sum  is  by  the  use  of  a  strip  of  paper 
and  a  sharp  pencil.  Starting  at  a  marked  zero  point  measure  the 
perpendicular  distance  from  the  curve  to  the  vertical  ic-deg.  zone 
at  the  5-deg.  point  (AB  Fig.  i)  marking  it  on  the  strip.  Then 
with  the  last  mark  as  a  starting  place  measure  the  distance  for  the 
second  zone,  CD  from  the  vertical  to  the  curve  at  the  1 5-deg. 
point,  and  so  on  adding  each  perpendicular  distance  for  every 
10  degrees  to  the  one  before,  over  the  whole  180  degrees.  Then  by 
using  the  candle-power  scale  of  the  curve  to  measure  the  total  length 
of  the  slip  of  paper  so  measured  off  and  adding  10  per  cent.,  the 
numerical  value  of  the  lumens  emitted  in  any  zone  or  for  the  entire 
sphere  o  to  180  degrees  is  quickly  ascertained.  Obviously  the  same 
method  applies  to  any  one  or  more  of  the  lo-deg.  zones  into  which 
the  sphere  is  divided  by  this  method,  so  that  the  lumens  can  be 
thus  determined  for  any  one  or  more  lo-deg.  zones. 

The  brightness  over  the  area  of  the  source  of  light  (or  of  the  source 
of  light  with  its  enclosing  equipment  such  as  a  globe  or  reflector) 
is  of  much  importance  in  connection  with  the  hygiene  of  the  eye  in 
designing  interior  illumination.  Such  brightness  has  been  ex- 
pressed in  many  units,  such  as  candles  per  square  centimeter,  candles 
per  square  inch,  candles  per  square  foot,  etc.,  but  practice  is  rapidly 
settling  to  the  new  unit  approved  by  our  Society,  namely,  the  "lam- 
bert"  and  its  loooth  part,  the  millilambert.  The  latter  is  about 
equal  to  the  brightness  of  white  blotting  paper  when  illuminated 
with  1.25  foot-candles.  Table  i  shows  the  relation  of  various 
brightness  units. 


TABLE  I. — CONVERSION  TABLE  FOR  VARIOUS  BRIGHTNESS  VALUES 


Values  in  units  in  this  column  X 
conversion  factor  «•  value  in 
units  at  top  of  column 

1. 

.** 

V 

h 

1$ 

at  u 
O 

Candles  per 
sq.  meter 

Candles  per 
sq.  foot 

Lamberts  (apJ 
parent  lum. 
per  sq.  cm.) 

Ft.-candles(apJ 
parent  lumend 
per  sq.  foot) 

Millilamberts 

Candles  per  sq.  cm. 

I 

6.451 

10,000 

929  .  03 

3  .  14 

2918 

3141.6 

Candles  per  sq.  inch 

155 

I 

1550 

144 

.4867* 

45* 

486.7 

Candles  per  sq.  meter  
Candles  per  sq.  foot  

Lamberts    (apparent    lumens    per 
sq.  cm.) 

.0001 
.00108 

318 

.  00064- 
51 
.0069 

2.O54 

10.70 
3180 

.0929 
I 

295  .8 

.000314 

.00330- 
12 

I 

.2918 
3   14 

929.03 

31416 
3   3912 

1000 

Foot-candles     (apparent     lumens 
per  sq.  foot) 

000343 

.00214 

3.40 

..318 

.00108 

1  .076 

Millilamberts 

.  0003  I  8 

.002054 

3.  180 

•  2958 

.001 

.929 

I 

ILLUMINATING   ENGINEERING   PRACTICE 


Luminous  Output  of  Bare  Light  Sources. — Although  in  good  practice 
in  the  lighting  of  interiors,  the  lamps  are  seldom  used  bare  without 
reflectors,  shades  or  globes  of  any  kind,  it  is  nevertheless  of  funda- 
mental importance  to  the  engineer  to  know  the  luminous  output  of 
the  various  sources  of  light  without  auxiliary  equipment.  Then  he 
can  proceed  with  his  calculations  by  allowing  the  proper  percentage 
of  loss  for  whatever  equipment  is  used  around  the  lamps. 

The  luminous  output  of  different  kinds  of  lamps  per  unit  of  input 
has  been  rapidly  changing  during  the  past  few  years  owing  to  im- 
provements in  the  art  and  will  probably  continue  to  change  so  that 
any  data  given  here  must  be  taken  with  the  idea  that  they  must  be 
revised  from  various  reliable  sources  at  frequent  intervals. 

Table  II  shows  the  lumens  and  the  lumens  per  watt  for  a  number 

TABLE  II. — LUMENS  OUTPUT  OF  AMERICAN  TUNGSTEN  INCANDESCENT  LAMPS 

JULY  i,  1916 


Watts 

Watts  per 
spherical  c.p. 

Lumens  per 
watt 

Total  lumens 

IO 

15 

20 

105-125  VOLT  MAZDA  B  LAMPS 

1.67 
1.47 
1.41 

7-50 
8-55 
8.90 

75 
128 
I78 

25 
40 
50 

i-35  " 
1.32 

i-3i 

9-30 
9-50 
9.60 

234 
380 
480 

60 

1.28 

9.80 

590 

IOO 

75 

IOO 

I  .  22 

10.3 

1,030 

105-125  VOLT  MAZDA  C  LAMPS 

1.09 
i  .00 

n-5 

12.6 

865 
1,260 

20O 

0.90 

14.0 

2,8oo 

300 
4OO 
500 

0.82 
0.82 
0.78 

15-3 
15-3 
16.1 

4,600 
6,150 
8,050 

750 
I,OOO 

0.74 
o.  70 

17.0 
18.0 

I2,8oo 
18,000 

NOTE. — 220  Volt  lamps  are  about  10  per  cent,  less  efficient. 


PRINCIPLES   OF   INTERIOR   ILLUMINATION  41 

of  the  commonest  sizes  and  types  of  tungsten  filament  incandescent 
lamps,  new,  as  made  and  used  in  the  United  States,  August,  1916, 
when  operated  at  a  voltage  giving  an  average  rated  life  of  1000  hours. 
From  this  it  is  seen  that  the  lumens  per  watt  range  from  7.5  for 
the  lo-watt  size  to  18  for  the  loco-watt  size. 

Gas  mantle  burners,  new,  and  properly  adjusted  range  in  specific 
output  from  200  to  325  lumens  per  cubic  foot  of  gas  per  hour  in 
sizes  giving  400  to  3000  lumens.  These  figures  vary  with  the  compo- 
sition of  the  gas  and  many  other  factors. 

The  amount  of  light  obtained  from  the  old-fashioned  open  flame 
burner  gas  jet  depends  upon  the  richness  of  the  gas  in  certain  hydro- 
carbons which  produce  a  yellow  flame  in  the  open  jet.  This  quality 
is  commonly  known  as  the  candle-power  of  the  gas  and  was  at  one 
time  the  common  standard  by  which  gas  was  rated.  With  the  gas 
mantle,  however,  the  candle-power  according  to  the  old  standards 
has  nothing  to  do  with  the  light  output  of  the  burner  which  in  this 
case  depends  on  the  composition  of  the  gas. 

The  efficiencies  of  lamps  burning  acetylene,  Blau  gas,  alcohol, 
kerosene  and  gasoline  vary  considerably,  depending  upon  the  design 
of  the  burner,  the  purity  of  the  illuminant  and  the  conditions  of 
supply.  The  following  figures  have  been  actually  obtained  under 
favorable  conditions,  but  do  not  necessarily  represent  the  maximum 
obtainable.  On  the  other  hand,  the  average  results  in  the  case  of 
kerosene  and  gasoline  are  probably  much  below  the  stated  values. 


Lumen,  hours, 
per  cu.  ft. 

Acetylene  (open  flame)  
Acetylene  (mantle) 

500 
QOO 

Blau  gas  (mantle) 

4OO 

Kerosene    (round  wick  open  flame)                                         .    . 

Per  gallon 
0,OOO 

Kerosene    (mantle)  

24,000 

Kerosene    (mantle-pressure  tvpe)  

8o,OOO 

Gasoline    (mantle-low  pressure).                                   

8o,OOO 

Alcohol       (mantle) 

l6,OOO 

Kerosene  lamps  in  particular  suffer  a  considerable  decrease  in 
efficiency  during  burning. 

The  older  carbon  filament  incandescent  lamp  gave  a  specific 
output  of  from  2.5  to  4  lumens  per  watt. 


44  ILLUMINATING   ENGINEERING   PRACTICE 

Prismatic  reflectors  offer  a  control  of  light  which  approaches  that 
of  the  mirror.  Considerable  light  passes  through  the  reflector  at  the 
tops  and  bottoms  of  the  prisms. 

For  indirect  lighting  and  semi-indirect  with  dense  reflectors  it  can 
be  shown  theoretically  that  the  best  reflector  for  the  purpose  would 
distribute  light  evenly  over  the  whole  ceiling  area  served  from  one 
fixture. 

That  is,  in  a  small  room,  with  one  central  fixture,  the  whole  ceiling 
would  be  evenly  illuminated;  or  in  a  large  room  with  a  fixture  in  the 
center  of  each  bay  each  reflector  would  evenly  illuminate  that  bay. 
By  confining  a  considerable  portion  of  the  light  flux  to  the  center 
of  the  ceiling  with  a  fixture  hung  in  the  middle  of  the  room,  more  of 
the  light  flux  will  reach  the  working  plane  after  one  reflection  from 
the  ceiling  than  if  the  distribution  over  the  ceiling  were  more  uniform. 
The  more  even  the  distribution  the  greater  the  amount  of  light  lost 
by  absorption  at  the  walls.  However,  from  the  standpoint  of  the 
desk  worker  there  is  some  advantage  in  having  the  ceiling  evenly 
illuminated  as  there  is  some  tendency  to  specular  reflection  from  the 
brightest  portions  of  the  ceiling  causing  a  slight  veiling  glare.  This 
glare  is  not  so  pronounced  if  the  ceiling  is  evenly  illuminated.  An 
indirect  reflector  giving  uniform  ceiling  distribution  must  be  of  the 
deep  bowl  type,  but  this  type  has  a  very  sharp  "  cut-off  "  or  transition 
from  high  to  low  illumination  at  the  edge  of  the  reflector.  This 
causes  a  shadow  on  the  ceiling  which  is  objectionable  and  calls  for 
some  modification  of  uniform  ceiling  distribution.  Two  principal 
ways  of  overcoming  this  have  been  worked  out  in  practice  which 
work  well  with  non-concentrated  light  sources.  One  is  to  use  a 
shape  similar  to  the  deep  bowl  distributing  type  for  the  lower  part 
of  the  reflector  and  a  flaring  bell-shaped  one  for  the  upper  part.  The 
other  plan  is  to  use  a  large  reflector  of  the  shallow  bowl-shape.  The 
former  plan  is  used  mainly  with  mirrored  reflectors  where  it  is  desir- 
able on  account  of  first  cost,  to  keep  down  the  size  while  the  other 
plan  is  used  with  white  enamel  reflectors  and  for  semi-direct  lighting 
with  large  glass  bowls.  While  it  may  be  immaterial  for  the  engineer 
who  plans  the  lighting  installation  how  the  result  of  eliminating 
dark  shadows  from  the  ceiling  is  accomplished  it  must  nevertheless 
always  be  kept  in  mind  that  good  design  calls  for  the  elimination  of 
these  shadows  to  a  large  extent  by  tapering  off  the  brightness  from 
the  center  to  the  edges  of  the  illuminated  area  covered  by  each 
reflector. 

For  semi-indirect  lighting  a  plain  bowl  somewhat  shallower  than 


PRINCIPLES   OF   INTERIOR  ILLUMINATION  45 

a  hemisphere  is  likely  to  give  the  best  results  in  efficiency.  Orna- 
mental designs  in  which  the  maximum  diameter  of  the  bowl  is  greater 
than  the  diameter  at  the  top  cause  considerable  loss  of  light  because 
of  the  light  which  is  intercepted  by  the  part  of  the  bowl  projecting 
inward.  Therefore  when  such  designs  are  used  this  extra  loss  should 
be  recognized  in  the  calculations  and  a  decision  reached  whether  the 
ornamental  effect  attained  is  sufficient  to  justify  the  loss. 

While  the  placing  of  lamps  and  shaping  of  semi-indirect  bowls  is 
not  as  important  as  in  the  case  of  indirect  reflectors  of  the  opaque 
type,  it  is  not  by  any  means  a  matter  of  indifference.  The  lamps 
should  be  placed  in  a  position  not  to  cause  undue  shadows  on  ceilings 
or  walls  or  too  uneven  illumination  on  the  bowls  as  viewed  from 
below. 

Angle  reflectors  may  be  obtained  giving  a  number  of  different 
types  of  distribution  for  special  purposes  such  as  show  window  light- 
ing, bulletin  board  lighting  and  other  cases  where  more  light  is 
wanted  on  one  side  of  the  plane  through  the  lamp  axis  than  on  the 
other.  They  cannot  be  classified  into  general  types  as  there  is  such 
a  variety.  Makers  data  should  be  thoroughly  studied  as  to  the  forms 
available. 

Shifting  the  position  of  a  lamp  in  a  reflector  by  the  use  of  different 
forms  of  shade  holders  may  materially  change  the  light  distribution. 

In  the  selection  of  reflectors  for  any  purpose  it  is  always  well  to 
remember  the  fundamental  principle  that  control  of  the  light  flux 
is  the  end  to  be  desired  if  the  flux  is  not  to  be  wasted  by  escaping  to 
places  where  it  is  not  needed  or  positively  undesirable.  The 
larger  the  percentage  of  the  total  flux  of  light  from  the  lamp  which 
the  reflector  intercepts  and  reflects  in  desired  directions  the  higher 
the  efficiency;  unless,  however,  the  natural  undirected  flux  from  the 
lamp  approximates  the  distribution  desired.  With  reflectors  which 
must  confine  the  light  flux  of  the  lamp  within  rather  restricted  areas 
as  in  show  windows  and  for  localized  lighting  of  work  benches  and 
the  like  it  is  important  to  use  reflectors  large  enough  to  intercept 
a  considerable  portion  of  the  light  flux.  There  is  apt  to  be  a  tend- 
ency to  cut  down  reflector  sizes  to  save  first  cost  but  such  reduction 
usually  means  a  permanent  impairment  of  efficiency.  This  is  also 
true  in  the  lighting  of  a  large  high  room  of  the  armory  or  coliseum 
type  where  the  lamps  must  be  placed  high  and  all  light  eminating  from 
reflectors  at  angles  only  a  little  below  the  horizontal  is  likely  to 
undergo  serious  loss  by  striking  dark  roof  and  walls. 

Sky  Brightness  Characteristics  useful  for  design  of  natural  illumina- 


44  ILLUMINATING   ENGINEERING   PRACTICE 

Prismatic  reflectors  offer  a  control  of  light  which  approaches  that 
of  the  mirror.  Considerable  light  passes  through  the  reflector  at  the 
tops  and  bottoms  of  the  prisms. 

For  indirect  lighting  and  semi-indirect  with  dense  reflectors  it  can 
be  shown  theoretically  that  the  best  reflector  for  the  purpose  would 
distribute  light  evenly  over  the  whole  ceiling  area  served  from  one 
fixture. 

That  is,  in  a  small  room,  with  one  central  fixture,  the  whole  ceiling 
would  be  evenly  illuminated;  or  in  a  large  room  with  a  fixture  in  the 
center  of  each  bay  each  reflector  would  evenly  illuminate  that  bay. 
By  confining  a  considerable  portion  of  the  light  flux  to  the  center 
of  the  ceiling  with  a  fixture  hung  in  the  middle  of  the  room,  more  of 
the  light  flux  will  reach  the  working  plane  after  one  reflection  from 
the  ceiling  than  if  the  distribution  over  the  ceiling  were  more  uniform. 
The  more  even  the  distribution  the  greater  the  amount  of  light  lost 
by  absorption  at  the  walls.  However,  from  the  standpoint  of  the 
desk  worker  there  is  some  advantage  in  having  the  ceiling  evenly 
illuminated  as  there  is  some  tendency  to  specular  reflection  from  the 
brightest  portions  of  the  ceiling  causing  a  slight  veiling  glare.  This 
glare  is  not  so  pronounced  if  the  ceiling  is  evenly  illuminated.  An 
indirect  reflector  giving  uniform  ceiling  distribution  must  be  of  the 
deep  bowl  type,  but  this  type  has  a  very  sharp  "  cut-off  "  or  transition 
from  high  to  low  illumination  at  the  edge  of  the  reflector.  This 
causes  a  shadow  on  the  ceiling  which  is  objectionable  and  calls  for 
some  modification  of  uniform  ceiling  distribution.  Two  principal 
ways  of  overcoming  this  have  been  worked  out  in  practice  which 
work  well  with  non-concentrated  light  sources.  One  is  to  use  a 
shape  similar  to  the  deep  bowl  distributing  type  for  the  lower  part 
of  the  reflector  and  a  flaring  bell-shaped  one  for  the  upper  part.  The 
other  plan  is  to  use  a  large  reflector  of  the  shallow  bowl-shape.  The 
former  plan  is  used  mainly  with  mirrored  reflectors  where  it  is  desir- 
able on  account  of  first  cost,  to  keep  down  the  size  while  the  other 
plan  is  used  with  white  enamel  reflectors  and  for  semi-direct  lighting 
with  large  glass  bowls.  While  it  may  be  immaterial  for  the  engineer 
who  plans  the  lighting  installation  how  the  result  of  eliminating 
dark  shadows  from  the  ceiling  is  accomplished  it  must  nevertheless 
always  be  kept  in  mind  that  good  design  calls  for  the  elimination  of 
these  shadows  to  a  large  extent  by  tapering  off  the  brightness  from 
the  center  to  the  edges  of  the  illuminated  area  covered  by  each 
reflector. 

For  semi-indirect  lighting  a  plain  bowl  somewhat  shallower  than 


PRINCIPLES    OF    INTERIOR   ILLUMINATION  45 

a  hemisphere  is  likely  to  give  the  best  results  in  efficiency.  Orna- 
mental designs  in  which  the  maximum  diameter  of  the  bowl  is  greater 
than  the  diameter  at  the  top  cause  considerable  loss  of  light  because 
of  the  light  which  is  intercepted  by  the  part  of  the  bowl  projecting 
inward.  Therefore  when  such  designs  are  used  this  extra  loss  should 
be  recognized  in  the  calculations  and  a  decision  reached  whether  the 
ornamental  effect  attained  is  sufficient  to  justify  the  loss. 

While  the  placing  of  lamps  and  shaping  of  semi-indirect  bowls  is 
not  as  important  as  in  the  case  of  indirect  reflectors  of  the  opaque 
type,  it  is  not  by  any  means  a  matter  of  indifference.  The  lamps 
should  be  placed  in  a  position  not  to  cause  undue  shadows  on  ceilings 
or  walls  or  too  uneven  illumination  on  the  bowls  as  viewed  from 
below. 

Angle  reflectors  may  be  obtained  giving  a  number  of  different 
types  of  distribution  for  special  purposes  such  as  show  window  light- 
ing, bulletin  board  lighting  and  other  cases  where  more  light  is 
wanted  on  one  side  of  the  plane  through  the  lamp  axis  than  on  the 
other.  They  cannot  be  classified  into  general  types  as  there  is  such 
a  variety.  Makers  data  should  be  thoroughly  studied  as  to  the  forms 
available. 

Shifting  the  position  of  a  lamp  in  a  reflector  by  the  use  of  different 
forms  of  shade  holders  may  materially  change  the  light  distribution. 

In  the  selection  of  reflectors  for  any  purpose  it  is  always  well  to 
remember  the  fundamental  principle  that  control  of  the  light  flux 
is  the  end  to  be  desired  if  the  flux  is  not  to  be  wasted  by  escaping  to 
places  where  it  is  not  needed  or  positively  undesirable.  The 
larger  the  percentage  of  the  total  flux  of  light  from  the  lamp  which 
the  reflector  intercepts  and  reflects  in  desired  directions  the  higher 
the  efficiency;  unless,  however,  the  natural  undirected  flux  from  the 
lamp  approximates  the  distribution  desired.  With  reflectors  which 
must  confine  the  light  flux  of  the  lamp  within  rather  restricted  areas 
as  in  show  windows  and  for  localized  lighting  of  work  benches  and 
the  like  it  is  important  to  use  reflectors  large  enough  to  intercept 
a  considerable  portion  of  the  light  flux.  There  is  apt  to  be  a  tend- 
ency to  cut  down  reflector  sizes  to  save  first  cost  but  such  reduction 
usually  means  a  permanent  impairment  of  efficiency.  This  is  also 
true  in  the  lighting  of  a  large  high  room  of  the  armory  or  coliseum 
type  where  the  lamps  must  be  placed  high  and  all  light  eminating  from 
reflectors  at  angles  only  a  little  below  the  horizontal  is  likely  to 
undergo  serious  loss  by  striking  dark  roof  and  walls. 

Sky  Brightness  Characteristics  useful  for  design  of  natural  illumina- 


46  ILLUMINATING   ENGINEERING   PRACTICE 

tion  are  given  in  Table  III.     It  will  be  seen  that  there  is  an  enormous 
variation  in  the  brightness  of  the  sky  during  what  are  ordinarily 

TABLE  III. — SKY  BRIGHTNESS 


Sky,  with  light  clouds 

Sky,  clouds  predominating,  generally  cumulus .  . 

Sky,  blue  predominating,  clouds  cirrus 

Sky,  cloudless,  either  clear  blue  or  hazy 

Sky,  cloudy,  storm  near  or  present 

Walls,  typical  rooms,  ordinary  range  diffused 
daylight  through  window 


Brightness  in 
millilamberts 


2,OOO 
1,900 
1,500 
I,OOO 

700  to  70 


50  to 


considered  daylight  hours.  Calculations  of  daylight  illumination 
of  interiors  should  therefore  be  made  on  the  basis  of  maximum  and 
minimum  values. 

The  sky  is  the  principal  source  of  daylight  illumination  of  in- 
teriors, as  the  illumination  obtained  directly  from  the  sun  may  be 
considered  as  purely  incidental  and  frequently  avoided  by  the  use 
of  shades. 

In  connection  with  daylight  we  have  first  to  consider  the  amount 
of  sky  exposure  through  side  or  ceiling  windows;  the  amount  of 
illumination  (excluding  reflection  from  walls  and  other  buildings 
on  any  point)  varying  directly  according  to  the  area  of  the  exposure 
as  projected  from  the  point  in  question.  Part  of  the  window  area 
may  be  obstructed  by  buildings  and  in  certain  cases  the  reflection 
of  light  from  these  buildings  (or  in  other  words  their  brightness) 
must  also  be  taken  into  account  as  well  as  that  of  the  sky. 

Illumination  from  Direct  Sunlight  in  the  open  has  been  found  to 
reach  approximately  9000  foot-candles,  in  Virginia,  during  the 
summer  months  as  measured  on  a  horizontal  plane.  Extensive 
measurements  made  there  by  Prof.  Herbert  H.  Kimball,  of  the  U.  S. 
Weather  Bureau,  show  that  the  total  illumination  from  sun  and  sky 
during  the  middle  of  the  day  consists  of  about  20  per  cent,  skylight 
and  80  per  cent,  direct  sunlight.  Sunlight  shining  into  interiors 
therefore  may  have  about  80  per  cent,  of  its  outdoor  value. 

With  clear  glass  windows  the  only  sky  brightness  which  is  useful 
for  illumination  of  the  room  is  that  directly  visible  from  the  interior 
of  the  room.  If  the  window  is  obstructed  by  buildings  the  sky 
brightness  is  not  available.  Where  a  window  is  exposed  to  sky 


PRINCIPLES   OF   INTERIOR   ILLUMINATION  47 

area  either  above  or  at  one  side,  and  the  illumination  from  such  area 
does  not  reach  back  into  the 'room  far  enough,  prisms  and  diffusing 
glasses  of  various  kinds  are  applicable.  The  action  of  the  prism 
glass  window  is  to  bend  the  light  rays  so  that  they  strike  back  further 
into  the  room  than  if  a  clear  glass  window  were  used.  Rough  and 
ribbed  glasses  accomplish  the  same  end  with  less  precision  and 
effectiveness.  They  diffuse  the  light  rays  passing  through,  and  a 
certain  portion  of  such  rays  are  directed  back  into  the  room.  For 
some  locations  louvers  or  shutters  consisting  of  partially  or  wholly 
opaque  strips  which  can  be  tilted  at  any  angle  make  it  possible  to 
regulate  the  relative  amount  of  sun  and  skylight  or  cut  out  direct 
sunlight  without  too  serious  a  reduction  in  the  skylight.  The  com- 
mon method  of  controlling  sunlight  is  by  translucent  shades  but  this 
method  for  some  interiors  (such  as  art  galleries)  does  not  offer  very 
accurate  control. 

The  Brightness  or  Intrinsic  Brilliancy  of  Various  Artificial  Light 
Sources  and  also  the  brightness  of  some  sources  equipped  with  dif- 
fusing glassware  for  the  protection  of  the  eyes  is  shown  in  Table  IV. 


TABLE  IV.— BRIGHTNESS  or  ARTIFICIAL  LIGHT  SOURCES 

Brightness  in 
milHlamberts 

Crater,  carbon  arc 40,800,000 

Flaming  arc,  clear  globe . . . . . . .  2,435,000 

Magnetite  arc,  clear  globe 1,945,000 

Gas-filled  tungsten  electric  light  filament 1,400,000 

Incandescent  electric  tungsten,  1.25  watts  per 

candle 516,000 

Quartz  tube,  mercury  vapor  arc , 486,700-292,000 

Incandescent  electric  carbon  filament,  3.1  watts 

per  candle 236,000 

Acetylene  flame  (i  foot  burner) 25,800 

Welsbach  mantle 15,080 

Cooper  Hewitt  glass  tube  mercury  vapor  lamp.  .  6,800 

Kerosene  flame 1,946-4,380 

25  watt  frosted  tungsten  lamp,  side 2,920 

Candle  flame 1,460-1,945 

Gas  flame  (fish  tail) 1,314 

10"  opal  ball,  over  100  watt  tungsten  lamp 306 

Ceilings   over  indirect  lighting   fixtures   (usual 

range,  brightest  part  as  viewed  by  occupants 

of  room) 73~4 

Glass  bowls  used  for  semi-direct  lighting 1,000-35 


48 


ILLUMINATING   ENGINEERING   PRACTICE 


Depreciation  due  to  dirt  on  glass  and  reflecting  surfaces  and  to 
inherent  characteristics  of  the  lamp's  used  must  be  recognized  in 
design. 

Both  the  total  lumens  and  the  lumens  per  watt  of  tungsten  fila- 
ment electric  lamps  drop  with  use,  partly  by  the  blackening  inside 
the  bulb  and  partly  by  disintegration  and  increase  in  resistance  of 
the  filament.  Such  lamps  operated  at  the  specific  outputs  shown  in 
Table  II,  fall  off  in  lumens  output  about  15  per  cent,  in  1000  hours 
service.  With  electric  arc  lamps  and  gas  mantle  burners  so  much 


12  16  20  24 

Elapsed  Time  in  Weeks 

Fig.   2. — Depreciation  caused  by  dirt. 


32 


36 


40 


depends  upon  the  adjustment  and  other  variable  factors  that  no 
depreciation  figure  inherent  in  the  lamp  can  be  given,  but  unless 
maintenance  is  especially  good  more  must  be  allowed  than  for  the 
internal  depreciation  of  the  tungsten  filament  electric  lamp. 

The  accumulation  of  dirt  on  the  surrounding  glassware  and  on  the 
globe  or  reflector  is  an  important  cause  of  loss  of  light  and  should 
also  always  be  reckoned  with  in  preliminary  calculations.  It  is 
necessary  to  assume  some  probable  maximum  depreciation  figure 
from  this  cause  and  in  making  such  an  assumption  of  course  the  sur- 
rounding conditions  and  the  probable  frequency  of  cleaning  must  be 
considered.  In  Table  V  is  given  a  compilation  of  results  of  various 
tests  made  in  different  places  by  different  observers  on  the  effect 
of  the  accumulation  of  dirt,  and  Fig.  2  shows  the  depreciation  over 
an  extended  period  for  a  given  set  of  reflectors. 

The  effect  of  accumulation  of  dirt  on  side  and  ceiling  windows 
is  probably  about  the  same  as  on  lamps. 

Utilization  of  the  Generated  Light  Flux. — There  are  various  methods 
of  calculating  the  resultant  illumination  at  the  desired  point  with  a 


PRINCIPLES   OF   INTERIOR   ILLUMINATION 


49 


TABLE  V. — Loss  OF  LIGHT  BY  ACCUMULATION  OF  DIRT 


Authority  and 
reference 

Conditions  and 
surroundings 

Lamps,  globes  and 
reflectors 

3-ga 

|6S 

ill 

"1G 

(x  > 

8 

£|° 

°*       0. 

Durgin    &    Jackson, 

Down  town   Chicago 

Semi-direct,         dense 

3wk. 

76 

35  o 

Trans.  I.  E.S.,  1915. 

office  building  dust- 

bowls and  tungsten 

p.  707. 

iest  rooms.                    1    lamps. 

Aldrich      &      Malia, 

Office  building  in  Chi- 

Prismatic    reflectors, 

12  wk. 

25 

8.5 

Trans.  I.E.  S..  1914. 

cago  Stock  yards. 

satin  finish  and  tungs- 

p. 112. 

ten  lamps.     Direct. 

Do. 

Do. 

Mirror  reflectors  and 

9wk. 

25 

II  .0 

tungsten  lamps.    In- 

direct. 

Do. 

Do. 

Opal    bowl    reflectors 

2wk. 

5 

IO.O 

and  tungsten  lamps. 

Semi-indirect. 

C.    E.    Clewell,    Fac- 

Suburban factory 

Prismatic,  satin  finish 

14  wk. 

42 

13  5 

tory  Lighting,  p.  46. 

office. 

reflectors  and  tungs- 

ten lamp.     Direct. 

Do. 

Do. 

Prismatic  clear  reflec- 

17 wk. 

17 

45 

tors     and     tungsten 

lamps. 

Do. 

Suburban  factory. 

Do. 

9wk. 

28 

14.0 

Do. 

Do. 

Do. 

ii  wk. 

29 

II.  5 

Do. 

Do. 

Do. 

13  wk. 

40 

13-4 

Edwards  &  Harrison, 

Office  corridor  Subur- 

Enclosing prismatic. 

8  wk. 

II 

6.2 

Trans.  I.  E.  S.,  1914. 

ban  district,    Cleve- 

p. 176. 

land. 

NOTE. — Since  depreciation  is  more  rapid  at  first,  as  shown  by  the  curves  the  decline  per 
month  here  given  would  not  apply  to  longer  periods. 

NOTE. — For  extensive  additional  tests  see' paper  by  A.  L.  Eustice,  Trans.  I.  E.  S.,  1909, 
p.  849- 

given  generated  light  flux.  Before  making  such  calculations  it  is 
of  course  important  to  reach  an  intelligent  decision  as  to  the  points 
where  the  desired  illumination  is  needed  and  whether  it  is  best  to 
consider  the  illumination  measured  in  a  horizontal  plane,  or  vertical 
plane,  or  a  plane  in  some  other  angle,  suited  to  the  particular  re- 
quirements in  question.  Common  practice  in  calculating  and 
measuring  the  illumination  in  most  interiors  is  to  ascertain  the 
illumination  in  a  horizontal  plane  from  2.5  to  3.5  ft.  above  the  floor, 
or  about  the  height  of  desks,  counters  and  benches.  For  the 

4 


50  ILLUMINATING   ENGINEERING   PRACTICE 

majority  of  interiors  this  consideration  of  the  horizontal  plane  serves 
the  purpose  sufficiently  except  for  special  localized  lighting  around 
machinery.  If  the  illumination  in  the  horizontal  plane,  commonly 
known  as  the  "working  plane"  is  to  be  taken  as  the  criterion,  it  is 
possible  to  measure  the  average  illumination  in  this  plane  over  an 
entire  room  by  measuring  the  illumination  with  a  portable  photom- 
eter at  the  center  of  a  number  of  equal-sized  rectangles  into  which 
the  room  may  be  divided.  Dividing  this  average  light  flux  by  the 
light  flux  generated  by  the  lamp  gives  what  is  known  as  the  per- 
centage efficiency  of  utilization,  or  utilization  factor.  Of  course  any 
other  plane  might  be  used  for  figuring  efficiency  of  utilization  pro- 
vided the  position  of  the  plane  were  the  position  where  the  light  was 
wanted.  For  example  in  an  Art  Gallery  the  efficiency  of  utilization 
might  well  be  figured  from  the  light  flux  incident  upon  wall  spaces 
devoted  to  pictures  and  in  a  show  window  it  would  be  figured  from 
the  flux  through  a  curved  surface  corresponding  to  the  line  of  trim 
of  the  window. 

The  point-by-point  method  of  calculation  (that  is,  if  dealing  in 
English  units,  dividing  the  candle-power  by  the  square  of  the  dis- 
tance in  feet  to  the  point  in  question  and  multiplying  this  by  the 
cosine  of  the  angle  of  the  incident  ray  to  the  surface  in  question  to 
get  the  foot-candles  incident  illumination)  is  now  chiefly  used  only 
for  calculating  the  illumination  at  a  few  points  from  a  single  or  small 
number  of  light  sources.  It  is  too  time-consuming  and  laborious 
a  method  for  the  calculation  of  the  illumination  of  large  interiors  with 
many  light  sources.  It  has  the  further  limitation  that  it  takes  no 
account  of  reflection  from  ceiling,  walls  and  floors  and  considers  only 
the  illumination  direct  from  the  lamp  and  its  accessories. 

The  point-by-point  method  may  be  of  considerable  use  in  forecast- 
ing the  differences  in  daylight  illumination  and  at  different  points  of 
interiors  where  the  sky  exposure  and  reflection  coefficient  of  the 
buildings  visible  from  any  point  in  question  are  definitely  known. 
The  foot-candles  illumination  at  various  points  as  one  proceeds  back 
into  a  room  from  a  window  with  unobstructed  sky  exposure  may  for 
the  rough  purpose  of  practical  calculations  be  taken  as  inversely 
proportional  to  the  square  of  the  distance  from  the  window  to  the 
given  point.  In  applying  this  rule  the  fact  should  be  kept  in  mind 
that  frequently  the  window  is  far  from  an  unobstructed  sky  exposure 
and  that  the  sky  exposure  changes  as  seen  from  various  points  further 
back  into  the  room.  The  effective  exposure  is  the  projected  area  of 
the  sky  seen  by  one  looking  at  the  window  from  the  given  point. 


PRINCIPLES    OF    INTERIOR   ILLUMINATION  51 

A  practical  short-cut  in  the  use  of  the  point-by-point  method  in 
calculating  horizontal  illumination  which  obviates  the  necessity  of  a 
table  of  cosines  and  makes  possible  calculations  with  only  the  aid  of 
a  polar  candle-power  curve  of  the  light  sources,  is  the  following, 
which  is  a  graphic  method  of  applying  the  cosine  factor.  In  the  usual 
rule  for  getting  horizontal  illumination  the  illumination  is  equal  to 
the  candle-power  at  the  given  angle  divided  by  the  square  of  the  dis- 
tance multiplied  by  the  cosine  of  the  angle  between  the  ray  in 
question  and  the  vertical.  Now  if  we  draw  a  perpendicular  from  the 
photometric  curve  at  the  angle  in  question  to  the  vertical  and  take  as 
the  candle-power  the  candle-power  scale  reading  at  the  point  where 
this  perpendicular  intersects  the  vertical,  we  apply  the  cosine 
factor  at  the  outset  and  by  simply  dividing  this  candle-power  at  the 
intersection  with  the  horizontal,  by  the  square  of  the  distance  the 
illumination  is  determined. 

In  calculations  of  illumination  by  the  zone  flux  method  all  of  the 
lumens  emitted  in  a  certain  zone,  say  from  o  to  60  degrees  or  from 
o  to  70  degrees,  are  figured  as  falling  upon  the  working  plane  in 
the  general  lighting  of  an  interior.  This  method,  of  course,  takes  no 
account  of  the  uniformity  of  illumination  and  where  approximate 
uniformity  is  desired  must  be  used  only  with  lamps  and  reflectors 
giving  a  type  of  distribution  which  will  be  sufficiently  uniform.  The 
zone  flux  method  is  chiefly  applicable  to  illumination  calculations 
with  opaque  reflectors  where  ah1  of  the  flux  is  emitted  in  downward 
directions  and  little  reliance  is  placed  upon  walls  and  ceilings  to  bring 
up  the  general  illumination.  Some  industrial  plants  and  foundries 
present  such  conditions.  In  the  application  of  this  method  care 
must  be  taken  not  to  select  such  a  large  zone  as  a  basis  that  too  much 
of  the  light  strikes  walls  or  other  obstructions.  At  the  same  time  in 
large  interiors  it  is  not  necessary  to  confine  the  zone  to  simply  those 
which  would  cover  the  floor  near  by.  In  show-window  lighting  if  the 
reflector  selected  is  such  as  to  confine  its  flux  to  the  plane  it  is  desired 
to  illuminate  the  method  may  sometimes  be  used  for  approximation. 

Empirical  methods  of  calculation  based  on  actual  experience  and 
tests  of  existing  installations  form  by  far  the  most  important  basis  for 
most  calculations.  With  the  other  methods  certain  assumptions  are 
necessary  which  may  or  may  not  be  correct.  With  the  empirical 
method  based  on  experience,  the  only  sources  of  error  are  those  due  to 
erroneously  assuming  conditions  in  the  case  to  be  calculated  to  be 
similar  to  those  in  the  tested  cases.  Tables  VI  and  VII  and  Figs.  3 
to  9  inclusive  give  utilization  factors  or  ratio  of  generated  lumens  to 


ILLUMINATING   ENGINEERING   PRACTICE 
TABLE  VI. — UTILIZATION  FACTORS 


Ceiling,  reflection  coefficient 

Light  70  per  cent. 

Medium   50  per 
cent. 

Walls,  reflection  coefficient 

Light 
50 
per  cent. 

Medium 
35 
per  cent. 

Dark 

20 

per  cent. 

Medium 
35 
per  cent. 

Dark 

20 

per  cent. 

Lighting  Equipment: 

Direct,  Prismatic  

6"? 

6l 

^o 

<8 

<6 

W  J 

40 

37 

oV 
36 

o" 
36 

ou 
35 

Direct,  Light  Opal  

cj7 

C-2 

CQ 

48 

46 

o  / 
33 

oo 

28 

ow 

27 

<^W 

26 

*TW 

24 

Direct,  Dense  Opal  

61 

58 

57 

56 

53 

40 

35 

34 

34 

32 

Direct,  Steel  Bowl,  Enamel  or  Alu- 

minum. 

57 

55 

54 

54 

53 

39 

36 

35 

35 

34 

Direct,  Steel  Dome,  Enamel  

70 

67 

65 

67 

65 

46 

42 

39 

42 

39 

Totally  indirect,  Mirrored 

4.O 

^8 

36 

27 

26 

T.V-; 

24 

o" 
21 

o" 

20 

z  / 

IS 

H 

Semi-indirect,  Light  Opal  

47 

45 

43 

39 

35 

30 

25 

24 

22 

20 

Semi-indirect,  Dense  Opal.  .  .  . 

42 

4-1 

4b 

31 

•20 

T"O 

27 

T-A 

25 

T-W 

22 

OA 

18 

O^ 
17 

Totally  enclosing  

46 

42 

40 

38 

35 

Light  Opal 

25 

19 

18 

18 

15 

The  values  in  this  table  have  reference  to  square  rooms  equipped  with  a  sufficient  number 
of  lighting  units  and  so  placed  as  to  produce  reasonably  uniform  illumination.  In  each  case 
the  upper  figure  applies  to  an  extended  area,  namely,  one  in  which  the  horizontal  dimension 
is  at  least  five  times  the  distance  from  floor  to  ceiling.  The  lower  figure  applies  to  a  con- 
fined area,  one  in  which  the  floor  dimension  is  but  five-fourths  of  the  ceiling  height.  The 
utilization  factor  for  a  rectangular  room  is  approximately  the  average  of  the  factors  for 
two  square  rooms  of  the  large  and  small  floor  dimension  respectively. 

lumens  incident  upon  the  working  plane  for  a  number  of  typical  con- 
ditions. A  study  of  these  tables  shows  the  marked  influence  of  size 
of  room  and  ceiling  and  wall  colors  on  efficiency.  The  figures  on 
utilization  factors  Figs.  3  to  9  will  hold  for  all  rooms  of  the  same 
relative  proportions,  as  to  shape,  without  regard  to  sizes. 


PRINCIPLES   OF   INTERIOR   ILLUMINATION 


53 


HYGIENE 

The  hygienic  aspect  of  illumination  is  chiefly  that  of  the  effect 
on  the  eyes.  It  is  also  known  that  sunlight  and  other  kinds  of 
light  having  ultra-violet  rays  have  a  germicidal  effect  useful  in  kill- 
ing disease  organisims.  There  is  also  a  psychological  effect  of  light. 

TABLE  VII. — UTILIZATION  FACTORS  OBTAINED  BY  LANSINGH  &  ROLPH 

Page  586.  Transactions  I.  E.  S.,  1908.  Room  11.5  by  10.1  ft.  high.  All 
lamps  at  ceiling.  Reflectors  (where  used)  were  of  clear  prismatic  type. 


Ceiling 

Walls 

Floor 

Per  cent, 
utilization 

i  Bare  lamp  

Dark 

Dark 

Dark 

16  4 

i  Lamp  in  reflector 

Dark 

Dark 

Dark 

Ti  6 

i  Bare  lamp  

Light 

Dark 

Dark 

20  4 

i  Lamp  in  reflector  
i  Bare  lamp 

Light 
Light 

Dark 
Light 

Dark 
Dark 

42.0 

a.8  6 

i  Lamp  in  reflector  
i  Bare  lamp 

Light 
Light 

Light 
Light 

Dark 
Light 

55-0 
60  o 

i  Lamp  in  reflector  
3  Bare  lamps 

Light 
Dark 

Light 
Dark 

Light 
Dark 

79-o 

14.   O 

3  Lamps  in  reflector  
3  Bare  lamps 

Dark 
Light 

Dark 
Dark 

Dark 
Dark 

26.0 

26  o 

3  Lamps  in  reflector  
3  Bare  lamps 

Light 
Light 

Dark 
Light 

Dark 
Park 

34-o 
4.6  o 

3  Lamps  in  reflector  

Light 

Light 

Dark 

^o.o 

3  Bare  lamps          .      . 

Light 

Light 

Light 

e6    O 

3  Lamps  in  reflector 

Light 

Light 

Light 

66  o 

The  germicidal  effect  of  sunlight  has  led  to  legislation  requiring 
sunlight  in  living  and  sleeping  rooms  in  some  cities.  It  is  evident, 
however,  that  an  intelligent  application  of  this  to  design  requires 
considerable  definite  knowledge  as  to  the  amount  of  sunlight  in 
a  room  which  will  cause  appreciable  germicidal  effect  and  on  this 
scientific  evidence  is  still  lacking. 

As  to  the  psychological  effects  there  is  a  still  greater  need  of  defi- 
nite knowledge.  Points  which  may  be  considered  psychological 
by  some  are  taken  up  later  under  the  head  of  aesthetic  effects. 

The  eye  is  concerned  chiefly  with  two  things  (a)  sufficient  bright- 
ness of  visualized  objects,  resulting  from  sufficient  illumination  and 
(b)  with  the  distribution  of  brightness  within  the  entire  field  of  vision. 

Ordinary  requirements  for  efficient  vision  are: 

i.  Sufficient  quantity  of  steady  diffusely  reflected  light  from  the  object 
viewed. 


54 


ILLUMINATING   ENGINEERING   PRACTICE 


2.  Minimum  flux  of  light  emitted  in  the  direction  of  the  eye  by  specular 
or  spread  reflection  from  the  objects  viewed. 


Per  cent  Utilization 

oS88SS§2 

.£ 

T 
j& 

1 

Walls  I 

i— 

1 

"36 

J" 
1 

Roor 
Rati 

Walls  Med 

ieflection  Coefficients 
Ceiling  Varied 
Walls  Black          4.3* 
•  •      Medium    42.5? 
"      White        81.0* 
Flooj  Wood          I4.0SJ 

Q   A 

j     Spacing  of  Units 

=  1.59 
ite  -  81* 

1 

rt 

K-13'6-->j 
Slack-  4.3*' 

Height  above  Plane 
urn  -42.5*     Walls  Wt 

. 

0D 



+* 

~ 

I 

ire1 

t 

I)i 

rect 

X^ 

>»*V 

/" 

B 

a.  — 

9 

TStt 

——  • 

*& 

?•» 

L^i 

ifc^ 

^ 

•>  V> 

']>? 

^ 

%2 

<? 

^ 

^f^ 

XI 

l.O. 

w. 

^ 

<4  i,°- 

.v. 

^ 

•     L 

£ 

ir. 

0      20     40    60     80    100    0      20     40     60     80    100     0      20    40    60     80    100 
Reflection  Coefficleat  ol  Ceiling 

Fig.  3. — Utilization  factors. 


70 

T 

i 

T 

'CO 

7 

.i. 

Walls  1 



i 

Re 
c 
11 

F 
Room 
Ratio 

Walls  Medium 

flection  Coefficients 

eiling  Varied 
^alls  Black           4.3* 
»>      Medium     42.5* 
..     White        81.0* 
loor  Wood         14.0* 

B 

Spacing  of  Units 

=  1.04 
81* 

n      a 

Height  above  Plane 
-42.5*      Walls  White  - 

?lack-4.3* 

§40 

I- 

£20 
10 

rcct 

-—-*-* 

^ 

j^_ 

a  — 

1) 

irec 

a-  — 

T*i 

—  — 
cct 

-»  •— 

<*> 

^ 

^ 

"*^7P" 

^. 

9 

£ 

*/ 

° 

^i 

H 

-»  — 
^ 

£• 

V 

^ 

* 

; 

.0. 

>r. 

ftffns 

.0. 

»r. 

^L 

D.at.     i 

.0. 

r. 

0      20    40    60     80    100    0       20    40    00    80    100    0       20    40 
Reflection  Coefficient  ol  Ceiling 

Fig.  4. — Utilization  factors. 


3.  Absence  of  violent  brightness  contrasts  within  the  field  of  vision. 

4.  Freedom  from  sharp  shadows. 


PRINCIPLES   OF   INTERIOR   ILLUMINATION 


55 


Glare  Defined. — The  1915  Committee  on  Glare  of  the  Illuminating 
Engineering  Society  in  its  report  on  Interior  Illumination,  page 
36, 1.  E.  S.  Transactions,  1916,  tentatively  offered  the  following  defi- 


i I  ------ 


3 

B 

H 

ti 

D 

? 

h«6'9*^ 
B         H 

tr 

D 

Reflection  Coefficients 

Ceiling  Varied 

Walls  Black  4.3* 
»  Medium  42.5* 
,,  White  81.0* 

Floor  Wood          14.0* 


Boom  O 
Ratio 


Walls  Black -4.3* 


Height  above  Plane 
Walls  Mediom- 42.5*     Walls  White -81* 


0      20     40    «0     80    100   0      20     40    60    80   100    0      20     40    60    80    100 
Reflection.  Coefficient  ol  Ceiling 

Fig-  5- — Utilization  factors. 


-1 

------  -°—  °- 

_L2'2"           Beflection  Coefficients 

f                     Ceiling  Varied 
Walls  Black          4.3* 

T~ 
I 

*£'&-"   "   B    H 

»      White        81.0* 
Floor  Wood          14.0* 

T 

i 

Ratio   si>acinK  of  Tnits 

<               27                 >• 

Height  above  Plane 

Walls  Black-4.3*        Walls  Medium -42.5*     Walls  White-81* 


100    0      20     40    60     80    100    0      20     40    60     SO    100 
Reflection  Coefficient  of  Ceiling 

Fig.  6. — Utilization  factors. 


nitions  which  express  more  definitely  than  heretofore  attempted 
what  constitutes  glare.  Three  alternative  definitions  were  offered 
as  follows: 


ILLUMINATING  ENGINEERING  PRACTICE 


A            A  

? 

36' 

f 

Reflection  Coefficients 

Ceiling  Varied 

Walls  Black  4.3* 

M  Medium  42.5* 

'•  White  81.0* 

Floor  Wood  14.0* 


20     40     60    SO    100    0      20     40    60     80   100    0      20    40    60    80     100 
Reflection  Coefficient  of  Ceiling 

Fig.  7. — Utilization  factors. 


Roorn-A-lsYx  ISY*  u'    1  Unit 


Eoom-D- 13  6  x  27  x  6    18  Units 


Reflection  Coefficients 


Walls  Varied 

Ceiling  Black         -4.3* 

"        Dark  Gray -33* 

••       Light  Gray-64*        Reflection  Coefficients 

"       White         -81* 
Floor  Wood 


Walls  Varied 

Ceiling  Black  -4.3% 
"  Dark  Gray-33jt 
»»  Light  Gray-64«t 
»»  White  -81% 

Floor  Wood  -14% 


0      20     40    60      80   100    0      20     40     60     80    100 
Reflection  Coefficient  of  Walla 

Fig.  8.— Utilization  factors. 


0      20    40     60     80     100    0       20     40     60    80    100 
Reflection  Coefficient  of  Walls 

Fig.  9. — Utilization  factors. 


PRINCIPLES   OF   INTERIOR  ILLUMINATION  57 

Glare. — i.  Brightness  within  the  field  of  view  of  such  excessive 
character  as  to  cause  discomfort,  annoyance,  or  interference  with 
vision. 

2.  Excess  brightness  of  or  flux  of  light  from  the  whole  or  any  por- 
tion of  the  field  of  view,  resulting  in  reduced  vision,  fatigue  or  dis- 
comfort of  the  eye. 

3.  Light  shining  into  the  eye  in  such  a  way,  or  of  sufficient  quantity, 
as  to  cause  discomfort,  annoyance  or  interference  with  vision. 

Contrast  glare  is  a  kind  of  glare  commonly  experienced  in  de- 
fective lighting  of  interiors.  That  is,  the  contrast  between  the 
brightness  of  the  sources  of  light  and  other  objects  in  the  visual 
field  is  so  great  as  to  cause  discomfort,  annoyance  or  interference 
with  vision.  As  far  as  we  know  there  is  no  measurable  interference 
with  vision  when  the  glaring  bright  source  of  light  is  removed  25 
to  30  degrees  away  from  the  center  of  vision.  It  may,  however,  cause 
discomfort,  annoyance  and  eye  fatigue  if  it  is  anywhere  within  the 
visual  field.  Therefore  while  a  design  which  removed  the  lamp  more 
than  25  degrees  from  the  ordinary  range  of  the  center  of  vision  might 
be  satisfactory  as  far  as  measurable  interference  or  reduced  ability 
to  see  is  concerned,  it  might  not  be  satisfactory  to  work  or  live  under 
continuously  because  of  the  fatigue  and  annoyance  resulting. 

A  review  of  all  of  the  available  data  and  observations  of  cases 
where  eye  fatigue  and  annoyance  have  been  complained  of  together 
with  numerous  eye  fatigue  tests  by  the  Ferree  method  indicates  that 
to  avoid  glare  effects  visible  light  sources  should  not  be  more  than  200 
times  as  bright  as  their  background  and  preferably  not  over  100 
times,  in  ordinary  artificial  lighting  of  interiors  where  the  average 
illumination  of  the  working  plane  is  from  3  to  6  foot  candles.  As 
most  of  the  tests  on  this  point  have  been  made  at  about  this  mag- 
nitude of  brightness  it  is  not  entirely  certain  what  ratio  should  be 
adopted  for  other  magnitudes,  but  from  tests  made  by  Nutting 
(I.  E.  S.  Transactions,  1916  Convention)  on  the  lower  limits  of 
annoying  glare  (which  limits  of  brightness  are  of  course  much 
higher  than  for  fatiguing  glare)  as  well  as  from  certain  well  known 
common  experience  there  is  reason  to  believe  that  for  higher  illu- 
minations than  6  foot  candles  this  limit  of  contrast  should  be  less 
than  loo  to  i  while  for  lower  limits  it  may  be  more  than  100  to  i. 

Brightness  for  bowls  and  globes  for  locations  where  they  are  con- 
tinuously within  the  field  of  vision,  with  from  3  to  6  foot  candles  on  the 
working  plane  should  be  kept  approximately  below  300  millilamberts 
in  rooms  with  light-colored  (50  per  cent,  reflection  coefficient)  walls  to 


58  ILLUMINATING  ENGINEERING   PRACTICE 

safely  conform  to  the  100  to  i  contrast  limit.  The  brightness  should 
be  diminished  as  the  reflection  coefficient  of  the  walls  is  decreased. 
Outdoors  where  brightness  magnitudes  are  much  higher  it  is  worth 
while  noting  that  contrasts  do  not  often  exceed  twenty  to  one;  while 
at  night,  outdoors,  much  greater  contrasts  are  well  known  to  be 
tolerable. 

Brightness  glare  is  glare  due  to  an  excessive  general  brightness  of 
the  field  of  view.  It  is  seldom  experienced  in  interior  illumination 
except  possibly  from  the  reflection  of  sunlight  from  a  sheet  of  white 
paper. 

Temporary  glare  resulting  from  flicker  is  a  condition  caused  by  the 
lack  of  brightness  accommodation  of  the  retina  of  the  eye  to  such 
sudden  changes  in  brightness. 

Specular  reflection  or  -veiling  glare  from  glossy  paper,  polished 
metal  work  and  the  like  are  very  common  conditions  with  all  sys- 
tems of  lighting  and  are  likely  to  be  especially  pronounced  with  arti- 
ficial illumination,  from  relatively  small  sources.  The  polished 
surface  reflects  a  glaring  image  of  the  source  of  light.  The  actual 
brightness  of  the  glare  on  the  paper  as  far  as  it  can  be  measured  is 
not  likely  to  be  over  1.5  times  that  of  the  background  but  this  seems 
to  be  enough  to  make  trouble  in  this  location  though  it  would  hardly 
be  noticed  elsewhere.  Frequently  the  ink  or  pencil  marks  on  paper 
have  more  specular  reflection  than  the  paper  and  in  the  glare  posi- 
tions these  marks  may  be  equally  as  bright  as  the  paper,  and  hence 
invisible  or  nearly  so. 

Shadows  may  cause  interference  or  trouble  with  work  if  the  illumi- 
nation in  the  shadow  is  insufficient  or  if  the  contrast  between  the 
parts  in  shadow  and  those  out  of  the  shadow  makes  the  shadowed 
places  appear  dark  by  contrast.  Shadows  caused  by  bright  light 
sources  with  direct  lighting  have  sharp  edges  and  may  cause  annoy- 
ance while  an  equal  shadow  with  a  large  source  of  light  or  indirect 
lighting  where  the  transition  from  the  middle  of  the  shadow  to  the 
edge  is  gradual  may  not  be  perceptible  e'xcept  to  the  expert. 

Shadows  are  to  be  most  carefully  considered  in  large  office  and 
factory  spaces  lighted  by  general  lighting,  where  the  location  of  the 
work  with  reference  to  the  light  cannot  be  adjusted  or  charged  and 
the  illumination  must  be  sufficiently  good  at  any  point  in  any  posi- 
tion to  permit  of  efficient  work. 

The  ratio  of  illumination  in  the  shadow  to  illumination  just  out- 
side of  the  shadow  with  large  sources  or  indirect  light  may  be  as 
high  as  one  to  two  without  causing  annoyance  provided  the  illu- 


PRINCIPLES    OF   INTERIOR   ILLUMINATION  59 

mination  in  the  shadow  is  sufficient  for  the  purpose  in  hand.  Be- 
cause of  the  nature  of  these  shadows  with  indirect  lighting  the  or- 
dinary person  is  apt  to  think  there  are  no  shadows  and  to  attempt  the 
closest  work  in  the  shadows  of  his  head  and  body,  not  realizing  that 
the  illumination  is  better  away  from  this  shadow.  With  the  sharper 
shadows  common  to  direct  lighting  systems  this  would  not  be  the 
case.  However,  owing  to  the  sharpness  of  these  latter  shadows  the 
same  shadow  ratio  might  sometimes  cause  some  annoyance. 

In  a  large  room  with  a  number  of  lighting  units  the  actual  magni- 
tude of  the  shadow,  that  is  the  ratio  of  illumination  in  the  shadow 
to  that  out  of  it,  is  likely  to  be  about  the  same  with  an  indirect  sys- 
tem as  with  a  direct,  provided  the  spacing  of  the  outlets  is  the  same 
in  both  cases.  The  direct  lighting  shadows  have  sharp  edges,  how- 
ever, which  makes  them  easily  apparent  where  the  others  are  not. 

Quantity  of  Illumination. — It  is  customary  to  discuss  problems 
concerning  the  quantity  of  illumination  required  for  different  pur- 
poses in  terms  of  the  illumination  incident  upon  the  work.  This  in- 
cident illumination,  however,  is  the  cause  which  produces  the  desired 
effect,  namely,  brightness  of  the  object  viewed,  and  it  is  this  effect 
that  is  the  real  end  desired.  Table  VIII  calculated  by  Dr.  P.  G.  Nutt- 
ing from  work  by  Konig  and  himself  shows  the  sensibility  of  the  eye 

TABLE  VIII. — EYE  SENSIBILITY  AT  DIFFERENT  MAGNITUDES  OF  SURROUNDING 

BRIGHTNESS 

P.  G.  Nutting 


Average  brightness 
magnitudes,  milli- 
lamberts 

Perceptible  percent- 
age difference  in 
brightness 

Exterior,  daylight  
Interiors,  daylight 

IOOO.O 
IO.O 

0.0176 
0.030 

Interiors,  night  

.....'  o.i 

O.  123 

at  different  typical  brightness  magnitudes.  As  a  matter  of  fact  of 
course  the  brightness  magnitudes  of  interiors  both  at  night  and  day 
vary  considerably  from  the  average  brightness  value  given.  From 
this  table  it  will  be  seen  that  increasing  the  illumination  one  hundred 
fold  from  a  rather  poor  lighted  interior  at  night  to  an  interior  by 
daylight  makes  the  eye  able  to  perceive  a  percentage  difference  in 
brightness  about  one-third  of  that  it  is  able  to  perceive  in  the  former 
case.  This  gain  is  apparently  rather  small  but  if  the  eye  is  working 
near  the  limit  it  may  be  important. 


60  ILLUMINATING   ENGINEERING  PRACTICE 

The  eye  sees  by  virtue  of  differences  of  brightness  and  color. 
The  question  of  a  sufficient  quantity  of  illumination  for  a  given  kind 
of  work  is  not  altogether  that  of  delivering  a  certain  number  of  foot- 
candles  on  a  certain  plane  where  the  work  is  being  done.  The  ques- 
tion is  fundamentally  one  of  producing  a  sufficient  contrast  of  bright- 
ness for  the  eye  to  perceive  readily  objects  with  a  given  brightness  of 
surroundings.  In  the  case  of  reading  printed  or  written  letters  on 
paper  we  have  a  considerable  contrast  between  the  paper  and  ink 
or  pencil  which  makes  them  easy  to  distinguish  with  any  kind  of 
illumination  which  does  not  produce  specular  reflection  or  glare  from 
the  paper  or  ink,  provided  the  illumination  is  of  sufficient  quantity. 
In  the  case  of  sewing  on  either  dark  or  light  goods  there  is  very  little 
contrast  between  the  thread  and  the  goods  so  that  the  problem  of 
producing  sufficient  shadows  and  specular  reflection  to  enable  the 
thread  and  the  texture  to  be  seen  easily  is  important.  For  this 
purpose  localized  lighting  coming  mainly  from  one  direction  is 
necessary. 

Many  tables  have  been  published  of  the  intensity  of  illumination 
required  for  various  purposes  but  all  should  be  used  with  allowance 
for  the  fact  that  color  and  direction  must  be  considered  as  must  also 
the  general  brightness  of  the  surroundings.  The  latter  is~  especially 
true  when  there  is  a  large  window  exposure  but  the  particular 
spot  to  be  illuminated  does  not  get  the  benefit  of  the  window 
illumination. 

The  indications  of  scientific  research  so  far  are  that  the  eye  works 
best  when  the  object  upon  which  vision  is  centered  is  of  about  the 
same  general  magnitude  of  brightness  as  the  surroundings.  This  is 
what  one  might  expect  from  the  conditions  under  which  the  eye  has 
been  evolved. 

Table  IX  shows  the  approximate  foot-candles  illumination  consid- 
ered about  right  by  a  number  of  authorities  for  various  classes  of 
interior  lighting. 

The  question  of  proper  quantity  of  illumination  for  reading  has 
been  investigated  much  more  thoroughly  than  that  for  other  pur- 
poses. Tests  show  considerable  difference  between  individuals 
although  the  same  individuals  show  consistent  repetition  of  the 
quantities  considered  sufficient.  If  the  direction  and  diffusion  of  light 
is  such  as  to  cause  veiling  glare  from  the  paper  or  ink  more  illumina- 
tion is  required  although  it  cannot  be  said  that  with  veiling  glare 
present  it  is  ever  possible  to  produce  as  satisfactory  and  comfort- 
able illumination,  no  matter  what  the  intensity,  as  can  be  obtained 
with  veiling  glare  practically  absent. 


PRINCIPLES   OF   INTERIOR   ILLUMINATION  6 1 

TABLE  IX. — ILLUMINATION  FOR  VARIOUS  PURPOSES 


Foot-candles 


Reading:  U.  S.  Government  Postal  Car  minimum  require- 


ments. 


Clerical  and  office  work 

Drafting 

Drafting,  tracing  on  blue  prints  or  faint  pencil  drawings. 

Factory  work,  coarse .'. 

Factory  work,  fine 

Corridors 

Stores,  ordinary  practice 

Stores,  first  floors,  large  cities 

Audience  rooms 

Show  windows 


2 . 8  Note  a. 

3-7 

5-10 

10-20  Note  6. 
1.25-2.5  Note  c 
3.5  -10  Note  c. 
0.25-1 

3-7 

5-10  Note  d. 

i-3 

5-40  Note  d. 


NOTES. — (a)  Some  individuals  are  satisfied  with  half  this  while  others,  especially  the 
aged  and  those  not  properly  fitted  with  glasses  and  those  whose  eyes  are  sub-normal  for 
any  reason  may  be  satisfied  only  with  values  considerably  higher  than  this;  perhaps  5  to 
10  foot-candles.  When  such  individuals  are  to  be  satisfied  this  fact  must  be  remembered 
in  the  design. 

(6)  Illumination  from  below  is  preferable,  using  a  translucent  table. 

{c)   Depends  also  on  color. 

(d)   Depends  on  surrounding  competition. 

As  a  result  of  extensive  tests  of  postal  clerks  and  others  on  the 
light  required  for  reading  under  postal  car  lighting  conditions  the 
United  States  Government  now  specifies  a  minimum  illumination  of 
2.8  foot-candles  at  points  where  reading  of  letter  addresses  is  to  be 
done  by  postal  clerks. 

There  is  no  conclusive  evidence  at  the  present  that  there  is  any 
marked  hygienic  advantage  in  color  of  one  artificial  illuminant  over 
another.  This  statement  refers  to  purely  physiological  results 
rather  than  to  aesthetic  effects.  An  exception  to  this  which  should 
be  noted,  however,  is  that  there  is  good  evidence  that  the  chromatic 
abberation  of  the  eye  causes  a  certain  lack  of  clearness  with  most 
natural  and  artificial  illuminants  so  that  for  seeing  fine  details  a  light 
which  is  nearly  monochromatic  like  the  mercury-vapor  light  is 
preferable. 

ESTHETIC  EFFECTS 

It  is  not  the  function  of  this  portion  of  the  lecture  to  give  a  dis- 
sertation on  art  but  rather  to  call  attention  to  methods  by  which 
certain  desirable  effects  can  be  produced  and  undesirable  ones 
avoided. 


62  ILLUMINATING   ENGINEERING   PRACTICE 

The  function  of  illumination  is  to  provide  light  and  shade,  as  it  is 
artistically  called,  on  various  objects.  From  the  standpoint  of 
appearance  much  depends  on  how  the  light  and  shade  are  regulated 
or  in  more  scientific  language  upon  the  direction  and  diffusion  of  the 
light.  By  diffused  light  is  here  meant  light  coming  from  many  di- 
rections or  from  large  surfaces  like  the  sky  or  illuminated  ceilings. 
Much  of  the  pleasing  or  displeasing  effect  of  a  design  of  interior  illumi- 
nation depends  upon  the  proper  use  or  misuse  of  shadows,  and  tastes 
differ  decidedly  as  to  what  light  and  shade  effects  are  most  pleasing. 
Heavy  shadows  are  produced  by  light  coming  mainly  from  one 
direction  with  very  little  general  diffused  light.  While  for  some  par- 
ticular purposes  extreme  contrasts  are  considered  desirable  by  some 
persons,  others  think  them  to  be  unpleasant.  It  is  possible  to 
eliminate  shadows  so  completely  by  having  light  coming  from  many 
directions  that  there  remains  only  the  difference  in  the  coefficient  of 
reflection  of  different  parts  of  the  illuminated  object  to  enable  the  eye 
to  distinguish  it.  If  the  object  is  a  piece  of  white  statuary  or 
moulding  of  uniform  color  and  reflecting  power,  perfectly  uniform  or 
diffuse  illumination  will  obliterate  all  details. 

Direct  lighting  systems  with  small  sources  produce  sharp  shadows 
like  those  produced  by  sunlight.  With  indirect  lighting  systems  and 
semi-direct  systems  with  very  dense  glassware  the  shadows  are  very 
similar  to  those  obtained  from  skylight.  They  differ  from  window 
daylight  in  direction  when  the  ceiling  is  the  main  reflecting  surface, 
but  if  the  wall  is  the  main  surface  window  direction  and  diffusion  is 
imitated.  Sky-  and  window-light  shadows  are  gradual  transitions 
from  light  to  dark. 

Exposed  light  sources  have  been  used  for  many  years  for  decora- 
tive effect  and  will  doubtless  continue  to  be  used.  It  is  for  the  illumi- 
nating engineer  to  recognize  this  fact  and  to  guide  the  use  into  the 
proper  hygienic  channels.  With  the  tiny  sources  of  light  available 
up  to  the  introduction  of  electricity  and  gas  mantle  burners  the  bad 
effects  of  glare  with  decorative  lighting  of  this  kind  were  not  much 
felt.  With  the  brighter  and  more  powerful  light  sources  now 
common,  adequate  shading  precautions  must  be  taken.  The  bare  light 
source  of  to-day  is  not  only  hygienically  bad  but  it  is  so  crude  as  to 
be  unartistic.  There  are  so  many  opportunities  to  produce  pleasing 
effects  with  diffused  light  by  the  use  of  colored  glass,  cloth,  or  paper 
shades,  leaving  the  main  light  for  useful  purposes  to  be  obtained  in 
other  ways  that  there  is  no  longer  much  excuse  for  the  type  of  fixture 
which  in  spite  of  its  great  expense  offers  nothing  better  for  light 


PRINCIPLES   OF   INTERIOR   ILLUMINATION  63 

diffusion  than  a  lot  of  loosely  hung  prisms  interspersed  with  bare 
lamps. 

In  the  use  of  lamps  for  decorative  effects  the  same  rule  as  to  low 
brightness  values  should  be  adhered  to  as  is  laid  down  under  the  head 
of  hygiene.  The  lamp  shade  or  globe  which  must  be  faced  continu- 
ally should  be  not  more  than  200  times  as  bright  as  its  background, 
and  no  light  source  of  this  kind  should  be  bright  enough  to  be  annoy- 
ing or  noticeably  glaring. 

Although  white  daylight  cannot  be  said  to  have  an  unpleasant 
effect  on  countenances  it  is  notable  that  among  lamps,  those  which 
give  light  yellowish  in  color  rather  than  those  offering  considerable 
green  and  blue  are  the  most  pleasant.  Red  and  yellow  light  bring 
out  the  agreeable  color  of  the  face  while  the  absence  of  those  colors 
and  prominence  of  blue  and  green  give  the  countenance  a  ghastly  hue. 

There  is  some  difference  of  opinion  as  to  how  far  red  and  yellow  and 
amber  colors  should  be  sought  in  light,  especially  in  residence  light- 
ing. Some  even  go  so  far  as  to  color  the  tungsten  lamp  purposely  to 
get  nearer  the  yellow  color  of  the  old  carbon  lamp.  However,  this 
result  can  also  be  obtained  by  the  use  of  ceiling  and  wall  colors  and 
proper  glassware.  If  indirect  or  semi-indirect  lighting  is  used,  the 
color  of  the  ceiling  has  much  to  do  with  the  resultant  illumination  in 
the  room.  The  ceiling  can  be  so  tinted  as  to  make  the  room  illumi- 
nation as  yellow  as  desired.  The  small  amount  of  illumination  which 
should  be  allowed  to  come  directly  through  the  bowl  of  a  semi- 
indirect  fixture  should  not  have  much  effect  in  the  general  total. 

A  very  yellow  light  like  that  of  the  old  carbon  incandescent  and 
open  gas  flame  or  more  modern  illuminants  with  yellow  globes  brings 
out  certain  yellowish  hues  in  decorations  and  paintings  so  as  to  give 
a  richer  effect  than  would  be  obtained  with  white  light.  At  the  same 
time  it  must  be  remembered  that  these  are  deficient  in  green  and  blue 
and  the  green  and  blue  in  paintings  and  decorations  suffer  accord- 
ingly. Either  the  decorations  should  be  suited  to  the  color  of  light 
or  the  color  of  the  light  to  the  decorations.  As  to  which  course 
should  be  pursued  depends  entirely  on  the  particular  conditions  of 
the  case. 

At  the  present  time  almost  any  color  desired  in  artificial  lighting 
can  be  obtained  with  a  sufficient  expenditure  of  money.  Where  it  is 
desirable  to  bring  out  all  of  the  colors  as  in  daylight  several  methods 
are  open.  The  Moore  carbon  dioxide  tube  lamp  and  the  intensified 
carbon  arc  lamp  uncorrected  give  practically  white  light.  The  gas 
filled  tungsten  lamp  and  the  gas  mantle  burner  with  a  special  mantle 


64  ILLUMINATING   ENGINEERING    PRACTICE 

can  be  used  with  a  glass  having  the  proper  selective  absorption  to 
filter  out  the  excess  of  certain  colors  and  give  a  white  light.  The 
same  process  can  be  used  with  other  yellow  illuminants.  Since  this 
process  involves  throwing  away  the  excess  yellow  over  and  above 
that  needed  to  maintain  a  proper  balance  for  white  light  it  is,  of 
course,  somewhat  wasteful. 

In  considering  the  question  whether  art  may  clash  with  hygiene 
and  utility,  it  may  be  appropriate  to  ask  whether  anything  can  be 
considered  artistic  which  is  unhygienic  and  ill-suited  to  the  use  for 
which  it  is  intended.  Nevertheless  it  may.be  proper  to  mention 
some  points  where  so-called  art  and  comfort  and  the  health  of  the 
user  may  clash.  When  an  architect  designs  an  interior  so  that  noth- 
ing but  exposed  glaring  lamps  on  brackets  will  satisfy  his  idea  of  the 
artistic  one  is  tempted  to  ask  where  the  art  conies  in  as  far  as  the 
user  of  the  room  is  concerned.  When  an  audience  room  or  council 
chamber  is  finished  in  dark  colors  with  elaborate  chandeliers  of  a 
design  which  permit  of  nothing  but  a  great  quantity  of  glare  one  is 
again  tempted  to  make  the  same  inquiry.  Cases  can  be  cited  with- 
out number  where  the  ideas  of  some  person  as  to  what  is  artistic  are 
given  precedence  over  health  and  comfort.  There  are  no  reasons 
why  these  three  elements  cannot  be  combined. 

PART  II.     THE  PROCESS  OF  DESIGN 

The  process  of  illumination  design  usually  consists  of  the  following 
steps: 

1.  Selection  of  the  general  scheme  of  lighting,  and  the  type  of 
lighting  units. 

2.  Calculations  of  the  quantity  of  light  flux  required. 

3.  Final  selection  of  the  location  and  size  of  the  lighting  units. 

In  making  each  of  these  steps  we  must  fall  back  upon  the  basic 
information  given  in  Part  I. 

The  selection  of  the  general  type  of  light  source  must,  of  course, 
depend  on  the  kind  of  lamps  available.  This  depends  on  local 
conditions  and  need  not  be  discussed  here.  Then  the  hygienic  and 
artistic  requirements  and  limitations  should  be  considered. 

Both  the  electric  incandescent  lamp  and  the  gas  mantle  burner 
are  adapted  to  the  illumination  of  almost  any  kind  of  interior  from 
the  roughest  to  the  most  refined.  For  the  illumination  of  offices  and 
industrial  plants  there  also  comes  up  for  consideration  the  mercury- 
vapor  lamp.  For  some  of, the  roughest  industrial  plants  such  as 
foundries  and  steel  mills  the  flame  arc  lamp  can  also  be  considered, 


PRINCIPLES   OF   INTERIOR  ILLUMINATION  65 

although  in  offices  and  stores  the  fumes  emitted  are  not  allowable. 
The  color  of  the  light  from  the  mercury-vapor  lamp,  of  course,  is  an 
objection  from  the  artistic  standpoint  although  hygienically  no  case 
has  been  found  against  it.  For  work  on  fine  black  and  white  detail 
the  better  visual  acuity  it  gives  tends  to  offset  the  psychological 
effect  of  its  color. 

In  choosing  between  the  tungsten  electric  and  gas  mantle  burner 
lamps  the  following  points  must  be  considered  for  each  case: 

'(a)  The  cost  per  1000  lumen  hours  for  electricity  versus  gas  at  the 
current  prices.  In  making  such  comparison  allowance  should  be 
made  for  the  probable  depreciation  of  the  lamp  below  the  labora- 
tory performance  figures  given  in  Part  I.  In  the  case  of  elec- 
tricity there  is  a  blackening  and  increase  in  resistance  in  the  lamp 
internally  and  the  accumulation  of  dirt  externally  to  cause  depre- 
ciation, and  in  the  case  of  gas  in  practice  the  burner  adjustment  is 
seldom  as  good  as  that  obtained  in  the  laboratory  and  there  is  the 
possibility  of  worn  and  defective  mantles.  These  depreciation  figures 
can  easily  lower  the  electric  lamp  output  by  from  20  to  50  per  cent, 
below  laboratory  figures  and  the  gas  lamp  output  to  by  from  30 
to  60  per  cent,  below  the  laboratory  figures.  Of  course,  the  engineer 
should  take  into  account  the  maintenance  conditions  that  are  likely 
to  exist  in  the  completed  installation.  The  better  the  mainte- 
nance the  lower  the  necessary  percentage  allowance  for  depreciation. 

(b)  The  relative  convenience  of  control  under  the  two  methods. 
If  the  gas  installation  is  to  be  arranged  for  a  control  practically 
equivalent  to  that  of  electric,  the  comparative  total  cost  of  the  two 
systems  should  be  considered. 

(c)  Additional  blackening  of  ceilings  and  walls  with  gas  as  com- 
pared  to  electricity  should  be  weighed  against  the  cost  of  elec- 
tricity along  with  the  cost  of  gas. 

(d)  The  probable  relative  steadiness  of  the  two  illuminants  under 
the  particular  local  conditions  under  consideration.     The  voltage 
of  the  electric  system  may  be  very  unsteady  and  the  pressure  of  the 
gas  very  steady  or  the  reverse. 

(e)  The  cost  of  glassware  and  lamp  renewals  for  electric  lamps 
and  the  cost  of  glassware  and  mantle  renewals  or  maintenance 
service  for  gas  lamps  should  be  figured. 

No  illuminant  should  be  chosen  which  does  not  permit  the  use  of 
the  proper  globe,  shade  or  reflector  equipment  to  conform  to  the 
hygienic  requirements  spoken  of  later. 

Glare  Elimination. — The  necessity  of  the  elimination  of  glare  de- 


66  ILLUMINATING   ENGINEERING    PRACTICE 

pends  largely  on  the  .purpose  to  which  the  room  is  to  be  put.  In 
a  living  room  or  a  general  office  or  an  audience  room  where  persons 
sit  for  long  periods  in  one  position  it  is  of  first  importance  to  avoid 
glare  in  the  eyes  of  the  occupant.  On  the  other  hand  if  the  eye  is 
not  to  be  exposed  to  the  glare  for  long  periods,  some  temporary 
glare  is  permissible  in  many  cases  to  keep  down  the  cost  of  con- 
struction and  operation. 

Glare  may  be  kept  from  the  eyes  of  the  occupants  of  a  room  by 
limiting  the  brightness  contrast  ratios  to  which  the  eye  is  subjected. 
In  the  case  of  artificial  light  this  is  done  by  inserting  opaque  re- 
flectors or  a  diffusing  medium  between  the  lamp  and  the  possible 
positions  from  which  it  can  be  seen. 

Practically  all  sources  of  artificial  light  now  in  common  use  are 
too  bright  for  continuous  exposure  to  the  eye  with  the  background 
illuminated  no  better  than  is  common  practice  to-day. 

In  eliminating  glare  by  the  insertion  of  diffusing  glass  or  other 
material  between  the  light  source  and  the  eye  three  general  methods 
have  been  used.  An  opaque  reflector  or  one  of  dense  trarislucent 
glass,  cloth  or  paper  can  be  placed  over  the  lamp  far  enough  to  pro- 
tect the  eyes  of  occupants  of  the  room  and  yet  allow  direct  light  from 
the  lamp  and  reflector  to  fall  on  objects  under  and  near  the  lamp. 
Another  method  is  to  reverse  this  process,  putting  opaque  or  dense 
translucent  reflectors  under  the  lamp  to  reflect  the  light  to  a  light 
colored  ceiling  or  wall  and  so  obtain  a  diffused  light  from  the  ceiling 
or  wall.  As  the  light  is  spread  out  on  the  ceiling  its  brightness  is 
comparatively  low  and  the  brightness  contrast  ratios  are  cut  down  to 
bring  them  within  the  limit  of  tolerance  of  the  eye.  A  third  method 
is  to  put  around  the  lamp  an  enclosing  globe  that  will  diffuse  the 
light  going  in  all  directions.  While  this  is  a  very  common  method  it 
is  an  incomplete  solution  of  the  problem  of  the  most  modern  illumi- 
nants  because  a  diffusing  globe  which  will  cut  the  brightness  down 
to  a  proper  figure  is  either  so  large  as  to  be  prohibitively  expensive 
or  so  dense  as  to  cause  a  prohibitive  loss  of  light. 

The  second  method,  that  of  using  indirect  lighting,  or  semi-direct 
lighting  with  bowls  of  very  low  brightness,  is  the  only  reasonably 
economical  and  practical  method  which  conforms  fully  to  the  hy- 
gienic requirements  in  most  cases  where  low  brightness  of  the  units 
is  required.  Even  if  the  ceiling  is  dark  in  color  it  may  be  more  feas- 
ible to  light  the  room  indirectly  from  a  dark-colored  ceiling  than  to 
put  in  enough  outlets  to  supply  general  illumination  from  the  en- 
closing globes. 


PRINCIPLES    OF   INTERIOR   ILLUMINATION  67 

A  method  which  partially  eliminates  glare,  adopted  in  many  cases 
in  which  indirect  lighting  would  be  considered  too  expensive  on 
account  of  the  poor  reflecting  qualities  of  the  ceiling,  involves  the  use 
over  the  lamps  of  reflectors  deep  enough  to  hide  most  of  the  source  of 
light,  the  lamp  being  placed  as  high  as  possible  to  get  it  out  of  the 
ordinary  range  of  vision.  This  method  is  extensively  employed  both 
with  translucent  reflectors  of  various  types  of  opal  and  with  opaque 
reflectors  of  white  enamel  steel,  aluminum-finished  metal  and  mir- 
rored glass.  This  method  is  necessarily  an  incomplete  solution  of  the 
problem  of  eliminating  glare  because  it  is  possible  to  see  the  lamp 
filaments,  mantles  or  frosted  tips  of  the  lamp  and  the  interior  sur- 
faces of  the  reflectors,  any  of  which  is  bright  enough  to  cause  contrast 
glare.  It  is  however  much  more  efficient  and  less  glaring  than  the 
use  of  bare  lamps  or  flat  reflectors. 

In  the  lighting  of  industrial  plants  where  the  ceilings  are  consider- 
ably broken  up  and  not  very  white,  and  in  large  rooms  of  the  coli- 
seum or  armory  type  with  high  roof  and  open  roof  trusses,  the 
operating  expense  of  indirect  lighting  would  be  usually  considered 
prohibitive,  and  the  method  of  using  bowl  reflectors  of  various 
depths  with  lamps  placed  high  is  the  most  common  in  the  best 
practice  to-day. 

Opinion  differs  somewhat  as  to  whether  the  bowl  reflectors  used 
in  this  way  should  be  opaque  or  translucent  like  opal.  Opaque  re- 
flectors have  been  extensively  used  partly  because  of  the  greater 
strength  of  the  opaque  metal  reflector  and  partly  because  it  was  felt 
that  light  striking  such  dark  colored  ceilings  would  be  so  largely 
wasted  that  a  reflector  directing  all  of  the  light  flux  below  the  hori- 
zontal might  better  be  used.  The  latter  is  a  mistaken  view.  A 
dense  opal  reflector  directs  as  much  light  flux  below  the  horizontal  as 
a  good  white  enameled  reflector,  so  that  the  light  passing  through 
the  opal  reflector  to  light  the  ceiling  and  upper  walls  represents  clear 
gain.  Illuminating  the  ceiling  and  upper  walls  reduces  the  contrast 
glare,  makes  the  room  more  cheerful,  and  adds  to  the  diffused  light. 

In  an  armory  or  a  coliseum  type  of  building  there  is  another 
method  of  partially  reducing  the  contrast  glare  effect  which  combines 
some  of  the  elements  of  the  methods  previously  mentioned.  This 
is  to  use  reflectors  of  an  extra  deep  bowl  type  confining  most  of  the 
light  flux  within  about  40  degrees  of  the  vertical.  This  of  course 
reduced  the  number  of'  light  sources  which  are  within  the  field  of 
vision  at  one  time  and  those  sources  which  can  be  seen  are  near  the 
edge  of  the  visual  field.  With  such  deep  reflectors  a  mirrored  sur- 


68  ILLUMINATING   ENGINEERING   PRACTICE 

face  is  more  necessary  to  the  exact  control  of  light  and  high  efficiency 
than  where  the  reflectors  are  shallower.  The  reflector  should  not 
be  too  concentrating  or  the  illumination  on  vertical  surfaces  will  be 
poor. 

Along  with  this  plan  of  using  deep  reflectors  in  buildings  of  this 
type  it  is  frequently  considered  desirable  to  provide  for  some  illumi- 
nation of  the  roof  and  upper  walls  to  reduce  the  contrast  glare 
effect  between  the  illuminated  interior  at  the  lower  part  of  the  re- 
flector and  the  roof  background.  This  can  be  done  by  providing 
indirect  lighting  for  the  roof  from  separate  lamps  and  reflectors  but 
is  most  easily  accomplished  by  simply  allowing  enough  light  to  es- 
cape out  of  the  top  of  the  deep  reflector  to  illuminate  the  roof. 

The  avoidance  of  glare  with  natural  lighting  from  side  and  ceiling 
windows  is  partly  a  matter  of  the  proper  selection  of  window  glass, 
louvres  and  shades  but  it  is  also  very  much  dependent  upon  the 
general  arrangement  and  color  scheme  of  the  room. 

Diffusing  glass  of  various  kinds  such  as  ribbed,  prism,  frosted, 
corrugated  and  roughed  glass  have  been  used  to  some  extent  to  in- 
crease the  illumination  in  a  room  (as  already  explained  in  Part  I) 
and  may  do  this  very  effectively  if  they  are  kept  clean.  In  the 
application  of  such  glass  care  should  be  taken  not  to  place  diffusing 
glass  below  the  eye  level.  In  an  ordinary  type  of  window  where 
the  sill  is  much  below  the  eye  level  the  lower  sash  should  not  be 
provided  with  diffusing  glass.  The  effect  of  diffusing  glass  is  to 
receive  light  from  the  sky  and  transmit  it  by  diffusion  into  the  room. 
The  result  is  a  great  increase  in  brightness  of  the  lower  window,  to 
such  an  extent  that  the  brightness  is  much  greater  than  that  to 
which  the  eye  is  accustomed  in  such  a  location.  While  the  eye  is 
accustomed  to  the  brightness  of  the  sky  and  clouds  above  a  hori- 
zontal plane  it  is  not  accustomed  to  such  a  high  order  of  brightness 
below  the  horizontal  plane.  Although  it  is  occasionally  subjected  to 
it  when  outdoors  with  sunlight  on  snow  or  on  white  macadam  roads 
or  desert  sand  all  of  these  conditions  cause  eye  discomfort. 

It  is  quite  possible  for  the  architect  to  render  glare  unavoidable 
either  by  night  or  by  day  and  so  defeat  all  later  attempts  at  good 
lighting.  Conditions  are  more  easily  controlled  as  regards  artificial 
illumination,  however,  than  as  regards  natural  illumination.  In 
the  case  of  artificial  illumination,  interiors  with  a  very  dark  finish 
with  corners  where  there  is  a  small  amount  of  illumination  introduce 
large  contrasts  which  are  uncomfortable,  if  lighted  by  ordinary 
methods  with  exposed  lamp  or  lamps  with  enclosed  globes.  Such 


PRINCIPLES   OF  INTERIOR   ILLUMINATION  69 

interiors  can  be  lighted  by  the  expenditure  of  sufficient  luminous 
energy  upon  dark  ceilings  and  walls  to  bring  up  the  general  illumina- 
tion to  a  satisfactory  point.  This  method,  however,  is  not  in  ac- 
cordance with  the  general  scheme  of  design  of  such  interiors.  The 
only  method  of  treatment  of  such  interiors  which  is  satisfactory  and 
is  in  accordance  with  the  general  architectural  scheme  is  the  use  of 
localized  light  from  thoroughly  shaded  sources  and  this  usually 
means  that  there  must  be  a  large  number  of  sources. 

In  the  case  of  daylight  illumination  from  windows,  one  of  the  prin- 
cipal things  to  be  avoided  is  an  architectural  arrangement  which 
makes  it  necessary  for  persons  to  be  seated  facing  windows  with  a 
bright  sky  visible  through  the  window  in  contrast  to  a  dark  space 
around  the  window.  Facing  the  window  may  not  be  objectionable 
when  seated  very  near  to  the  window  so  that  the  sky  occupies  a 
considerable  portion  of  the  field  of  vision  but  as  one  recedes  into  the 
room  the  sky  occupies  a  smaller  portion  of  the  visual  field  and  in 
painful  contrast  with  it  are  the  walls  of  the  room  which  are  very 
much  less  bright. 

In  office  work  the  direction  of  diffusion  of  light  has  much  to  do 
with  the  amount  of  glare  from  papers  on  desk  tops.  Daylight  com- 
ing from  windows  at  one  side  of  the  desk  gives  the  best  working 
conditions,  partly  because  of  the  large  diffusing  surface  (the  sky) 
from  which  the  light  comes  and  partly  because  of  the  fact  that  it 
comes  from  one  side  so  that  all  of  the  veiling  glare  on  the  paper  is 
in  a  direction  where  it  is  not  often  observed  by  the  worker. 

The  most  effective  method  of  eh'minating  veiling  glare  in  office 
work  with  either  daylight  or  artificial  lighting  is  the  use  of  nothing 
but  matte  or  soft  finish  paper.  Of  course  this  is  not  feasible  in  most 
cases  at  the  present  time.  Under  present  conditions  such  glare  can 
be  eliminated  only  by  so  placing  the  source  of  light  that  the  angle 
from  the  source  to  the  paper  can  never  equal  the  angle  from  the  paper 
to  the  eye.  Under  these  conditions  the  only  veiling  glare  present 
is  that  due  to  a  reflection  from  the  paper  of  the  moderate  illumina- 
tion from  the  walls  and  ceilings.  Such  a  position  is  usually  only 
feasible  with  a  drop  cord  or  wall  bracket  lamp  placed  at  one  side  and 
slightly  back  of  the  worker.  With  any  kind  of  local  desk  lamp  near 
the  work  it  is  difficult  to  avoid  glare  from  the  paper  altogether  as 
there  are  so  many  positions  from  which  the  light  can  be  received. 
Furthermore  with  either  a  desk  lamp  or  a  wall  bracket  lamp  properly 
placed  for  one  worker,  direct  glare  from  the  lamp,  or  glare  by  re- 
flection from  the  paper,  is  almost  sure  to  be  experienced  by  other 


70  ILLUMINATING   ENGINEERING    PRACTICE 

workers  in  the  room.  With  indirect  lighting  for  general  office  work 
a  slight  amount  of  veiling  glare  consisting  of  reflection  of  the  ceiling 
from  the  paper  is  received  in  many  working  positions  but  this  is  not 
so  serious  as  the  glare  with  the  other  arrangements  described. 

Complaint  is  sometimes  made  that  daylight  and  artificial  light  do 
not  mix  well  in  color  or  direction  and  that  there  is  a  period  at  dusk 
when  there  is  likely  to  be  trouble  with  an  artificial  lighting  arrange- 
ment that  is  satisfactory  after  dark.  This  trouble  is  usually  due 
simply  to  insufficient  artificial  light  for  the  best  work,  but  is  some- 
times further  aggravated  by  the  presence  of  sky  areas  visible  to  the 
worker  but  shaded  from  the  work.  In  the  latter  case  the  eye  is 
adapted  to  the  sky  brightness  rather  than  the  desk  top  brightness. 

As  already  seen  most  of  the  available  modern  light  sources  are 
very  bright.  In  order  to  conform  to  the  hygienic  requirements,  if 
the  reflectors  or  shades  used  are  not  opaque,  they  must  at  least  be 
dense.  Semi-direct  lighting  usually  requires  a  bowl  which  is  rather 
thick,  not  only  to  withstand  the  mechanical  strain  but  to  give  a 
sufficient  thickness  of  glass  to  cut  down  the  brightness.  Various 
glass  mixtures  have  been  compounded  for  such  bowls.  Some  of  the 
blown  glass  bowls  for  this  purpose  consist  of  two  or  three  layers, 
forming  what  is  technically  known  as  a  cased  glass.  Specific  limita- 
tions for  bowl  brightness  have  already  been  noted. 

From  the  efficiency  standpoint  the  prime  requisite  for  a  semi- 
direct  lighting  bowl  is  a  pure  white  highly  polished  interior  surface 
which  will  give  a  high  percentage  of  reflection  from  its  surface  and 
a  sufficiently  dense  glass  medium  so  that  the  light  that  is  not  re- 
flected shall  be  considerably  reduced  in  brightness. 

In  the  manufacture  of  heavy  diffusing  glasses  of  this  kind  there  is 
much  opportunity  for  development  of  pleasing  artistic  effect  by  the 
use  of  tints  and  coloring.  To  most  people  yellowish  tints  are  more 
pleasing  than  those  of  blue  or  green. 

The  eyebrows  of  the  average  person  shade  the  eyes  from  rays 
falling  as  near  perpendicular  as  25  degrees  from  the  vertical  or  less, 
but  for  rays  emanating  from  light  sources  above  this  angle  artificial 
shading  must  be  provided  if  the  lamp  is  overhead.  If  the  edge  of 
the  lamp  shade  is  below  or  near  the  level  of  the  eye  any  kind  of  shade 
which  will  intercept  all  rays  above  the  horizontal  will  protect  the  eye. 

With  daylight  illumination  it  is  common  for  window  curtains  and 
draperies  to  cut  off  about  50  per  cent,  of  the  total  light  and  for  the 
roller  shade  to  be  left  where  it  will  cut  off  from  30  per  cent,  to  40 
per  cent,  of  the  remainder.  Large  effective  window  areas  in  pro- 


PRINCIPLES    OF    INTERIOR   ILLUMINATION  71 

portion  to  the  size  of  the  room  are  conducive  to  the  most  hygienic 
daylight  conditions.  Dark  curtains  or  draperies  around  the  edges 
of  a  window  tend  to  increase  the  contrast  glare  effect.  The  bright- 
ness of  the  sky  seen  through  the  central  part  of  the  window  is  not 
changed  by  such  draperies  and  the  total  illumination  in  the  room 
is  materially  changed  so  that  the  contrast  between  the  sky  and  the 
interior  surface  of  the  room  is  increased.  Practices  of  this  kind 
should  be  discouraged  for  hygienic  reasons.  For  similar  reasons  large 
window  spaces  are  desirable.  Legislation  for  schoolroom  construc- 
tion frequently  names  a  window  area  of  from  %  to  %  of  the  floor 
area. 

In  selecting  a  window  shade  it  is  well  to  consider  the  purpose  for 
which  the  shade  is  most  likely  to  be  used.  A  dark  dense  shade  is 
frequently  objectionable  for  shutting  out  sunlight  in  an  office  build- 
ing, factory  or  schoolroom  because  it  shuts  out  altogether  too  much 
light.  If  a  very  dark  shade  is  used  to  shut  out  sunlight,  a  small 
area  of  brightly  illuminated  space  is  left  near  the  window  while  the 
rest  of  the  room  is  in  strong  contrast  to  this  bright  space  and  the 
effect  is  to  introduce  contrast  glare  and  make  the  illumination  of 
the  room  seem  insufficient.  Moreover,  such  a  preponderance  of 
brightness  below  the  eye  level  is  unnatural,  as  before  explained,  and 
will  of  itself  cause  discomfort  if  sufficiently  pronounced.  If  on  the 
other  hand,  use  is  made  of  light  colored  window  shades  which  allow 
considerable  diffused  light  to  pass  through,  the  illumination  sent 
back  into  the  room  is  not  so  seriously  interfered  with  when  they  are 
pulled  down  and  the  contrasts  of  brightness  within  the  room  are 
not  so  great  and  the  whole  effect  is  more  comfortable  and  hygienic. 

Having  selected  a  general  type  of  lighting  source  to  be  employed 
and  the  lamp  equipment  to  be  installed  the  next  step  is  the  selection 
of  the  exact  size  and  location  of  the  lighting  units.  Two  general 
characters  of  problems  are  presented  in  practice.  One  of  these  is 
where  the  general  illumination  is  desired  within  minimum  and  maxi- 
mum limits  and  the  other  is  where  the  principal  consideration  is  a 
local  illumination  of  a  certain  intensity  without  much  regard  to  the 
quantity  of  illumination  elsewhere  in  the  room. 

In  problems  of  the  latter  class  where  the  illumination  at  some 
particular  point  is  the  main  thing  desired,  the  point-by  point  method 
of  calculation  has  its  advantages.  If  it  be  assumed  that  a  certain 
number  of  foot-candles  illumination  is  required  at  a  certain  point 
this  illumination  multiplied  by  the  square  of  the  distance  in  feet  will 
give  the  candle-power  which  must  be  emitted  from  the  unit  in  that 


72  ILLUMINATING   ENGINEERING    PRACTICE 

direction.  The  general  type  of  unit  and  its  shading  equipment  hav- 
ing been  already  selected  it  then  becomes  a  matter  of  determining 
what  size  of  lamp  will  most  nearly  give  the  candle-power  required 
at  that  particular  angle.  This  is  done  from  photometric  curves 
of  the  lamp  equipment.  If  curves  are  not  available  for  all  sizes  of 
lamps  that  can  usually  be  calculated  with  sufficient  approximation 
from  curves  made  with  one  size  of  lamp. 

Most  of  the  problems  however  are  those  requiring  a  certain  aver- 
age general  illumination.  The  selection  of  such  an  average  however 
always  implies  that  the  minimum  in  the  working  area  shall  not 
fall  too  far  below  the  average.  In  modern  practice  it  is  comparatively 
easy  to  keep  this  minimum  within  25  per  cent,  of  the  average  with 
proper  design.  Having  assumed  the  average  illumination  required  and 
assuming  also  that  the  spacing  to  be  selected  will  be  such  as  to  give 
a  reasonable  degree  of  uniformity  the  next  step  is  the  calculation  of 
the  total  light  flux  required  to  be  generated  by  the  lamp.  Using 
the  empirical  method.  This  is  obtained  by  the  simple  formula 


Where  i  equals  foot-candles  average  illumination  upon  the 
working  plane. 

a  equals  area  of  the  working  plane  in  square  feet. 

e  equals  the  efficiency  or  utilization  factor  or  percentage  of  lumens 
generated  which  become  effective  upon  the  working  plane  with  the 
lamp  equipment  and  room  conditions  under  consideration. 

L  equals  the  total  lumens  to  be  generated  by  the  lamp. 

In  the  foregoing  formula,  ia,  of  course,  equals  the  total  lumens  ef- 
fective upon  the  working  plane. 

In  applying  the  foregoing  formula  of  course  the  important  thing 
is  to  select  the  proper  value  for  e,  the  efficiency  or  utilization  factor. 
This  can  best  be  done  by  consulting  the  various  tables  and  curves 
of  utilization  factors,  Tables  VI  and  VII  and  Figs.  3  to  9  or  any  other 
good  authority  and  selecting  conditions  which  most  nearly  corre- 
spond with  those  in  the  room  under  calculation.  In  applying 
these  factors  they  should  be  reduced  by  the  amount  corresponding 
to  the  depreciation  due  to  dirt  and  age  of  lamp.  Such  depreciation 
figures  for  various  conditions  have  already  been  noted. 

If  the  value  of  e  is  not  obtainable  from  experience  and  use  is 
to  be  made  of  opaque  direct  reflectors,  e  can  be  determined  for  most 
large  interiors  from  the  distribution  curve  of  the  lamp  and  reflector 


PRINCIPLES    OF   INTERIOR   ILLUMINATION  73 

by  dividing  the  total  lumens  emitted  by  the  lamp  by  the  lumens 
emitted  in  the  zone  from  o  to  70  degrees.  For  smaller  rooms  a 
smaller  zone  should  be  used. 

Having  determined  the  total  lumens  required  to  be  generated  by 
the  lamp  by  the  foregoing  formula  there  remains  the  determination 
and  decision  as  to  how  this  total  flux  is  to  be  divided,  or  in  other 
words  the  sizes  of  the  lamps  and  their  locations. 

In  most  cases  there  are  certain  natural  divisions  of  the  rooms  by 
ceiling  panels  or  other  architectural  features  so  that  it  is  necessary 
in  the  interest  of  good  appearance  to  make  the  lighting  outlets 
symmetrical  with  reference  to  these  panels.  The  ideal  condition 
to  be  sought  after  is  to  divide  the  ceiling  into  a  number  of  squares 
with  an  outlet  at  the  center  of  each  square.  Frequently  it  is  not 
possible  to  do  this,  but  it  is  well  to  maintain  the  divisions  as  nearly 
squares  as  possible.  In  other  words  if  an  oblong  division  is  necessary 
long  and  narrow  rectangles  should  be  avoided. 

Height. — To  secure  proper  uniformity  either  with  indirect  light 
or  with  direct  lighting  reflectors  giving  the  most  extensive  type  of 
distribution  the  height  of  the  sources  of  light  should  not  be  less  than 
half  their  distance  apart,  taking  the  height  of  the  sources  of  light  as  the 
height  of  the  ceiling  in  the  case  of  indirect  lighting  and  as  that  of  the 
lamp  in  the  case  of  direct  light.  Spacing  at  shorter  intervals  than  the 
maximum  permissible  is  desirable  both  in  the  case  of  direct  and  in- 
direct lighting  in  order  to  secure  greater  uniformity,  freedom  from 
annoying  shadows,  and  a  reduction  in  the  amount  of  specular 
reflection  or  veiling  glare  from  papers  and  polished  metals.  Shorter 
spacing  is  imperative  if  concentrating  direct  reflectors  are  used. 

When  the  spacing  has  been  determined  in  a  way  which  will 
fit  in  symmetrically  with  the  architecture  and  at  the  same  time  an- 
swer the  uniformity  requirements,  the  number  of  outlets  is  ascer- 
tained and  this  number,  divided  into  the  total  lumens  to  be  generated 
by  the  lamp,  gives  the  lumens  per  lamp.  From  the  proper  up-to- 
date  manufacturer 's  information  the  lamp  size  most  nearly  answer- 
ing the  requirements  must  be  selected. 

Indirect  fixtures  should  be  hung  a  sufficient  distance  from  the 
celling  to  avoid  a  very  spotted  lighting  effect.  The  nearer  to  the 
ceiling  they  hang  the  greater  the  concentration  of  light  under  the 
fixture. 

EXAMPLES  OF  THE  PROCESS  OF  DESIGN 

The  following  typical  examples  on  the  process  of  design  are  given 
to  illustrate  the  principles  that  have  been  laid  down. 


74  ILLUMINATING   ENGINEERING   PRACTICE 

Example  i. — A  large  room  area  100  by  100  feet  with  14.5  foot  ceil- 
ing used  for  general  office  purposes  and  clerical  work,  having  light 
colored  walls  and  ceilings.  The  entire  area  is  covered  by  desks  and 
filing  cases.  Since  practically  the  entire  room  has  to  be  illuminated 
sufficiently  for  working  purposes,  localized  lighting  is  not  to  be 
considered  except  possibly  for  a  few  billing  machines  having  lamps 
on  portions  of  the  machine  that  might  be  in  shadow.  In  order  to 
avoid  glare  the  system  must  be  indirect  or  nearly  so,  so  that  the  semi- 
indirect  with  very  dense  bowls  will  be  selected,  as  the  office  is  of  a 
prominent  concern  where  the  decorative  effect  of  the  illuminated 
bowls  is  desirable.  As  it  is  necessary  to  seek  first  the  highest  effi- 
ciency of  the  employees  (as  saving  in  the  consumption  of  energy  for 
lighting  would  be  a  very  small  percentage  of  the  amount  spent  for 
pay-roll)  the  lighting  intensity  should  be  such  as  to  be  beyond 
criticism  or  question  as  to  sufficiency.  An  average  illumination  of 
6  foot-candles  will,  therefore,  be  selected  with  the  understanding  that 
the  minimum  is  not  to  fall  below  4.5. 

From  the  utilization  factor  table  we  see  that  a  large  interior  of  this 
kind  has  a  utilization  factor  of  about  48  per  cent,  before  allow- 
ing for  depreciation  and  dirt.  We  will  allow  15  per  cent,  deprecia- 
tion by  dirt  on  electric  lamps  and  reflectors,  and  assume  that  the 
system  of  cleaning  and  maintenance  will  be  such  that  this  will  be 
a  maximum  figure.  We  will  also  allow  10  per  cent,  depreciation 
for  falling  off  in  luminous  output  of  the  lamp.  This  gives  a  total 
figure  of  25  per  cent,  to  be  allowed  for  dirt  and  depreciation  in 
service,  so  that  our  48  per  cent,  utilization  factor  is  reduced  to  36 
per  cent. 

The  room  having  a  floor  area  of  10,000  square  feet,  multiplying 
this  by  6  foot-candles  average  illumination  gives  60,000  lumens  re- 
quired on  the  working  plane.  60,000  lumens  divided  by  36  per 
cent,  gives  166,600  lumens  to  be  generated  at  the  lamps.  Taking 
up  the  spacing  of  the  lamps  we  find  the  room  divided  into  bays 
20  X  20  feet  and  as  those  bays  are  not  too  large  to  give  good  uni- 
formity with  an  outlet  in  the  middle  of  each  bay  with  this  ceiling 
height  we  will  put  an  outlet  in  the  middle  of  each  bay.  With  this 
division  25  outlets  will  be  required.  The  total  166,600  lumens  at 
the  lamps  divided  among  25  outlets  equals  6660  lumens  per  lamp. 
The  nearest  sizes  to  this  in  electric  lamps  are  the  400- watt  6150 
lumen  lamp  and  the  5oo-watt  8050  lumen  lamp.  In  gas  lamps  an 
inherent  depreciation  figure  of  20  per  cent,  more  than  the  electric 
had  probably  better  be  assumed.  An  output  of  325  lumens  per 


PRINCIPLES    OF   INTERIOR   ILLUMINATION  75 

cubic  foot  per  hour  less  20  per  cent,  equals  260  lumens.  Twelve 
inverted  mantles  taking  2.5  cu.  ft.  of  gas  each  per  hour  would  then 
give  7800  lumens. 

The  size  of  lamps  having  been  determined,  the  bowl  can  be  selected 
for  the  semi-direct  lighting  fixture  of  a  glass  having  a  density  prefer- 
ably such  that  the  bowl  brightness  will  not  be  over  300  millilamberts, 
as  that  will  not  be  over  100  times  as  bright  as  of  the  3  millilamberts 
on  the  wall  illuminated  to  about  6  foot-candles.  The  brightness  of 
a  wall  in  millilamberts  equals  the  incident  foot-candles  times  1.07 
times  the  coefficient  of  reflection  of  the  wall. 

Example  2. — A  small  office  room  10  feet  wide  and  10.5  ft.  high  by 
20  feet  deep  with  light  ceilings  and  walls,  typical  of  thousand  of 
rooms  in  large  office  buildings.  The  character  of  the  occupancy 
cannot  be  predicted  but  the  usual  arrangement  is  desks  near  the 
window  facing  each  side-wall.  These  desks  maybe  either  flat  or  roll 
top.  Another  possible  arrangement  is  to  place  the  desks  so  that  the 
back  of  the  worker  is  to  the  window.  There  would  also  probably  be 
a  typewriter  desk  farther  back  in  the  room,  usually  along  one  of 
the  walls.  The  building  is  to  be  provided  with  electricity  only  for 
lighting.  The  two  plans  for  artificial  lighting  for  such  an  office  which 
must  naturally  receive  consideration  are  the  following: 

A,  General  lighting,  supplemented  by  local  desk  lighting.  B, 
General  lighting  for  all  purposes  without  localized  lamps.  The 
economy  of  modern  lamps  has  done  away  with  much  of  the  necessity 
of  using  localized  lighting  for  the  sake  of  economy  as  formerly. 
For  most  office  work  localized  lighting  is  not  as  satisfactory  as  general 
lighting,  because  of  the  veiling  glare  from  papers,  etc.  However,  if 
general  lighting  is  depended  upon  alone  use  must  be  made  of  a  system 
which  will  not  cause  annoyance  from  shadows. 

If  this  is  in  a  typical  modern  office  building  it  is  desirable  to  have 
as  few  outlets  as  possible  on  partitions  as  the  occupancy  and  loca- 
tion of  partitions  may  change.  If  general  lighting  is  to  be  ac- 
complished from  ceiling  fixtures  centrally  located  a  system  indirect 
or  nearly  so  will  provide  for  most  contingencies  in  variation  of  desk 
location,  etc.,  and  if  the  desks  are  located  facing  each  wall  the 
shadows  of  heads  will  cause  the  least  annoyance.  On  account  of  the 
importance  of  reducing  the  shadows  to  their  lowest  terms  an  in- 
direct system  will  be  selected,  rather  than  semi-direct.  The  office 
can  conveniently  be  assumed  as  divided  into  squares  each  10  by  10 
feet  and  an  outlet  located  in  the  center  of  each  square.  This  arrange- 
ment provides  for  ample  illumination  of  the  rear  of  the  room 


76  ILLUMINATING   ENGINEERING   PRACTICE 

farthest  from  the  windows.  On  account  of  the  possibility  of  shadows 
and  veiling  glare  being  more  annoying  with  only  two  sources  of 
light  and  with  the  possibilities  of  workers  being  seated  with  their 
backs  to  the  illuminated  ceiling  so  as  to  cause  maximum  shadows, 
more  light  should  be  provided  at  the  lamp  per  square  foot  of  floor 
area  than  in  the  case  of  the  general  office  in  Example  i.  How- 
ever, if  there  were  only  one  desk  in  the  room  and  that  located 
directly  under  a  lighting  unit  the  reverse  would  be  true  and  less  light 
would  have  to  be  provided,  because  the  maximum  light  would  be 
received  directly  under  the  outlet. 

In  this  case,  therefore,  we  will  allow  for  an  average  illumination  of 
7  foot-candles  which  may  fall  to  4  or  5  foot  candles  along  the  walls 
in  shadows.  Seven  foot-candles  times  200  square  feet  equals  1400 
total  lumens  to  be  generated  and  delivered  upon  the  working  plane. 
The  efficiency  of  utilization  in  such  a  room  will  probably  be  around 
29  per  cent.,  which,  when  reduced  by  25  per  cent,  for  dirt  and  lamp 
depreciation  as  in  Example  i,  would  mean  a  factor  of  22.5  per  cent. 
The  1400  lumens  needed  divided  by  the  22.7  per  cent,  equals  6600 
lumens  to  be  generated  at  two  outlets  or  3300  lumens  per  outlet. 
The  nearest  single  lamp  to  this  in  output  is  the  28oo-lumen,  200- 
watt  lamp.  Since  we  have  been  rather  liberal  in  our  allowances  as 
to  the  foot-candles  required  at  the  start  the  use  of  this  lamp  would  be 
permissible. 

If  a  room  of  this  type  were  a  little  wider  it  would  be  best  to  have 
two  rows  of  fixtures  in  spite  of  the  spacing  rule  given  because  of  the 
desirability  of  minimizing  the  shadows  at  the  desks  near  the  walls. 

Example  3. — If  the  general  office  of  Example  i  were  an  industrial 
plant  having  the  same  dimensions  but  with  a  darker,  more  broken 
up  ceiling  the  method  of  treatment  would  be  the  same  except  that 
deep  bowl  opal  reflectors  or  opaque  shallow  or  deep  bowl  reflectors 
might  be  used.  The  utilization  factor  would  be  changed  and  per- 
haps more  allowance  should  be  made  for  dirt. 

Working  Out  Cost  Comparisons. — In  making  comparison  of  oper- 
ating and  maintainance  cost  for  different  illuminants  or  systems  of 
lighting  the  following  items  should  enter  for  any  given  period. 

(a)  Cost  of  energy  or  fuel  (electricity,  gas,  oil,  etc.). 

(b)  Renewals  of  lamps,  lamp  parts  or  burners  (mantles,  lamps, 
trimmings,  etc.). 

(c)  Cost  of  cleaning  lamps  and  accessories. 

(d)  Cost  of  cleaning  or  redecorating  walls  and  ceilings. 

(e)  Interest  and  depreciation  on  cost  of  system  in  building. 


PRINCIPLES  OF  EXTERIOR  ILLUMINATION 

DR.    LOUIS   BELL 

Exterior  illumination  is,  speaking  broadly,  the  generalized  case 
of  application  of  artificial  light.  In  interior  lighting,  that  applied, 
for  example,  within  the  limitations  of  a  room,  the  light  flux  is  con- 
fined within  the  bounding  surfaces  where  it  is  subject  to  reflection 
and  absorption,  the  amount  of  which  has  to  be  taken  into  rigorous 
account  in  reckoning  the  final  result  in  lumens  available  for  service. 
There  are  no  such  restrictions  necessary  in  exterior  lighting  since  its 
problems  have  to  be  dealt  with  chiefly  in  terms  of  the  radient  un- 
limited by  artificial  boundaries.  In  general  the  case  is  that  of  a 
luminous  source  required  to  produce  a  certain  flux  density  on  a 
single  arbitrary  plane  which  may  be  horizontal,  as  in  the  case  of 
street  lighting,  or  vertical,  as  when  one  illuminates  the  facade  of 
a  building.  In  rare  instances  one  deals  with  both  a  horizontal  and 
one  or  more  vertical  planes  simultaneously,  as  when  light  is  directed 
into  a  street  or  public  square  of  limited  extent,  but  there  is  always 
one  general  direction  and  more  commonly  several  in  which  no  limiting 
surfaces  are  interposed  and  the  solid  angles  pertaining  to  which 
must  be  regarded  as  representing  regions  of  complete  absorption. 

The  use  of  reflectors  with  exterior  illuminants  is  merely  an  effort 
to  limit  this  absorption  angle  by  the  partial  interposition  of  a  reflect- 
ing surface  effective  roughly  in  proportion  to  the  solid  angle  which 
it  subtends  from  the  source.  On  this  point  of  view  it  is  immediately 
evident  why  in  employing  such  reflectors  their  equivalent  solid  angle 
is  a  matter  of  great  importance,  so  that  it  frequently  happens  that 
the  last  few  inches  of  radius  on  a  reflector  determine  whether  it  is 
to  be  good  or  bad  in  redirecting  the  light.  Further,  whether  one  or 
more  bounding  surfaces  must  be  taken  into  account  in  planning  for 
exterior  illumination,  the  effect  on  the  conditions  of  illumination  is 
altogether  different  from  that  found  in  interior  lighting.  Here  the 
surfaces  are  frequently  fairly  light  so  that  they  present  low  coefficients 
of  absorption.  One  surface,  the  ceiling,  is  almost  always  light  and 
one,  the  floor,  is  generally  dark,  but  no  darker,  however,  than  the 
ground  which  serves  as  the  working  plane  in  exterior  lighting.  In 
this  latter  case,  one  works  under  serious  disadvantages  in  the  appli- 

77 


78  ILLUMINATING   ENGINEERING   PRACTICE 

cation  of  the  light,  since  the  surface  to  be  illuminated  is  usually 
rather  dark  with  high  absorption.  The  remainder  of  the  surfaces 
toward  which  light  flux  is  directed  are  practically  also  of  high  absorp- 
tion, save  in  exceptional  cases,  so  that  one  cannot  depend  in  exterior 
lighting  upon  that  measure  of  assistance  often  equivalent  to  an  in- 
crease of  from  50  to  100  per  cent,  in  the  effective  flux. 

One  is  generally  dealing  out  of  doors  with  directed  light  flux  from 
the  radient  somewhat  modified  by  the  shades  or  reflectors  that  may 
be  applied  thereto,  and  as  a  rule  only  one  or  a  few  such  radients  have 
to  be  considered.  Hence  the  numerical  computations  in  the  case 
of  exterior  lighting  are  fairly  simple,  and  the  working  out  of  exterior 
problems  is  rendered  fairly  easy  by  the  fact  that  the  intensity  of 
illumination  demanded  is  generally  less  than  with  interior  lighting 
and  the  conditions  with  respect  to  uniformity  are  also  considerably 
less  severe.  Within  doors  the  illumination  demanded  is  determined 
by  the  things  which  have  to  be  done  by  its  aid  and  some  of  these  are 
tasks  which  require  close  vision  on  unfavorable  details,  so  that  com- 
mon intensities  of  illumination  run  all  the  way  from  10  to  50  lux 
(i  to  5  foot-candles),  and  in  rare  instances  much  higher.  In  exterior 
lighting,  save  for  deliberately  scenic  purposes,  10  lux  is  rarely  ex- 
ceeded and  the  usual  standard  intensities  run  from  about  0.5  to 
perhaps  5  lux  (0.05  to  0.5  foot-candle).  Broadly,  in  exterior  lighting 
the  conditions  of  distribution  are  less  favorable  than  in  interior 
lighting,  but  the  requirements  of  intensity  and  uniformity  are  much 
less  severe. 

The  amount  of  illumination  required  in  exterior  work  depends  on 
its  use,  but  this  is  never  such  as  to  call  for  illumination  good  enough 
to  facilitate  the  observation  of  fine  detail.  At  most  one  may  have 
to  read  a  program  or  an  address  card.  Ordinarily  it  is  sufficient  to 
distinguish  people  and  vehicles  easily,  to  note  obstructions  on  the 
roadway  or  sidewalk,  to  recognize  persons  and  things  at  a  moderate 
distance,  and  perform  other  simple  tasks  requiring  no  close  discrimi- 
nation. One  recognizes  objects  on  road  or  sidewalk  chiefly  by  their 
shadows.  If  their  color  tone  be  nearly  that  of  the  road  surface  they 
are  almost  invisible,  except  when  so  illuminated  as  to  show  a  shadow. 
One  also  sees  at  night  the  contrast  of  light  and  dark  masses,  like 
the  silhouette  of  a  cart  against  an  illuminated  roadway,  or  of  a 
white-clad  person  against  a  hedge  or  fence.  The  eye,  therefore,  is 
not  called  upon  to  do  any  fine  work  and  hence  does  not  require  a 
degree  of  illumination  sufficient  greatly  to  develop  its  full  discrimina- 
tory powers. 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          79 

Only  in  such  exterior  work  as  has  to  do  with  the  deliberate  illumi- 
nation of  particular  objects,  as  in  some  spectacular  •  lighting,  is  it 
necessary  to  push  the  intensity  near  to  the  point  common  in  interior 
lighting.  This  is  fortunate  since  with  immense  spaces  to  light  and 
unfavorable  conditions  as  regards  reflecting  surfaces  exterior  lighting 
is  only  economically  possible  in  virtue  of  the  modest  necessities  of 
the  case.  Luckily  the  human  eye  works  about  equally  well  for  the 
purpose  of  seeing  over  a  very  wide  range  of  illumination.  From  the 
full  sun  shine  of  noon  to  twilight,  the  illumination  may  vary  in  the 
ratio  of  1000  :  i  and  yet  the  eye  can  do  most  of  its  work  comfortably 
at  either  extreme.  It  is  not  the  absolute  amount  of  light  which 
counts,  but  the  relative  amount  as  between  two  things  to  be  dis- 
criminated. Speaking  in  general  terms  one  can  distinguish  as 
varying  in  shade  two  adjacent  surfaces,  the  illumination  of  which 
varies  by  a  little  less  than  i  per  cent.,  whether  the  actual  intensity  of 
the  lighting  be  of  the  order  of  magnitude  of  10  or  1000  lux.  A  con- 
trast of  10  per  cent,  is  conspicuous  even  when  the  illumination  falls 
much  below  10  lux.  The  power  of  the  eye  to  discriminate  both 
shades  and  small  details  even  in  black  and  white  falls  off  rapidly 
under  ordinary  visual  conditions,  so  that  at  a  few  tenths  lux  (or 
hundredths  of  a  foot-candle)  even  a  contrast  of  25  or  30  per  cent, 
between  surface  and  surface  may  not  be  easily  visible  unless  the 
surfaces  are  on  a  very  large  scale,  and  one  fails  to  read  even  very 
coarse  type.  In  such  lighting  obstacles  are  difficult  to  see,  persons 
difficult  to  recognize  and,  while  one  can  still  see  to  move  about,  the 
conditions  are  bad  if  any  traffic  is  to  be  considered.  Such  is  the 
situation  even  in  pretty  good  moonlight  which  may  run  to  say  from 
o.i  to  0.25  lux  (o.oi  to  0.025  foot-candle). 

One  of  the  many  valuable  properties  of  the  eye  is  that  it  possesses, 
however,  an  extraordinary  power  of  adaptation,  that  is,  of  getting 
used  to  great  variation  in  the  intensity  of  the  lighting  and  still 
being  able  to  see  fairly  well.  It  may  be  light-adapted,  as  when  the 
pupil  shrinks  to  its  minimum  diameter,  and  the  eye  adjusts  itself 
to  a  very  bright  light,  or  it  may  be  dark-adapted,  when  the  pupil 
opens  very  wide  and  the  retina  itself  becomes  adjusted  to  condi- 
tions of  very  low  illumination.  This  latter  process  is  largely  a 
physiological  one  which  requires  some  little  time  to  accomplish, 
but  it  is  tremendously  efficient.  After  ten  or  fifteen  minutes  in 
complete  darkness,  for  instance,  the  eye  is  many  hundred  times 
more  sensitive  to  faint  illumination  than  in  its  light-adapted  condi- 
tion, and  as  a  matter  of  fact  with  this  long  dark  adaptation  the  same 


80  ILLUMINATING   ENGINEERING    PRACTICE 

keenness  of  discrimination  which  ordinarily  exists  at  10  to  50  lux 
may  be  found  even  at  a  hundredth  of  this  amount.  This  is  par- 
ticularly true  as  regards  vision  of  surfaces  differing  slightly  in  illu- 
mination, less  true  for  the  observation  of  detail  like  printing. 

It  is  for  this  reason  that  one  can  see  much  better  by  moonlight,  to 
which  one  gradually  gets  adapted  in  the  absence  of  other  illumina- 
tion than  is  possible  with  artificial  lighting  where  one  continually 
comes  under  the  more  intense  illumination  near  lamps.  The  power 
of  adaptation  of  the  eye,  therefore,  rises  to  considerable  practical 
importance  in  external  lighting.  If  dark  adaptation  is  not  spoiled 
by  glaring  sources  of  light  one  can  see  astonishingly  well  at  low 
illumination.  Hence  under  lighting  conditions  where  one  has  to 
work  with  a  meager  amount  of  light,  a  source  which  would  be 
entirely  unobjectionable  in  the  case  of  the  brilliant  illumination 
found,  for  instance,  in  a  public  square,  becomes  unpleasantly  glaring 
and  unfits  the  eye  for  good  vision.  This  is  what  happens  when  one 
drives  an  automobile  under  a  low  hung  and  brilliant  street  lamp. 
The  vision  must  again  adjust  itself  to  the  less  brilliantly  illuminated 
regions,  only  to  get  another  rebuff  from  the  next  lamp. 

This  would  look  as  though  uniformity  in  lighting  roads  and  other 
large  areas  might  be  very  important.  Its  value  is  lessened  by  the 
fact  already  referred  to,  that  is,  that  we  see  objects,  generally,  in  a 
moderate  illumination,  chiefly  through  their  shadows.  A  perfectly 
uniform  low  illumination,  could  it  be  attained  conveniently,  would 
be  good  from  the  standpoint  of  adaptation  and  bad  from  lack  of 
contrast  due  to  shadows.  The  contrast  directly  under  a  strong  light 
source  may  be  actually  much  less  than  in  a  faint  light  directed  cross- 
wise so  that  the  visible  contrast  is  not  between  the  object  itself  and 
its  surroundings,  but  between  its  shadow  or  shadowed  parts  and 
the  surroundings.  These  facts  were  very  beautifully  brought  out 
in  experiments  tried  a  couple  of  years  ago  for  the  National  Electric 
Light  Association. 

For  most  purposes  of  exterior  lighting  the  best  results  are  obtained 
by  lamps  rather  well  shaded,  so  as  to  reduce  the  intrinsic  brilliancy 
of  fairly  good  power,  and  so  located  as  to  produce  only  a  moderate 
amount  of  uniformity  in  the  resulting  illumination.  In  situations 
where  the  intensity  for  one  reason  or  another  must  be  considerably 
increased,  the  value  of  directed  light  as  against  flat  uniformity  is 
very  considerable.  The  front  of  a  building,  for  example,  can  be 
flattened  distressingly  by  too  uniform  lighting  or  brought  out  with 
brilliant  effect  by  a  little  judicious  cross-illumination,  a  condition 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          81 

precisely  analogous  to  that  found  in  the  interior  lighting,  for  instance, 
of  a  church,  in  which  the  high  altar  requires  oblique  illumination  to 
bring  out  its  relief.  The  same  practical  application  of  contrast 
appears  in  that  class  of  exterior  illumination  which  has  to  do  with 
decorative  and  spectacular  illumination  of  places  or  things. 

Now  and  then  most  remarkable  effects  can  be  produced  by  close 
attention  to  regulating  the  quantity,  quality  and  direction  of  the 
light  applied.  This  is  on  a  large  scale  exactly  what  is  done  in  the 
setting  of  theatrical  scenes  where  effects  of  spaciousness  or  of  dis- 
tance are  produced  upon  the  very  limited  area  of  a  stage.1  Brilliant 
and  uniform  illumination  tends  to  give  an  effect  of  nearness  and 
lack  of  relief.  Faint  and  carefully  directed  lighting  on  the  contrary 
may  be  made  to  produce  an  effect  of  vague  distance.  When  many 
light  sources  are  in  view  a  decreasing  spacing  gives  a  spurious  effect 
of  distance,  uniform  or  increasing  spacing  from  the  foreground  back, 
the  reverse  effect.  Lamps  of  decreasing  brilliancy  along  the  line 
of  view  likewise  produce  an  impression  of  far  perspective,  while 
increasing  brilliancy  gives  an  illusion  of  nearness.  The  cases  where 
these  principles  need  to  be  applied  are  not  common  enough  to  justify 
going  into  the  matter  to  any  great  extent,  but  most  astonishing 
results  can  be  reached  in  the  way  of  forced  perspective  wherever 
scenic  effect  is  desirable. 

The  problems  encountered  in  exterior  lighting  are  of  a  very  diverse 
character  involving  many  different  sets  of  conditions,  each  of  which 
must  be  met  in  a  systematic  and  definite  way.  One  can  divide  the 
total  roughly  so  that  each  group  possesses  somewhat  similar  char- 
acteristics, for  instance,  perhaps  the  simplest  case  of  exterior  light- 
ing is  that  of  a  public  square  which  presents  a  somewhat  close 
analog  to  certain  types  of  interior  lighting.  Here,  as  a  rule,  from 
the  nature  of  the  surroundings  and  the  density  of  the  traffic,  the 
illumination  has  to  be  considerably  higher  than  usual,  rising  even 
to  10  or  20  lux  (one  or  several  foot-candles)  and  averaging  there- 
fore almost  as  high  as  certain  interiors.  A  square  roughly  approxi- 
mates a  large  and  not  very  high  interior  having  a  very  dark  ceiling 
and  side  walls  of  rough  texture  and  very  varied  reflecting  power. 
For  all  practical  purposes  the  sky  above  is  almost  completely  absorb- 
ing, while  the  entering  streets  take  the  place  of  a  few  great  windows, 
from  which  practically  no  light  is  reflected,  but  which  may  receive 
a  little  from  the  outside,  that  is,  from  down  the  street.  If  such  a 
square  is  illuminated  from  sources  provided  with  over  head  reflectors 
having  angles  wide  enough  to  intercept  rays  which  pass  above  the 


82  ILLUMINATING   ENGINEERING   PRACTICE 

house  tops,  the  full  downward  flux  from  all  the  lamps  may  be 
considered  as  concentrated  on  the  walls  and  floor.  Absence  of 
ceiling  reflection  somewhat  diminishes  the  amount  of  aid  given  to 
the  general  illumination  by  secondary  reflections.  If  then,  we  know 
the  efficiency  of  the  reflector  system  which  keeps  the  light  from  going 
skyward  and  therefore  the  total  downward  flux  of  the  lamps,  the 
illumination  on  the  working  plane  can  be  reckoned  practically  as  in 
a  case  of  interior  lighting.  In  the  latter  case  suitable  reflecting 
systems  will  turn  quite  half  the  total  light  flux  from  the  sources  upon 
the  working  plane,  from  which,  knowing  the  area,  the  average 
illumination  can  be  found  at  once  by  a  process  which  will  be  out- 
lined later.  The  uniformity  of  the  lighting  will  be  determined 
by  the  number  and  place  of  individual  radiants  and  the  light  curve 
derived  from  each.  A  pure  flux  method  leads  to  the  general  average 
illumination,  a  point-by-point  method  to  the  maximum  and 
minimum. 

Second  on  the  list  comes  street  lighting,  so  important  that  it 
will  be  dealt  with  in  this  course  by  special  lectures.  Here  the 
interior  analog  would  be  a  very  long  hall  with  a  black  ceiling  and 
it  is  usually  necessary  to  determine  the  illumination  by  consider- 
ing the  effect  of  individual  radiants,  since  they  are  seldom  close 
enough  together  to  require  the  addition  of  the  luminous  effects  from 
more  than  a  very  few  lamps.  As  a  rule  the  average  lighting  of  a 
street,  except  in  the  case  of  one  carrying  very  heavy  traffic,  does  not 
require  the  intensity  desirable  in  public  squares.  Indeed  the  neces- 
sary illumination  in  certain  classes  of  streets  may  fall  to  a  point 
where  the  lamps  are  little  more  than  markers  of  the  way.  Only 
in  densely  built  regions  can  any  gain  be  counted  upon  from  reflec- 
tion from  sides  of  buildings  which,  however,  it  is  sometimes  desirable 
to  light  rather  brightly  for  the  general  effect.  As  the  lamps  are 
usually  placed  considerably  lower  than  the  buildings  the  solid  or 
spherical  angle  subtended  by  the  reflector,  if  there  is  one,  may  be 
considerably  less  than  in  the  case  of  large  open  spaces. 

Next  in  order  comes  the  lighting  of  building  exteriors  which  in 
the  case  of  public  squares  and  of  streets  is  incidental  rather  than 
primary.  The  lighting  of  the  facade  of  a  building  for  utilitarian 
or  decorative  purposes  or  the  lighting  of  a  public  monument  is  a 
case  of  direct  illumination,  in  which  the  light  from  one  or  more 
reflecting  systems  is  concentrated  on  a  definite  area,  be  it  large  or 
small,  to  produce  specific  results  over  that  surface.  In  the  same 
category  falls  the  lighting  of  spaces  like  railroad  yards,  docks  and 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          83 

work  of  construction.  Such  lighting  may  have  to  rise  to  a  bril- 
liancy as  great  as  or  greater  than,  that  desirable  in  public  squares, 
or  may  fall  to  the  average  of  rather  mediocre  street  lighting  accord- 
ing to  the  purpose  intended,  but  in  all  cases  it  is  directed  for  special 
rather  than  general  results.  It  requires  ordinarily  lighting  units 
equipped  with  reflectors  of  comparatively  large  spherical  angle,  so 
as  to  direct  a  large  percentage  of  the  luminous  flux,  and  if  the 
properties  of  the  reflectors  are  approximately  known  the  results  can 
be  calculated,  as  will  be  presently  shown,  very  easily,  by  a  simple 
flux  method. 

Finally,  one  has  to  meet  the  special  conditions  imposed  by  parks 
and  other  very  large  open  spaces.  These  are  peculiar  in  that  no 
help  can  be  received  from  any  lateral  surfaces  and  in  that  conserva- 
tion of  resources  demands  generally  so  small  an  amount  of  illumina- 
tion that  the  preservation  of  suitable  dark  adaptation  in  the  eye 
becomes  of  paramount  importance.  Only  now  and  then  is  high 
intensity  desirable  and  that  only  locally,  where  the  lighting  may 
assume  something  of  a  decorative  aspect. 

To  take  up  in  more  detail  the  illumination  necessary  in  these  dif- 
ferent cases  the  highest  limit  is  touched  by  the  lighting  of  public 
squares.  In  such  areas,  which  are  generally  centers  of  streets  carry- 
ing dense  traffic,  the  average  illumination  on  the  reference  plane, 
three  or  four  feet  above  the  ground,  should  be  as  shown  by  ex- 
perience, one  or  several  lux  (a  few  tenths  of  a  foot-candle).  In  some 
cases  it  has  been  pushed  even  to  10  lux  over  a  considerable  area. 
The  exact  density  required  is  evidently  determined  by  the  nature  of 
the  situation,  but  any  average  less  than  one  lux  (o.i  foot-candle) 
must  be  regarded  as  undesirably  low.  In  practice  the  average 
should  generally  run  to  at  least  double  this  amount  in  order  to  pre- 
serve a  suitable  minimum  while  using  only  a  moderate  number  of 
lighting  units.  Certainly  anything  less  than  0.5  lux  must  be  re- 
garded as  unsatisfactory  as  a  minimum  figure  and  it  is  not  easy  to 
secure  in  a  large  area  this  minimum  without  having  an  average 
exceeding  i  lux,  and  a  considerable  area  of  maxima  of  at  least  double 
this  amount.  One  needs  to  see  well  in  a  public  square  where  many 
people  congregate  and  many  vehicles  pass,  and  the  amount  of 
illumination  must  therefore  be  pushed  to  a  high  limit  with  some 
special  effort  at  uniformity  in  order  to  prevent  the  appearance  of  dark 
areas  in  the  general  effect.  Likewise  in  such  situations  the  buildings 
deserve  more  than  the  usual  illumination  since  they  are  commonly 
of  importance  and  of  decorative  value  when  properly  lighted. 


84  ILLUMINATING   ENGINEERING   PRACTICE 

As  in  every  case  of  exterior  illumination  the  actual  amount  of 
light  to  be  furnished  in  a  public  square  depends  on  the  nature  of 
the  situation.  The  figure  just  given  ought  to  be  regarded  as  an 
irreducible  minimum  for  areas  in  which  there  is  any  considerable 
amount  of  traffic  even  of  pedestrians.  Where  vehicles  are  frequent 
and  the  space  generally  more  crowded  it  is  necessary  to  increase 
the  illumination  considerably,  rising  as  high  perhaps  as  5  lux  or 
more  under  extreme  conditions.  Large  open  areas  through  which  a 
continuous  stream  of  street  'cars,  automobiles,  and  pedestrians  are 
pouring,  particularly  in  the  evening  hours,  can  hardly  be  too  strongly 
illuminated  for  safety  and  convenience. 

The  method  selected  to  provide  the  illumination  depends  intrin- 
sically on  the  particular  area  to  be  dealt  with.  As  a  general  rule 
the  lamps,  whatever  their  size,  should  be  carried  relatively  high  in 
order  to  secure  a  fairly  even  light  distribution  without  going  to  an 
abnormal  number  of  supporting  structures.  It  is  a  good  rule  to  keep 
a  public  square  as  free  of  lamp  posts  and  other  obstructions  as 
possible,  which  indicates  the  wisdom  of  avoiding  a  multiplicity  of 
small  lamps  and  of  utilizing  a  few  tall  standards  which  may  be  given 
a  high  decorative  importance  and  lead  to  a  simpler  and  more  effective 
installation.  In  a  few  instances  where  the  open  area  is  large  and 
the  traffic  is  chiefly  around  its  margin,  lamps  carried  on  the  curbs 
as  in  ordinary  street  lighting  prove  to  be  the  best  sources  of  distribu- 
tion. In  a  case  of  this  kind  the  light  should  be  where  the  traffic  is, 
and  consequently  the  lighting  of  the  center  of  the  area  can  be  reduced 
in  intensity  while  that  on  its  bounding  streets  should  be  correspond- 
ingly increased.  For  simplicity  of  installation  and  efficiency  of 
light  production  large  units  are  desirable  in  this  as  in  every  case  of 
a  requirement  for  brilliant  illumination.  Arc  lamps  of  1000  or 
2000  candle-power  or  incandescent  lamps  of  nearly  or  quite  equiva- 
lent candle-power  lend  themselves  readily  to  this  particular  use. 
They  should  always,  of  course,  be  enclosed  in  diffusing  globes,  and 
it  should  be  remembered  that  the  gas-filled  incandescent  lamp  is 
almost  as  glaring  as  an  unshielded  arc.  If  in  such  a  square  there  are 
any  important  monuments,  as  sometimes  happens,  the  illumination 
should  be  directed  high  enough  to  include  them.  There  is  no  need 
here  to  deal  with  the  details  of  calculating  the  illumination,  since 
this  subject  has  been  admirably  handled  in  a  previous  lecture. 
Broadly  the  problem  can  be  attacked  along  two  related  lines. 

First,  the  area  and  the  desired  illumination  gives  at  once  the 
effective  lumens  required  to  obtain  the  average  result.  It  will 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          85 

generally  be  found  that  the  figure  thus  given  is  a  minimum  since 
the  ordinary  criterion  of  proper  illumination  considers  not  alone 
the  average  but  also  the  minimum  so  that  the  required  light  flux 
must  be  distributed  so  as  fully  to  meet  the  minimum  requirement, 
when  incidentally  it  will  carry  a  somewhat  high  maximum.  The 
simplest  way  of  solving  the  problem  is  to  determine  from  the  gen- 
eral light  flux  required  the  type  of  unit  which  will  be  desirable  to 
use,  that  is,  one  not  so  large  as  to  involve  great  difficulty  in  proper 
placement  or  so  small  as  to  require  undue  multiplication  of  supports. 
From  the  polar  candle-power  curve  of  such  a  unit  the  equilucial 
lines  corresponding  to  the  minimum  permissible  illumination  can 
be  plotted  for  various  heights  of  placement  and  then  these  areas 
arranged  so  as  to  overlap  enough  to  insure  keeping  comfortably 
above  the  minimum  at  all  points.  With  ordinary  light-sources  and 
reflecting  equipment  one  will  rarely  select  a  height  of  placement  less 
than  30  ft.  in  open  squares,  although  this  figure  may  be  somewhat 
reduced  in  cases  where  the  margin  of  the  area  is  the  chief  region 
of  traffic.  As  a  rule  the  more  powerful  the  unit  the  higher  it  may 
be  advantageously  placed. 

As  to  whether  single  or  multiple  units  should  be  used  on  a  single 
standard,  the  decision  is  chiefly  a  matter  of  taste.  With  the  large 
incandescent  lamps  in  particular  the  efficiency  varies  very  little 
with  output  so  that  one  may  freely  use  standards  carrying  two  or 
more  lamps  at  comparatively  slight  loss  of  efficiency.  Clusters  are 
generally  not  to  be  recommended  since  the  several  globes  with  their 
supports  are  in  each  other's  way;  moreover,  the  low  lying  clusters, 
which  have  been  frequently  used  in  the  past,  are  seldom  either  effi- 
cient or  artistic.  In  so-called  ornamental  lighting  both  arc  and 
incandescent  lamps  are  generally  mounted  much  too  low  for  efficient 
light  distribution,  a  few  distinguished  exceptions  like  the  recent 
exposition  lighting  at  San  Francisco  to  the  contrary.  The  rule  of 
artistry  in  the  lighting  of  public  places  is  to  keep  the  lamp  carriers 
in  scale  with  the  general  architectural  environment  and  to  place 
lamps  of  sufficient  candle-power  to  give  the  necessary  result  in 
illumination  approximately  as  here  indicated.  There  is  a  particular 
need  of  studying  such  problems  in  lighting  since  they  cannot  well 
be  solved  by  the  ordinary  apparatus  of  street  lighting,  placed,  as  it 
is,  usually  along  the  curbs  near  to  the  fagades  of  buildings,  and  de- 
signed to  be  at  its  best  in  illuminating  a  rather  narrow  street.  The 
public  square  is  a  place  by  itself  as  regards  the  requirements  for 
illumination. 


86  ILLUMINATING   ENGINEERING   PRACTICE 

In  street  lighting  proper  the  chief  area  of  illumination  is  that 
of  the  street  surface  itself  where  the  vehicular  traffic  is  located. 
Secondarily,  sidewalks  and  crosswalks  must  be  adequately  lighted, 
and  finally,  where  the  building  line  is  near  the  street,  the  fronts 
of  buildings  themselves  cannot  be  left  out  of  consideration;  first 
because  they  need  to  be  lighted  for  the  general  effect;  and  second, 
because  they  may,  if  light  in  color,  add  something  to  the  general 
effectiveness  of  the  street  illumination.  The  commonest  mistake 
made  in  street  lighting  is  to  follow  a  uniform  method  and  type  of 
illuminant  irrespective  of  the  individual  needs  of  the  street.  A  very 
common  method  of  lighting  in  the  earlier  days  consisted  in  placing 
at  each  street  intersection,  irrespective  of  the  length  of  the  block 
or  the  character  of  the  street,  an  arc  lamp,  usually  of  insufficient 
candle-power.  This  gave  a  fine  uniformity  of  lighting  units,  but 
extremely  bad  illumination  except  in  parts  of  the  city  where  the 
intersections  were  very  close  together.  The  almost  inevitable 
result  later  has  been  the  thrusting  of  incandescent  or  gas  lamps  into 
the  intermediate  spaces,  finally  producing  a  mixture  of  kinds  and 
sizes  of  lamps  both  bad  in  appearance  and  unsatisfactory  for  the 
purpose  intended. 

If  a  street  is  to  carry  dense  traffic  for  a  considerable  period  each 
night,  that  street  requires  thoroughly  good  illumination,  as  good 
even  as  the  better  class  of  public  squares.  If  the  traffic  is  not  heavy 
and  pedestrians  are  occasional,  vehicles  are  chiefly  to  be  considered, 
the  same  degree  of  lighting  is  totally  unnecessary  as  well  as  waste- 
ful. An  active  business  street  for  this  reason,  even  if  not  of  the 
first  class,  demands  brighter  illumination  than  an  ordinary  residence 
street,  and  this  in  turn  better  illumination  than  a  suburban  road. 
Speaking  generally  the  minimum  requirement  for  street  lighting  is 
that  demanded  for  proper  policing,  the  maximum,  that  required 
for  active  business  accompanied  by  dense  traffic  after  darkness  falls. 
The  attempt  to  illuminate  all  streets  in  approximately  the  same  way 
and  to  about  the  same  amount  means  that  if  the  important  streets 
are  really  well  lighted  a  great  deal  of  waste  will  occur  in  the  unim- 
portant ones,  or  if  the  illumination  be  standardized  for  the  latter 
the  former  will  suffer  greatly.  One  of  the  most  difficult  tasks  in 
arranging  the  proper  illumination  of  a  city  is  to  bring  the  public  to 
the  appreciation  of  the  fact  that  light  is  for  general  civic  service  and 
not  for  the  uniform  distribution  of  lighting  expense  through  every 
mile  of  street  or  every  ward  and  precinct. 

In  his  own  practice  the  speaker  customarily  divides  streets  into 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          87 

first,  second  and  third  class,  with  respect  not  to  the  popular  idea  of 
their  merits  or  the  cost  of  the  buildings  upon  them,  but  strictly  on 
the  basis  of  the  needs  disclosed  by  their  use  during  the  hours  of 
darkness.  For  the  purpose  of  lighting,  a  first-class  street  may  be 
a  boulevard  leading  through  the  center  of  the  city  and  containing 
many  of  the  important  business  houses,  or  a  street  running  along 
the  water  front,  congested  with  vehicles  and  streams  of  humans, 
or  even  a  side  street  through  which  from  one  necessity  or  another 
a  great  volume  of  traffic  passes. 

A  second-class  street  may  be  a  side  or  subsidiary  street  of  business 
houses,  a  residence  street  of  fine  mansions,  a  back  street  of  swarming 
tenements  or  a  long  road  running  out  of  the  city  but  constituting 
a  main  avenue  of  automobile  traffic.  The  third-class  streets  will 
form  the  residuum  after  the  first  two  classes  are  well  marked  out, 
the  ordinary  rank  and  file  of  city  and  suburban  streets  not  much 
frequented,  and  never  at  all  congested. 

Most  first-class  streets  are  thoroughly  obvious  except  for  those 
which  form  short  cuts  or  for  some  particular  reason  are  crowded  after 
nightfall.  These,  however,  are  easily  discovered  by  a  very  brief 
investigation  of  traffic.  The  second-class  streets  require  more  skill 
in  selection.  Some  of  them  suggest  themselves  at  once,  but  a  con- 
ference with  the  chief  of  police  will  usually  open  up  the  situation  in 
a  very  interesting  manner,  and  it  is  just  at  this  point  that  the 
greatest  difficulty  in  satisfying  the  public  occurs.  It  is  not  polite 
to  tell  the  alderman  of  the  Nth  ward  that  a  couple  of  shabby  streets 
in  his  district  needed  to  be  extremely  well  lighted  on  account  of  the 
semi-criminal  character  of  his  constituents,  while  the  quiet  residence 
street  on  which  he  lives  may  be  relegated  to  the  third-class.  Person- 
ally I  have  tried  to  make  a  practice  of  extending  good  second-class 
lighting  to  all  regions  of  churches  and  schools  and  other  districts 
where  for  one  reason  or  another  the  streets  might  be  much  used  at 
night.  Some  singular  anomalies  may  be  found  in  making  classifica- 
tions. For  example,  an  active  business  street  down  town,  which 
at  first  thought  would  be  put  in  the  first  class,  may  turn  out  to  be 
very  little  used  after  dusk  and  so  be  relegated  to  the  second.  The 
proper  classification  is  a  matter  of  tact  and  local  study. 

It  is  sometimes  wise  to  add  a  fourth  group  of  streets  and  roads  to 
those  already  mentioned,  in  which  the  street  lamps  are  hardly  more 
than  markers  of  the  way.  Almost  every  town  has  running  out  of 
it  long  roads  not  heavily  populated,  but  which  still  are  main  lines 
of  traffic  to  neighboring  districts.  To  light  these  even  as  a  third- 


88  ILLUMINATING   ENGINEERING    PRACTICE 

class  street  should  be  lighted  is  uneconomical,  but  a  very  modest 
equipment  indeed  may  work  great  improvement  in  the  traffic 
conditions.  It  is  extraordinary  how  much  even  small  lamps  widely 
spaced  will  do  to  assist  traffic  on  a  dark  night.  To  use  the  lamps  as 
markers  rather  than  for  illumination  then  becomes  practically  a 
rather  important  measure  of  public  convenience,  although  from  the 
standpoint  of  light  flux  the  provision  may  seem  almost  a  practical 
joke.  Some  engineers  attempt  even  a  somewhat  further  subdivi- 
sion, having  in  mind  a  gradation  between  the  second  and  third  class, 
but  it  is  not  generally  necessary,  since  there  is  trouble  enough  in 
establishing  a  sound  basis  of  classification  in  even  three  groups. 

As  respects  the  actual  amount  of  illumination  required  for  street 
work  the  figures  given  depend  on  the  agreement  as  to  the  way  in 
which  this  illumination  shall  be  measured.  Abroad  it  has  been  the 
custom  to  reckon  the  illumination  as  the  total  received  upon  and 
resolved  upon  a  horizontal  reference  plane  usually  taken  as  a  meter 
above  the  ground.  This  means  that  the  light  received  from  each 
source  must  be  resolved  according  to  the  cosine  law  on  the  plane 
and  the  total  received  from  all  the  sources  added.  In  American 
practice  it  has  been  the  custom  to  reckon  the  illumination  as  that 
received  from  one  direction  only  upon  a  plane  normal  to  the  ray. 
On  account  of  the  obliquity  of  the  illumination  the  former  method 
generally  gives  a  lower  numerical  value  for  the  illumination,  a  fact 
which  must  be  borne  in  mind  in  interpreting  foreign  specifications. 
For  the  special  case  in  which  only  two  lamps  are  considered, 
spaced  at  four  times  their  height,  the  numerical  results  will  coincide 
by  the  two  methods  and  such  relations  of  spacing  to  height  is  not 
uncommon  especially  abroad. 

For  street  lighting  proper,  the  writer  prefers  the  usual  American 
method  on  the  ground  that  the  most  trying  tests  of  street  lighting 
in  practice,  such  as  reading  an  address,  or  recognizing  a  person,  do 
not  depend  upon  supposing  either  page  or  person  to  be  extended  flat 
upon  the  ground,  but  do  depend  on  the  light  that  fairly  strikes  them 
from  the  lamp.  Either  method  of  reckoning  is  perfectly  safe  pro- 
vided it  is  consistently  used. 

Based  on  the  usual  American  reckoning  the  illumination  in  first 
class  streets  should  run  nearly  or  quite  as  high  as  in  public  squares, 
that  is,  should  amount  to  a  minimum  of  at  least  0.5  lux  (0.05  foot- 
candle)  and  preferably  double  this  amount,  with  an  average  two 
or  three  times  as  great.  The  chief  streets  of  well-lighted  foreign 
cities  before  the  war  averaged  fully  up  to  this  standard.  Indeed  I 


•   BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          89 

have  often  taken  as  a  rough  test  of  the  proper  illumination  the  ability 
to  read  a  Baedecker  when  walking  or  riding  along  the  street,  that 
excellent  volume  being  utilized  merely  as  one  generally  at  hand. 
First-class  streets  differ  among  themselves  to  a  considerable  extent, 
but  if  the  minimum  is  kept  up  to  i  lux  the  maximum  will  usually  run 
high  enough  to  give  an  average  of  from  2  to  4  lux  with  a  maximum 
anywhere  from  5  to  20. 

In  street  lighting  the  difference  between  the  minimum  and  maxi- 
mum illumination  is  generally  conspicuously  great.  Any  ratio  less 
than  i  :  10  requires  very  special  efforts  to  secure  uniformity  and 
streets  which  are  practically  very  well  lighted  indeed  may  show 
ratios  of  i  :  25  or  even  i  :  50.  In  such  instances  the  darkest  spots 
will  usually  be  very  small  in  area  and  due  to  special  circumstances 
and  the  average  will  be  high.  Streets  here  designated  as  second- 
class  ordinarily  require  about  half  the  intensity  ascribed  to  first- 
class  streets,  that  is,  an  average  of  0.5  to  i.o  lux.  Third-class 
streets,  again,  may  have  advantageously  about  half  the  intensity 
of  the  typical  second-class  streets,  with  the  proviso  that  anything  as 
low  as  0.25  lux  as  an  average  would  unquestionably  bring  a  mini- 
mum so  low  as  to  be  almost  negligible  midway  between  lamps. 
Finally  where  street  lamps  are  used  merely  to  mark  the  way  the 
illumination  will  be  so  small  except  near  the  lamps  as  to  be  hardly 
worth  considering,  the  function  of  the  lamps  being  to  define  rather 
than  to  illuminate  the  road.  A  committee  appointed  a  few  years 
ago  in  London  to  make  recommendations  as  to  street  illumination 
drew  up  the  following  recommendations: 


»                        Classification  of  streets 

Minimum    horizontal    illu- 
mination in  foot-candles 

Class  A  

O.OI 
0.025 
O.O4 
O.o6 
0.10 

Class  B.                                                        .... 

Class  C 

Class  D.         .                               

Class  E. 

which  correspond  pretty  closely  to  that  here  suggested. 

The  class  E  streets  of  this  table  correspond  to  the  first-class  streets 
just  described,  classes  C  and  D  include  the  second-class  streets,  and 
classes  A  and  B  the  third-class.  Bearing  in  mind  the  difference  in 
the  conventional  measurement  the  results  are  of  practically  the  same 
order  of  magnitude. 


90  ILLUMINATING   ENGINEERING   PRACTICE 

Variations  in  intensity  such  as  here  required  depend  on  two  things, 
the  height  and  the  spacing  of  the  illuminants  and,  other  things  being 
equal,  the  diversity  ratio  between  maximum  and  minimum  depends 
on  the  relation  of  the  height  to  the  spacing.  Powerful  light  sources 
need  to  be  placed  high  in  order  to  avoid  both  too  great  difference 
between  maxima  and  minima  and  too  great  obliquity  of  the  more 
distant  rays.  Small  lamps  may  be  placed  correspondingly  lower 
and  also  require  closer  spacing  to  meet  the  minimum  requirements. 
With  big  units  approximating  1000  candle-power  the  height,  of 
placement  should  be  not  less  than  25  ft.  to  obtain  the  most  useful 
distribution  of  light,  while  the  smaller  units  either  electric  or  gas 
are  usually  most  effective  when  placed  from  15  to  20  ft.  high  with 
the  spacings  ordinarily  employed.  Occasionally,  in  using  particularly 
well-screened  large  units  closely  placed  in  order  to  obtain  a  very 
powerful  illumination,  the  figures  here  given  may  be  somewhat 
reduced,  the  point  being  to  adjust  the  spacing  with  reference  to  the 
direction  of  maximum  intensity,  so  that  this  may  fall  nearly  at  the 
midway  point  between  lamps. 

Extreme  uniformity  of  illumination  is  in  general  not  worth  the 
effort  in  street  lighting,  the  main  point  being  that  for  streets  carry- 
ing any  material  amount  of  traffic  the  minimum  illumination  must 
be  high  enough  to  give  reasonably  good  results.  This  matter  will 
be  taken  up  in  the  lecture  dealing  with  the  technique  of  street  light- 
ing, so  that  I  need  not  further  mention  it  here  except  to  say  that  there 
is  definite  evidence  of  too  great  uniformity  tending  to  prevent  the 
quick  vision  of  obstacles,  and  also  tending  to  lessen  the  attentiveness, 
for  instance,  of  the  driver  of  a  motor  car.  This  point  was  admirably 
brought  out  in  the  psychological  work  done  under  the  direction  of 
Prof.  Munsterberg  for  the  N.E.L.A.  street  lighting  tests. 

As  to  the  conditions  of  placement  of  street  lamps  the  main  practi- 
cal factor  is  the  nature  of  the  street.  A  narrow  street,  particularly 
if  well  built  up,  may  be  admirably  lighted  in  the  usual  manner  by 
placing  the  lamp  posts  upon  the  curb.  A  street  much  shaded  by 
trees  loses  too  much  from  shadows  with  this  positioning  and  use 
must  be  made  of  long  brackets,  mast  arms,  or  cross  suspensions,  in 
this  country  usually  the  mast  arms.  Very  broad  streets  sometimes 
can  be  advantageously  lighted  by  means  of  a  row  of  posts  down  the 
center  perhaps  on  isles  of  safety,  in  extreme  cases  in  conjunction 
with  curb  lighting  as  well. 

A  word  here  with  reference  to  glare  from  street  lamps,  which 
sometimes  becomes  very  unpleasant,  especially  with  high  efficiency 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          91 

illuminants.  Lamps  mounted  high  are  in  general  much  less  trouble- 
some than  those  placed  low,  and  even  when  powerful  light  sources 
so  placed  fall  well  within  the  field  of  vision  at  the  midway  point 
between  them,  the  gross  intensity  of  the  light  reaching  the  eye  is  so 
considerably  reduced  as  not  to  be  serious.  One  does  not  par- 
ticularly mind  the  glare  from  even  a  very  powerful  arc  at  200  or  300 
ft.  distance,  while  it  may  be  most  offensive  when  nearer.  In  fact 
it  is  not  always  easy  to  tell  at  long  range  whether  a  high  power  lamp 
has  or  has  not  a  diffusing  globe,  but  in  either  case  the  light  does  not 
produce  serious  glare.  Even  the  smaller  lamps  now  used  for  street 
lighting,  especially  the  almost  universal  high-efficiency  incandescent 
lamps,  which  are  often  placed  low,  may  produce  very  offensive 
glare  and  seriously  hinder  the  utilization  of  the  illumination  derived 
from  them.  Certainly  in  the  larger  types  of  these  lamps  the  use  of 
frosted  bulbs  or  thin  diffusing  globes  is  highly  desirable  and  proves 
of  practical  benefit. 

Passing  now  from  street  lighting,  of  which  the  details  will  be  fully 
set  forth  in  a  separate  lecture,  we  come  to  a  rather  special  case  of 
illumination,  namely,  the  lighting  of  public  monuments  and  the 
facades  of  buildings.  I  will  not  here  enter  at  length  into  the  technique 
of  this  matter  since  it  will  form  part  of  the  subject  matter  of  another 
lecture  of  this  course,  but  will  merely  point  out  some  of  the  general 
requirements  and  the  means  for  meeting  them.  Where  the  facade 
of  a  building  is  to  be  illuminated  the  method  employed  has  to  depend 
on  the  character  of  the  building  and  its  distance  from  available  situa- 
tions for  lamps.  Sometimes  suitable  ornamental  lamp  posts  with 
powerful  illuminants  may  be  placed  on  the  curb  of  a  wide  sidewalk, 
fairly  high,  in  such  position  as  to  give  an  admirable  effect  in  lighting 
the  front  of  a  building.  In  cases  where  this  is  not  feasible  and  yet 
for  one  reason  or  another  good  illumination  of  the  facade  is  desired 
the  modern  projector  using  high-intensity  incandescent  lamps  meets 
the  requirements  with  admirable  effect.  The  difficulty  here  is  the 
proper  placement  for  the  lamps,  which  can  seldom  be  found  on 
the  building  itself  and  more  generally  has  to  be  sought  on  opposite 
or  near-by  roofs.  The  same  conditions  hold  for  the  task  occasionally 
required,  of  lighting  public  monuments.  Many  of  these  had  better 
be  left  under  the  concealing  wing  of  night,  but  occasionally  a  fine 
example  appears  which  fully  deserves  all  the  attention  that  can  be 
bestowed  upon  it. 

This  again  is  a  case  for  flood  lighting  which  can  rarely  be  carried 
out  from  the  base  of  the  monument  or  the  immediate  vicinity.  It 


Q2  ILLUMINATING   ENGINEERING   PRACTICE 

usually  required  a  suitable  placement  of  the  lamps  at  a  distance 
several  times  the  height  of  the  monument.  As  reflectors  for  in- 
candescent lamps  can  now  be  obtained  which  give  a  fairly  concen- 
trated beam,  a  suitable  point  of  attack  can  always  be  found,  even 
if  it  has  to  be  a  couple  of  hundred  feet  away.  It  is,  in  fact,  rather 
easier  to  get  a  projector  with  a  fairly  narrow  beam  than  it  is  to 
obtain  one  with  a  beam  of  moderately  great  angle,  well  distributed 
and  concentrated,  but  progress  is  now  being  made  to  assure  good 
results  in  almost  any  condition  that  can  be  found. 

I  will  not  here  go  at  length  into  the  topic  of  flood  lighting,  but 
will  content  myself  with  pointing  out  that  with  such  reasonably 
exact  knowledge  of  the  reflecting  system  as  should  be  at  hand  in  any 
well  designed  commercial  lamp  it  is  a  perfectly  simple  matter  to 
calculate  the  wattage  required  to  produce  any  given  amount  of 
illumination  which  circumstances  demand. 
In  doing  this  I  shall  merely  amplify  the  out- 
line of  the  theory  which  I  gave  in  the 
Baltimore  Lectures  of  six  years  ago,  apply- 
ing the  added  data  which  are  now  available. 
Consider  a  source,  x,  placed  in  the  focus 
of  an  approximately  parabolic  reflector. 
All  the  light  within  the  spherical  Z  <f>  passes 
out  in  a  scattered  secondary  beam.  The 
primary  beam  delivered  by  the  mirror  takes 

Fig.  i.  —  Beam  candle-power.      r  J  J 

the  light   from  an  Z  471-  —  (<£  +  8)   and  is 

diminished  by  the  absorption  and  scattering  at  the  mirror  surface. 
Let  the  beam  fall  normally  upon  a  surface  producing  a  circle  of 
illumination  of  radius  r.  Then 


Trr  r2 

Or,  if  the  circle  is  projected  into  an  ellipse, 


E  =  —  j-t  as  average. 

Here  rj  is  the  specific  efficiency  of  the  source  in  Us  reflecting  system 

and  w  is  watts  used,  17  =  apt. 

Where  a  is  the  specific  output  of  the  source  in  spherical  candles 

per  watt, 

p  is  the  coefficient  of  reflection  of  the  surface, 
K  is  percentage  of  total  sphere  effective  =  471-  —  (0  +  0). 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          93 

Now  obviously  the  larger  the  parabola  and  the  shorter  the  focal 
length  the  smaller  is  <£  but  care  must  be  taken  that  in  case  of  small 
focal  length  8  does  not  unduly  increase,  due  to  lamp  socket  or  sup- 
port. As  a  practical  matter  K  ranges  from  0.5  or  less  in  shallow 
parabolas  to  0.8  or  even  0.9  in  deep  ones  of  focus  say  of  only  one- 
tenth  the  diameter  of  the  opening. 

p  ranges  from  perhaps  0.6  with  cheap  metal  reflectors  to  0.8  or 
0.85  in  high  grade  silvered  glass  mirrors. 

<7  ranges  from  i  to  nearly  1.5  in  various  lamps.  A  lamp  is  at  its 
best  when  its  main  axis  of  filament  is  in  the  axis  of  the  mirror.  Its 
distribution  is  then  a  tore  (Fig.  2)  a  doughnut  with  a  very  small 
hole,  and  the  main  body  of  light  is  well  reflected. 

Reflectors  with  depolished  surface  or  fronted  with  a  depolished 
screen  scatter  the  light  from  each  element  of  surface  and  increase 
the  scattered  secondary  beam  at  the  expense  of  the  primary  closely 
directed  beam.  They  thus  may  throw  light  over  an  angle  of  90°  or 
so  and  cannot  give  high  concentration,  although  showing  very  low 
intrinsic  brilliancy  and  having  a  distinctly  useful  place  in  illumina- 
tion, for  instance  of  a  tennis  court. 

In  practical  flood  lighting  the  value  of  rj  is  likely    /       \(       j 
to  run  from  0.5  to  0.75,  more  nearly  the  former  figure    ^^^-^ 
when  dealing  with  reflectors  and  lamps  in  their  average     Fi&-  2.—  Light 
condition.     In  lamps  of  the  (arc)  carefully  designed 
search  light  type  77  may  rise  to  or  somewhat  above  unity  on  account 
of  the  large  proportion  of  light  delivered  from  the  advantageously 
placed  crater  to  the  mirror  and  the  small  proportion  of  light  ob- 
structed by  the  source. 

Assuming  rj  =  0.5  for  an  ordinary  flood  light  one  reaches  the  follow- 
ing very  simple  formulae  for  the  relation  between  illumination, 
energy  and  circle  of  illumination. 


2W 

r*  =  ^  (3) 

Dividing  lumens  by  area  gives  illumination  in  lux,  if  area  is  in 
square  meters  or  in  foot-candles  if  in  square  feet.  Hence  the  follow- 
ing examples. 

Required;  illumination  on  a  circle  of  10  in.  radius  from  a  1000  watt 
lamp  and  reflector. 


94  ILLUMINATING   ENGINEERING   PRACTICE 

2  X  1000 

h,  =  -  -  =  20  lux 

100 

Required,  watts  to  give  50  lux  on  a  circle  of  5  meters  radius 


Required,  circle  which  4000  watts  will  light  to  20  lux. 


„       8000 
r2  =  -     -  =  400 
20 


r  =  20  n. 


It  will  be  observed  that  the  distance  does  not  enter  these  reckonings, 
for  the  simple  reason  that  so  long  as  all  the  flux  of  the  primary  beam 
falls  on  the  required  surface  distance  does  not  count  save  as  it  may 
involve  atmospheric  absorption  which  is  of  small  moment  at  flood- 
lighting distances.  Only  when  the  spread  of  the  beam  gets  it  off  the 
object  does  distance  become  important  in  reckoning  the  illumination. 
At  very  short  range  the  secondary  beam  proceeding  directly  from 
the  source  may  not  be  negligible,  and  this  follows  the  ordinary  inverse 
square  law. 

Sufficient  has  been  said  already  to  outline  the  general  method  of 
reckoning  exterior  illumination.  The  fundamental  principle  is  to 
reckon  average  illumination  from  the  flux  theory  according  to 
methods  which  have  been  already  laid  down  in  a  previous  lecture, 
and  then  to  make  sure  of  a  sufficient  amount  of  uniformity  and  a 
sufficiently  large  minimum  by  computing  the  illumination  directly 
from  the  candle-power  curve  of  the  illuminants  concerned  at  any 
point.  In  open  squares  several  sources,  in  small  squares  all  the 
sources  have  to  be  considered.  In  street  work  one  need  very  rarely 
add  the  illumination  from  more  than  two  sources  on  one  side  of  the 
point  of  reckoning,  the  symmetrical  sources  on  the  other  side  being 
obvious  in  their  effect.  The  computations  involve  no  special  diffi- 
culties- and  are  fully  taken  care  of  by  the  general  theory  once  the 
candle-power  distribution  curve  of  the  source  is  known.  With 
reasonable  care  in  the  placement  of  lamps  a  very  good  estimate  of 
exterior  lighting  including  street  lighting  can  be  made  from  light 
flux  alone,  the  ordinary  practice  of  placing  and  spacing  the  lamps 
being  sufficient  to  secure  the  necessary  light  distribution. 

It  should  be  mentioned  in  this  connection  that  the  N.E.L.A. 
Committee,  which  investigated  the  details  of  street  lighting,  found 
that  for  practical  purposes  the  useful  illumination  was  pretty  nearly 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          95 

proportional  to  the  light  flux  as  might  have  been  anticipated.  In 
most  instances  it  is  the  lower  hemispherical  flux  which  is  concerned. 
In  narrow  streets  well  built  up  where  the  limiting  walls  have  a  per- 
ceptible effect  on  the  distribution  of  the  light  one  might  include  the 
total  flux  which  for  lamps  with  reflectors  is  roughly  proportional  to 
the  lower  hemispherical  flux  in  any  case.  Street  illuminants  there- 
fore can  rather  fairly  be  rated  in  terms  of  the  total  lumens  which 
they  give,  subject  to  the  requirement  that  reasonable  intelligence 
must  be  used  in  locating  the  sources. 

In  exterior  illumination  even  more  than  in  interior  it  is  the  adapta- 
tion of  means  to  ends  which  makes  the  difference  between  good  and 
bad  results.  One  cannot  safely  travel  on  a  hard  and  fast  theory  in 
such  matters.  He  cannot,  for  example,  say,  I  will  take  the  abed 
lamp  of  (n)  candle-power  as  my  standard  and  I  will  adapt  all  things 
to  it.  If  he  does  so  the  result  is  quite  certain  to  be  mediocre  in 
quality.  It  is  practically  necessary  in  meeting  the  great  range  in 
intensities  required  in  exterior  lighting  to  depend  upon  not  one  kind 
or  size  of  unit  but  at  least  several  sizes  and  perhaps  several  kinds. 

At  the  present  moment  for  light  sources  materially  below  some- 
where about  1000  candle-power  the  large  high  efficiency  incandescent 
lamps  have  the  call.  In  larger  outputs  than  this  the  big  luminous 
and  flame  arc  lamps  still  hold  their  own  well.  A  few  smaller  arc 
lamp  units  are  used  for  strictly  ornamental  lighting,  but  the  carbon 
arc  lamps  of  every  kind  and  even  the  smaller  flame  and  luminous 
arc  lamps  are  rapidly  passing  to  the  scrap  heap.  How  far  the  ten- 
dency just  indicated  can  go  on,  and  whether  the  arc  lamp  is  to  have  a 
permanent  place  in  exterior  lighting  is  somewhat  open  to  doubt. 
My  own  opinion  is  that,  particularly  on  account  of  the  conspicuous 
difference  in  color,  the  best  of  the  flaming  and  luminous  arc  lamps 
have  at  least  a  considerable  period  of  usefulness  still  before  them,  but 
in  the  smaller  sizes  the  hand-writing  is  certainly  upon  the  wall. 
Ordinary  public  lighting  is  generally  found  as  a  matter  of  practice 
to  include  the  use  of  at  least  three  sizes  of  units,  two  of  which  will 
generally  be  incandescent  electric  lamps  or  the  equivalent  gas 
mantles. 

For  the  lighting  of  public  squares  and  first-class  streets  the  big 
units,  whether  arc  or  incandescent,  .are  altogether  desirable.  For 
second-class  streets  one  may  either  retain  the  same  size  and  type  of 
unit  expanding  the  spacing  a  bit,  or  may  pass  to  a  smaller  unit.  The 
latter  is  the  more  common  practice,  although  some  transitional 
streets  may  be  very  well  treated  in  the  former  fashion.  The  smaller 


96  ILLUMINATING   ENGINEERING   PRACTICE 

units  may  pass  into  some  of  the  lighting  of  third-class  streets,  the 
distances  of  spacing  being  stretched  a  bit  in  response  to  the  smaller 
necessities.  It  is  quite  usual,  however,  to  employ  a  still  smaller 
unit  for  much  of  the  third-class  lighting  as  well  as  for  all  the  cases 
requiring  lamps  merely  as  markers.  More  than  three  sizes  of  lamp 
are  very  rarely  indicated,  and  extremely  good  work  can  be  done  with 
two,  although  the  gain  in  simplicity  so  attainable  does  not  amount 
to  much. 

In  shades  and  glassware  one  commonly  finds  that  each  type  of 
unit  has  its  own  requirement.  All  powerful  radiants  like  arc  lamps 
and  very  large  incandescent  lamps  should  be  provided  with  diffus- 
ing globes.  These  can  now  be  obtained  giving  good  diffusion  with- 
out much  loss  of  light.  Lighting  units  of  more  moderate  output, 
say  from  100  to  300  candle-power,  in  many  cases  require  screening 
to  obtain  the  best  results,  particularly  toward  the  upper  limit  of 
size  just  mentioned.  An  incandescent  lamp  of  a  couple  of  hundred 
candle-power,  unshielded,  is  rather  an  offense  to  the  eyes,  and 
diffusing  glassware  or  frosted  bulbs  very  much  improve  the  actual 
lighting  effect,  although  they  sometimes  create  an  entirely  false 
impression  of  insufficient  light.  Most  people  still  judge  a  street 
lamp  by  its  intrinsic  brilliancy  rather  than  its  actual  power,  and  this 
psychological  fact  must  be  kept  in  mind.  Lamps  of  smaller  output 
than  100  candle-power  seldom  need  screening,  for  while  they  may  be 
unpleasantly  bright  when  viewed  from  very  nearby,  in  the  position, 
actually  occupied  by  them  they  may  be  comparatively  inoffensive. 

COLOR  IN  LIGHTING 

In  the  lighting  of  buildings  and  monuments  and  flood  lighting 
problems  generally,  and  to  a  less  extent  in  some  types  of  street 
lighting,  the  matter  of  color  may  rise  to  considerable  importance. 
Save  in  rare  instances  color  in  illumination  can  only  be  obtained  at 
a  considerable  and  sometimes  almost  prohibitive  cost  of  energy. 
One  can  get  very  efficiently  a  bright  yellow  from  the  flame  arcs,  a 
color  perfectly  good  for  utilitarian  purposes,  but  not  lending  itself 
to  any  decorative  effects.  It  is  possible  to  produce  flaming  elec- 
trodes giving  striking  colors  at  some  loss  of  efficiency,  but  yet  at 
an  efficiency  probably  exceeding  anything  that  can  be  obtained  by 
screens  or  colored  globes.  At  the  Boston  Electrical  Show  of  1912 
red  and  green  flame  arcs,  owing  their  color  only  to  the  impregnation 
of  the  electrodes,  were  used  with  rather  beautiful  effect,  but  such 


BELL:  PRINCIPLES  OF  EXTERIOR  ILLUMINATION          97 

electrodes  cannot  be  obtained  commercially,  and  the  illuminating 
engineer  has  to  fallback  practically  upon  screens  for  obtaining  colored 
effects. 

Color  in  lighting  may  be  utilized  to  intensify  the  hue  of  objects 
already  colored  or  to  impart  color  to  things  not  already  possessing  it. 
Light  as  nearly  white  as  possible  brings  out  the  natural  color  values 
in  a  fairly  uniform  way.  A  single  color  gains  in  brilliancy  from  flood- 
ing with  the  same  color,  while  illumination  with  the  wrong  color 
may  utterly  spoil  the  effect.  These  things  are,  of  course,  perfectly 
familiar  in  interior  lighting.  The  decorative  value  of  color  has  been 
comparatively  little  appreciated  or  utilized  in  exterior  illumination. 
The  most  striking  instance  of  its  employment  on  a  large  scale  was  at 
the  Panama-Pacific  International  Exposition  of  last  year,  at  San 
Francisco,  in  which  for  both  day  and  night  effects  color  played  a 
predominant  part.  In  regular  flood  lighting  work  a  monument  or 
even  a  sign  may  be  so  tinted  as  to  gain  from  the  application  of  a 
particular  color  in  its  illumination.  But  instances  where  this  can 
be  advantageously  applied  are  rather  rare. 

Perhaps  of  more  general  importance  is  the  possibility  of  producing 
highly  decorative  results  in  the  illumination  of  facades  of  buildings 
by  giving  them  color  values  which  relieve  the  monotony  of  the  effect 
otherwise  attainable.  Comparatively  little  has  been  done  in  this 
line,  although  the  writer  tried  it  out  experimentally  on  the  facade 
of  the  Massachusetts  State  House  and  of  the  building  of  the  Edison 
Illuminating  Company  last  year  far  enough  to  learn  something 
of  its  possibilities.  The  chief  difficulty  in  such  work,  which  can  be 
carried  out  with  very  beautiful  effects,  is  to  obtain  the  necessary 
illumination  without  too  great  cost  in  energy.  Screens  of  the 
colored  film  used  in  theatrical  work  can  readily  be  arranged  in  con- 
junction with  lamps  for  flood  lighting.  In  the  case  of  the  experi- 
ments just  referred  to  the  screens  were  fitted  into  frames  in  racks 
just  in  front  of  the  lamps.  In  theatrical  working  the  areas  to  be 
covered  are  small  and  the  available  intensities  are  so  great  that  a 
considerable  range  of  color  can  be  successfully  employed.  This 
range  is  much  limited  in  the  larger  problems  of  exterior  lighting  unless 
at  great  cost  of  energy.  Light  yellow  screens  fail  to  produce  any 
striking  effect.  Even  amber  tints,  although  losing  considerable 
light,  do  not  seem  to  produce  a  good  hue  on  the  surface  illuminated. 
Light  reds  work  better  and  light  rose  pinks  also  are  very  successful. 
Greens  and  blues  are  not  very  striking  unless  deep  in  color  and 
consequently  wasting  much  light. 


g8  ILLUMINATING   ENGINEERING   PRACTICE 

In  general  terms  the  loss  of  light  in  colored  screens  of  hue  deep 
enough  to  produce  any  material  effect  is  from  50  to  80  per  cent,  so 
that  one  has  to  allow  from  3  to  4  or  5  times  the  intensities  which 
would  ordinarily  be  utilized  for  flood  lighting.  It  is  not  necessary 
to  fit  all  the  reflectors  with  screens  in  doing  such  work.  A  ground 
illumination  can  be  produced  in  the  oridinary  way  and  then  tints 
laid  on  by  banks  of  special  reflectors  directed  either  so  as  to  overlay 
the  whole  or  any  part  of  it  with  warm  color. 

Considerable  experimenting  is  needed  to  produce  screens  which 
will  give  the  maximum  of  tinting  effect  with  minimum  loss  of  light 
and  which  will  retain  their  color  without  fading.  Of  course,  the  films 
used  for  theatrical  purposes  will  not  withstand  moisture  so  that  when 
used  out  of  doors  they  must  either  be  screened  in  with  glass  or  with- 
drawn in  rainy  weather.  The  colored  applications  are  interesting, 
and  probably  will  be  made  an  important  adjunct  in  flood  lighting, 
but  the  whole  matter  is  still  in  the  experimental  stage. 


MODERN  PHOTOMETRY 

BY  CLAYTON  H.  SHARP 

The  present  lecture  is  to  be  looked  upon  as  in  a  measure  a  con- 
tinuation of  the  lectures  on  the  measurement  of  light  given  in  the 
1910  I.  E.  S.  course  at  Johns-Hopkins  University.  It  is  intended 
to  supplement  those  lectures  not  only  by  introducing  an  account  of 
the  developments  in  photometry  since  1910,  but  also  by  treating 
of  certain  matters  which  were  either  insufficiently  treated  or  were 
omitted  entirely  from  the  1910  lectures.  It  should  be  understood, 
however,  that  it  is  the  intention  of  the  lecturer  not  to  attempt  a 
complete  review  of  photometric  advance  during  recent  years,  but 
rather  to  confine  himself  to  the  practical  features  which  properly 
belong  in  this  essentially  practical  course. 

The  practice  of  to-day  in  the  measurement  of  light  involves  innova- 
tions and  improvements  which  the  change  of  conditions  since  1910 
has  brought  forward.  Since  1910  the  introduction  of  the  gas-filled 
tungsten  filament  incandescent  lamp  with  its  whiter  light  has  made 
the  photometric  difficulties  due  to  color  differences  a  more  important 
factor  in  the  art  and  has  been  a  direct  incentive  toward  the  prose- 
cution of  the  investigation  of  the  problem  of  heterochromatic 
photometry  and  of  the  introduction  of  means  to  solve  it,  while  the 
increasing  demand  for  accuracy  in  photometric  measurements,  and 
particularly  the  growth  of  the  idea  of  the  measurement  of  luminous 
output  of  all  lamps  in  terms  of  their  total  luminous  flux  rather  than 
in  terms  of  their  candle-power,  has  given  a  great  incentive  to  the  use 
of  the  integrating  sphere.  During  the  six-year  interval  new  and 
improved  types  of  apparatus  have  been  constructed  and  put  into 
use. 

PHYSICAL  PHOTOMETER 

The  physical  photometer,  an  apparatus  which  will  measure  the 
light  from  any  illuminant  and  give  the  result  in  terms  identical 
with  those  which  would  be  obtained  by  the  use  of  a  photometer  by 
a  person  of  normal  color  vision,  has  been  realized.  This  physical 
photometer  has  been  constructed  and  practically  used  by  Ives1 
who  uses  a  sensitive  thermopile  as  a  means  for  measuring  the  radiant 
energy.  He  has  two  methods  for  selecting  the  radiation  from  the 

99 


100  ILLUMINATING   ENGINEERING    PRACTICE 

lamp  in  accordance  with  the  luminosity  curve  of  the  average 
human  eye.  The  first  of  these  methods  involves  passing  the  light 
through  a  spectroscope  equipped  with  a  shield  or  screen  which  is 
cut  out  in  the  form  of  the  luminosity  curve.  The  spectrum,  which 
is  thereby  reduced  to  a  luminosity  curve  spectrum,  is  reunited,  and 
the  total  energy  passing  through  the  screen,  which  is  then  propor- 
tional to  the  light  of  the  lamp,  is  thrown  on  the  thermopile.  The 
second  method,  which  for  experimental  purposes  is  undoubtedly 
simpler,  involves  passing  the  light  through  a  glass  cell  having  a 
thickness  of  one  centimeter  containing  the  following  solution: 

Cupric  chloride 60 .  o  grams 

Potassium  ammonium  sulphate 14. 5  grams 

Potassium  chromate 1.9  grams 

Nitric  acid,  gravity  1.05 .  18.0  c.c. 

Water  added  to  make  one  liter. 

Between  the  solution  and  the  lamp  is  interposed  another  water  cell 
to  prevent  overheating  of  the  solution.  The  transmission  of  this 
solution  is  according  to  Ives  identical  with  the  luminosity  curve  of 
the  average  eye. 

FLICKER  PHOTOMETER 

Ives1  has  recommended  a  system  of  heterochromatic  photometry 
involving  the  use  of  a  standardized  form  of  flicker  photometer  and  the 
investigation  of  the  color  vision  of  the  observers  using  it.  The 
flicker  photometer  as  recommended  by  him  has  a  field  two  degrees 
in  diameter  with  a  surrounding  field  of  large  dimensions  illuminated 
to  approximately  the  same  degree.  As  the  standard  illumination 
for  the  flicker  field  he  recommends  25  meter-candles. 

A  simple  attachment  to  be  used  on  an  ordinary  Lummer-Brodhun 
photometer  to  convert  it  to  a  flicker  photometer  corresponding  to 
these  specifications  has  been  described  by  Kingsbury2  and  is  ex- 
pected shortly  to  be  commercially  available.  Ives  has  shown  both 
theoretically  and  experimentally  that  the  settings  of  observers  using 
a  flicker  photometer  are  affected  by  peculiarities  of  their  color  vision. 
He  has,  therefore,  proposed  a  criterion  for  normality  of  color  vision  of 
observers  using  the  flicker  photometer.  This  consists  in  measuring 
the  light  of  a  4-wpc.  carbon  lamp  through  a  one  centimeter  layer 
of  each  of  two  different  solutions.  The,  first  consists  of  72  grams 
of  potassium  bichromate  in  water  to  make  one  liter.  The  trans- 

1  Ives,  I.  E.  S.  Transactions,  1915,  page  315.     A  bibliography  of  the  subject  is  there  given. 

2  Kingsbury,  Journal  of  Franklin  Institute,  August,  ipiS- 


SHARP:  MODERN  PHOTOMETRY 


101 


mitted  light  with  this  solution  is  yellowish.  The  other  solution 
consists  of  53  grams  of  cupric  sulphate  in  water  to  make  one  liter. 
This  gives  a  bluish  color.  The  solutions  are  to  be  used  at  2O°C. 
Ives  has  shown  that  a  person  with  perfectly  normal  color  vision  will 
find  with  a  flicker  photometer  the  same  value  for  a  4-wpc.  lamp  with 
either  solution.  His  proposal  then  is  to  make  color  measurements 
using  the  flicker  photometer  and  a  group  of  observers  so  selected  that 
on  the  average  their  value  for  the  transmission  of  the  yellow  solu- 
tion is  the  same  as  that  of  the  blue  solution,  such  a  group  having 
according  to  his  measurements  normal  color  vision.  This  proposal 
has  been  thoroughly  investigated  by  Crittenden  and  Richtmyer3 
who  by  studying  the  peculiarities  of  a  large  number  of  observers 
using  a  Lummer-Brodhun  photometer  have  shown  that  identical 
photometric  results  are  obtained  by  a  selected  small  number  of 
observers  having  on  the  average  normal  color  vision  as  determined 
by  Ives'  criterion  and  using  a  flicker  photometer. 

CROVA'S  METHOD 

Ives4  has  shown  that  an  incandescent  gas  mantle  can  be  compared 
without  error  with  a  4-wpc.  carbon  lamp  using  an  ordinary  photo- 
meter and  interposing  between  the  eye  and  the  photometer  a  25 
mm.  layer  of  the  first  of  the  following  solutions.  To  effect  a  com- 
parison between  a  4-wpc.  carbon  lamp  and  other  incandescent  electric 
illuminants  the  second  of  the  following  solutions  is  used: 


For  mantle 
burners 

For  incandescent 
electric  lamps 

Cupric  chloride  
Potassium  bichromate 

90  grams 
•?o  grams 

86  grams 
60  grams 

Nitric  acid  (1.05  gravity)  
Water  added  to  make  one  liter. 

40  c.c. 

40  c.c. 

When  using  the  first  solution  with  a  mantle  burner  against  a 
4.85-w.p.scp.  carbon  standard,  the  standard  has  a  value  which  is 
one  divided  by  1.065  times  its  true  value.  No  correction  is  neces- 
sary in  using  the  second  solution.  The  use  of  this  solution  has  the 
great  advantage  of  eliminating  not  only  the  color  difference  between 
the  lights  as  seen  in  the  photometer  field,  but  also  the  effects  of  pecu- 

1  Crittenden  and  Richtmyer,  I.  E.  S.  Transactions,  vol.  n,  page  331,  1916. 
4  Ives,  Physical  Review,  page  716,   1915.     Ives  and  Kingsbury,  I.  E.  S.  Transactions, 
vol.  10,  page  716,  1915. 


IO2  ILLUMINATING   ENGINEERING   PRACTICE 

liarities  of  color  vision  on  the  part  of  the  observer.  It  suffers  from 
the  disadvantage,  which  under  many  conditions  is  a  very  serious 
one,  of  cutting  down  the  brightness  of  the  photometer  field  to  about 
one-tenth  the  value  which  it  otherwise  would  have.  This  necessi- 
tates either  a  rearrangement  of  the  photometric  apparatus  so  that 
the  photometric  field  shall  be  much  brighter  than  otherwise  is 
necessary,  a  procedure  which  is  attended  with  certain  practical 
difficulties,  or  requiring  the  observer  to  work  with  a  faint  field  and 
consequently  to  keep  his  eyes  shielded  from  extraneous  light  so  that 
their  photometric  sensibility  may  be  sufficiently  great. 

LIGHT  FILTERS 

The  difficulties  of  heterochromatic  photometry  may  be  effec- 
tually overcome  by  interposing  between  the  photometer  and  one 
of  the  light  sources  a  colored  screen  which  will  cause  the  illumination 
on  both  sides  of  the  photometer  disc  to  have  the  same  color.  The 
use  of  this  expedient  presupposes,  however,  that  the  amount  of 
light  absorbed  by  such  a  filter  when  used  with  the  light  in  question 
is  known.  The  determination  of  the  transmission  factors  of  light 
filters  involves  all  the  difficulties  of  heterochromatic  photometry,  but 
relegates  them  to  the  domain  of  the  standardizing  laboratory,  where 
they  can  be  overcome  by  the  experimental  means  at  hand.  The 
use  of  light  filters,  since  it  reduces  the  practice  of  the  compari- 
son of  lights  of  different  color  to  the  same  degree  of  simplicity 
as  the  comparison  of  lights  of  the  same  color,  and  by  means  at 
once  convenient  and  free  from  liability  to  error,  is  becoming  very 
extended  and  may  be  rightly  described  as  the  most  commonly 
accepted  method  in  practical  photometry.  These  light  filters  may 
be  of  translucent  solids  or  may  be  in  the  form  of  solutions.  Ives 
and  Kingsbury5'6  have  investigated  yellow  and  blue  solutions  for 
use  in  this  way  and  have  given  equations  whereby  their  transmission 
may  be  computed.  Such  solutions  may,  with  suitable  precautions 
be  used  as  reference  standards.  Mees7  has  produced  a  line  of  care- 
fully constructed  light  filters  using  colored  gelatins,  these  filters 
covering  the  entire  range  of  the  ordinary  lights  to  be  measured. 
It  is  found  practicable  to  get  colored  glasses  serving  as  light  filters 
for  nearly  all  purposes.  For  instance  a  blue  glass  may  be  obtained 
which  when  interposed  between  a  4-wpc.  standard  and  a  pho- 

*  Ives  and  Kingsbury,  I.  E.  S.  Transactions,  Vol.  9,  page  795,  1914. 

•  Ives  and  Kingsbury,  I.  E.  S.  Transactions,  Vol.  10,  page  253,  1915. 
7  Mees,  I.  E.  S.  Transactions,  Vol.  9,  page  990,  1914. 


SHARP:  MODERN  PHOTOMETRY  103 

tometer  will  give  a  color  match  with  a  i-wpc.  tungsten  lamp,  or  a 
pinkish  glass  may  be  found  which  when  interposed  between  a  i- 
wpc.  tungsten  lamp  will  give  a  color  match  with  a  4-wpc.  carbon 
standard.  Glasses  also  may  be  obtained  to  give  a  color  match  of 
gas-filled  tungsten  lamps  with  vacuum  tungsten  lamps,  etc. 

As  has  been  said  the  calibration  of  these  glasses  rests  with  a 
standardizing  laboratory  and  involves  all  the  difficulties  of  hetero- 
chromatic  photometry.  Through  an  extensive  set  of  measure- 
ments of  certain  light  filters  made  by  a  number  of  laboratories  under 
the  lead  of  the  Bureau  of  Standards,8  certain  light  filters  in  the  pos- 
session of  the  Bureau  of  Standards  have  come  to  have  an  unusually 
accurate  calibration.  It  is  possible  for  other  laboratories  to  have 
standards  calibrated  by  comparison  directly  with  those  at  the  Bureau 
of  Standards  or  indirectly  through  other  laboratories  deriving  their 
standards  from  the  Bureau.  Through  this  procedure  light  filters 
of  carefully  known  value  may  readily  be  obtained  by  any  photo- 
metrist,  and  by  the  use  of  these  filters  the  difficulties  of  hetero- 
chromatic  photometry  can  in  nearly  all  cases  be  overcome  and  the 
same  degree  of  concordance  attained  in  the  photometry  of  different 
colored  lights  which  is  expected  in  the  photometry  of  lights  of  the 
same  color. 

EXTRAPOLATION  OF  LAMP  VALUES 

Middlekauff  and  Skogland9  have  shown  that  a  curve  or  equation 
giving  the  relation  between  the  voltage  and  current  or  candle- 
power  of  tungsten  lamps  can  be  established  which  holds  within  close 
limits  for  tungsten  filament  lamps  of  all  ordinary  sizes  and  styles 
of  construction;  so  that  knowing  the  candle-power  of  any  normal 
tungsten  lamp  by  calibration  at  a  voltage  at  which  its  color  matches 
the  color  of  the  standard,  its  candle-power  at  some  other  voltage  at 
which  it  gives  a  color  corresponding  to  the  lamp  under  test  may  be 
accurately  computed.  This  method,  which  has  the  endorsement  of 
the  U.  S.  Bureau  of  Standards,  should  be  of  great  practical  utility. 

STANDARD  LAMPS 

Since  1910  the  drawn  wire  tungsten  lamp  has  supplanted  the 
pressed  filament  lamp,  and  lamps  of  drawn  wire  are  now  used  for 
purposes  of  photometric  standards.  In  the  smaller  sizes  of  lamps 

»  Middlekauff  and  Skogland,  I.  E.  S.  Transactions,  Vol.  9,  page  734;  also  Bulletin  of 
Bureau  of  Standards,  Vol.  3,  p.  287. 

•  Middlekauff  and  Skogland,  I.  E.  S.  Transactions,  Vol.  11,  page  164;  also  Bulletin  of 
Bureau  of  Standards,  Vol.  n,  p.  483. 


104  ILLUMINATING  ENGINEERING   PRACTICE 

small  variations  in  candle-power  are  likely  to  be  discovered  due  to  the 
variations  in  contact  between  the  filament  and  the  wire  supports. 
It  is  therefore  necessary  that,  for  the  smaller  sizes  at  least,  lamps 
should  be  of  special  construction,  avoiding  this  variation  in  contact 
with  its  consequent  variable  loss  of  heat  to  the  anchor  wires.  Either 
the  anchor  wires  are  pinched  tightly  over  the  filament  or  the  filament 
is  drawn  so  tightly  over  the  wires  that  no  variability  can  ensue.  The 
constancy  of  the  candle-power  of  the  drawn  wire  lamps,  together  with 
their  mechanical  strength,  etc.,  is  such  as  to  fit  them  eminently 
well  for  service  as  standards.  They  may  be  standardized  not 
only  at  a  voltage  approximating  their  operating  voltage,  but  also 
at  lower  voltages;  for  instance  at  such  a  voltage  that  they  give  a 
color  match  with  a  4-wpc.  carbon  standard.  It  is  a  question  whether 
all  things  considered,  the  tungsten  standards  are  not  more  reliable 
than  the  old  carbon  standards,  but  the  time  has  not  yet  come 
when  this  question  can  be  finally  answered.  It  is  to  be  noted,  how- 
ever, that  inasmuch  as  the  incandescent  lamps  most  used  to-day  are 
of  the  tungsten  class,  the  use  of  tungsten  standards  enables  photo- 
metric measurements  to  be  made  without  the  difficulties  of  hetero- 
chromatic  photometry.  A  photometric  laboratory  may  carry  side 
by  side  a  series  of  4-wpc.  carbon  standards  and  a  series  of  ap- 
proximately i.2-wpc.  tungsten  standards,  each  set  of  standards  to 
be  used  with  its  corresponding  class  of  lamps.  The  introduction 
of  the  gas-filled  lamp,  however,  has  given  rise  to  a  situation  where 
heterochromatic  photometry  is  difficult  to  avoid.  The  filaments  of 
these  lamps  are  made  up  in  the  form  of  fine  spirals.  The  candle- 
power  of  a  spiral  wound  filament  can  vary  not  only  because  of 
alteration  in  its  physical  state  or  in  the  conditions  surrounding  it 
in  the  bulb  (convection  currents,  etc.)  but  also  on  account  of  any 
change  in  the  spacing  of  the  spires  of  the  helices  in  which  the  filament 
is  formed.  If  on  account  of  sagging  at  the  high  temperature  at  which 
the  filaments  are  operated,  the  little  spirals  open  up  somewhat  at 
any  point,  the  candle-power  will  be  found  to  be  reduced  at  this 
point,  since  there  the  convection  currents  carry  off  more  heat. 
Moreover,  the  question  of  the  conduction  of  the  heat  from  the  filament 
by  the  anchor  wires  is  one  which  may  intervene  to  cause  variable 
candle-power.  Hence  it  is  that  it  is  a  more  difficult  thing  to  get  from 
gas-filled  lamps  the  entire  constancy  of  candle-power  at  given  voltage 
or  current  which  is  demanded  of  a  real  standard.  Lamps  of  this 
type  are  sometimes  calibrated  as  "  check  lamps,"  intended  to  be  used 
as  standards  in  the  industrial  photometry  of  gas-filled  lamps,  but 


SHARP:  MODERN  PHOTOMETRY  ,      105 

not  dignified  with  the  name  of  standards.  It  is  to  be  hoped  that 
methods  of  construction  will  be  found  whereby  entire  constancy  may 
be  insured  in  the  candle-power  of  gas-filled  lamps  specially  designed 
for  use  as  standards.  Until  this  is  done  the  real  standards  against 
which  gas-filled  lamps  have  to  be  compared  are  vacuum  tungsten 
lamps  and  this  comparison  involves  color  differences  which,  however, 
can  be  removed  by  the  use  of  suitable  light  filters. 

lo-C.P.  HARCOURT  LAMP 

This  important  primary  standard  has  been  subjected  to  thorough 
investigation  at  the  Bureau  of  Standards  and  there  has  been  found 
a  well-defined  difference  between  the  pentane  lamps  of  English  and 
American  manufacture.  Moreover,  the  newer  American  lamps  are 
differentiated  from  the  older  ones  by  certain  operating  requirements. 
For  instance,  the  time  required  for  the  lamp  to  reach  its  full  candle- 
power  is  less  than  15  minutes  in  the  case  of  the  English  lamp,  whereas 
20  minutes  must  be  allowed  with  the  newer  American  lamps  and 
30  minutes  with  the  older  ones.  The  Bureau  authorities  have  found 
that  the  control  of  the  density  of  the  pentane  is  of  considerable 
importance  and  that  to  empty  the  saturater  once  a  month,  as 
should  be  done  according  to  the  instructions  of  the  London  Gas 
Referees,  is  quite  insufficient  when  the  lamp  is  used  as  much  as  three 
times  a  day,  since  the  density  of  the  residual  pentane  would  be 
considerably  greater  and  its  candle-power  greater.  At  the  Bureau  of 
Standards  the  density  of  the  pentane  used  is  always  kept  below 
0.635.  The  saturater  should  be  from  one- third  to  two- thirds  full 
of  pentane  at  starting,  and  the  height  of  liquid  as  seen  against  the 
window  of  the  saturater  should  never  be  less  than  ^  inch.  It  is 
recommended  that  in  the  photometer  room  a  hood  or  chimney  should 
be  arranged  in  the  ceiling  above  the  lamp  in  such  a  way  as  to  carry 
the  products  of  combustion  directly  out  of  the  room.  The  correc- 
tion for  water  vapor  is  made  in  accordance  with  the  following: 

/  =  78[/  +  (8  -  /)o.oos67]. 

Where  7g  represents  the  candle-power  of  the  lamp  with  normal  water 
vapor  content,  namely,  8  liters  of  water  per  cubic  meter  of  dry  air, 
and  /  represents  the  actual  humidity.  In  order  to  determine  the 
value  /  a  hygrometer  of  the  wet  and  dry  bulb  type  is  used.  The 
most  precise  instrument  is  the  Assmann  psychrometer  which  con- 
sists of  two  finely  divided  mercury  thermometers,  mounted  side 
by  side  on  a  stand  and  with  a  tube  surrounding  the  bulb  of  each. 


106         ,  ILLUMINATING   ENGINEERING   PRACTICE 

At  the  top  is  a  small  spring-driven  suction  pump  which  draws  a 
rapid  current  of  air  over  both  bulbs.  One  bulb  is  surrounded  by  a 
cloth  which  is  wet  with  water,  while  the  other  is  dry.  From  the 
difference  between  the  readings  of  the  two,  by  reference  to  hygro- 
metric  tables,  such  as  for  instance  the  tables  issued  by  the  United 
States  Weather  Bureau,  the  pressure  of  the  water  vapor  is  determined. 
A  simpler  apparatus  than  the  Assmann  psychrometer  is  the  sling 
psychrometer,  used  extensively  by  the  Weather  Bureau.  This  is 
a  relatively  inexpensive  apparatus  consisting  of  two  thermometers 
mounted  on  a  handle  so  that  they  can  be  swung  rapidly  in  a  circle. 
One  of  them  being  wet  and  the  other  dry,  a  difference  is  obtained 
which  corresponds  to  the  humidity  of  the  atmosphere.  Knowing  e, 
the  partial  pressure  of  the  water  vapor,  and  the  barometric  height,  b, 
in  millimeters,  the  water  vapor  in  liters  per  cubic  meter  of  dry  air 
is  found  from  the  equation 

£> 

I  =T—     ~  X   1000 

b  —  e 

The  effect  of  variation  in  atmospheric  pressure  on  the  candle-power 
of  flame  standards  has  been  investigated  by  Butterfield,  Haldane 
and  Trotter  in  London  and  also  by  Ott10  in  Zurich.  Ott  confirmed 
the  old  formula  of  Liebenthal,  as  follows: 

/  =  1.049  ~~  °-°°55^  H~  0.06011(6  —  760) 

Butterfield,  Haldane  and  Trotter  found  a  relation  which  is  depicted 
in  curves  of  Fig.  i. 

A  late  investigation  by  the  U.  S.  Bureau  of  Standards11  yielded 
the  curves  of  Fig.  2. 

The  variation  of  standard  flames  with  barometric  pressure  is  of 
vital  importance  when  dealing  with  these  standards  in  places  located 
at  considerable  altitudes  above  sea  level,  and  affects  us  particularly 
in  this  country  where  there  are  a  number  of  important  cities  at 
relatively  high  altitudes. 

PORTABLE  ELECTRIC  STANDARD 

A  portable  electric  standard  lamp  outfit  which  has  been  used  to  a 
limited  extent  in  gas.  photometry  is  illustrated  in  Fig.  3.  The  lamp 
used  has  a  single  loop  tungsten  filament  and  is  of  such  a  rating  that 
when  burned  so  as  to  give  two  candle-power,  it  has  a  color  which 

10  Ott,  Journal  of  Gas  Lighting,  Nov.  16,  1915. 

11  Transactions  I.  E.  S.,  Vol.  X,  page  843,  1915. 


SHARP:  MODERN  PHOTOMETRY 


107 


,110 


iioo 


£ 


J£ 


40 


100 


Fig.   i.— 


50  60  70  80  90 

Barometric  Pressure  Cm.  of  Mercury 

Variations  of  flame  candle-power  with  atmospheric  pressure.     (Butterfield,  Haldane 
and  Trotter.) 


40 


110 


50  60  70  80  90          100 

Barometric  Pressure:  -  Cm   of  Mercury 
Fig.  2.  —  Variations  of  flame  candle-power  with  atmospheric  pressure,     (i)  Hefner  lamp;  (2) 
Pentane  lamp;  (3)  No.  7  Bray  slit  union  gas  burner.     (Bureau  of  Standards.) 


io8 


ILLUMINATING   ENGINEERING    PRACTICE 


Fig.   7. — Diagram  of  bridge  of  portable 
standard. 


matches  that  of  gas  burned  in  an  open  burner.  It  then  consumes 
approximately  one  ampere  with  four  volts  potential  difference  be- 
tween its  terminals.  The  current  for  the  lamp  is  furnished  by  a 
portable  six-volt  storage  battery  such  as  is  used  frequently  for  gas- 
engine  ignition  purposes  and  of  40  ampere-hour  rating.  The  bat- 
tery, therefore,  when  fully  charged  is  capable  of  supplying  current  to 
the  lamp  for  a  considerable  time.  On  the  controller  box  are  mounted 
suitable  rheostats  and  also  a  Wheatstone  bridge  and  galvanometer 

for  setting  the  lamp  to  its  standard 
candle-power.  As  is  well  known, 
the  tungsten  filament  has  a  large 
positive  temperature-resistance  co- 
efficient. If  the  lamp,  therefore,  is 
made  one  arm  of  a  Wheatstone 
bridge,  the  other  arms  being  con- 
stituted of  zero  temperature  coeffi- 
cient wire,  as  shown  in  Fig.  7,  the 
resistances  may  be  so  adjusted  that 
the  bridge  is  in  balance  when  the 
lamp  is  operating  at  its  normal 
candle-power.  As  soon  as  the  current  through  the  lamp  varies,  its 
resistance  also  varies  and  the  bridge  falls  out  of  balance.  The 
balance  of  the  bridge  is  indicated  by  a  pivoted  galvanometer  which 
is  shown  in  the  figure,  or  it  may  be  determined  by  means  of  a  tele- 
phone and  an  interrupter  which  gives  a  slight  click  in  the  telephone 
if  the  bridge  is  out  of  balance.  This  method  of  adjusting  the  stand- 
ard lamp  is  a  very  sensitive  one,  particularly  when  a  galvanometer 
is  used;  far  more  sensitive  than  a  direct  reading  ammeter  or  volt- 
meter, even  of  the  laboratory  standard  type.  In  order  to  permit 
adjustment,  there  is  a  portion  of  the  bridge  in  the  form  of  a  slide 
wire,  the  galvanometer  circuit  making  contact  with  this  slide  wire 
at  any  position  desired.  The  position  may  be  recorded  from  a 
divided  scale  with  which  the  slide  wire  is  equipped. 

The  apparatus  was  given  the  form  here  described  in  order  that  an 
unskilled  man  unfamiliar  with  electrical  apparatus  should  be  able 
to  operate  the  standard.  He  has  merely  to  close  the  switch  and 
adjust  the  rheostat  until  the  galvanometer  comes  to  zero.  It 
should  be  noted,  however,  that  in  a  general  case  a  test  of  candle- 
power  of  gas  will  agree  with  a  test  made  against  a  lo-c.p.  pentane 
lamp  only  when  the  conditions  of  atmospheric  humidity  are  standard 
for  the  lamp,  that  is,  eight  liters  of  water  per  cubic  meter  of  dry  air. 


Fig.  3. — Portable  electric  standard. 


Fig.  4. — One-meter  sphere  for  industrial  photometry  of  incandescent  lamps. 

(Facing  page   107.) 


Fig.  s.— Industrial  sphere  photometer,  showing  arrangement  of  sphere  doors  and  photom- 
eter scale. 


FIG.  6. — Sharp- Millar  calibrator  in  position  on  photometer. 


SHARP:  MODERN  PHOTOMETRY  109 

If  the  hygrometric  condition  is  different  from  this  a  different  result 
will  be  obtained.  It  is,  therefore,  necessary  in  using  the  electric 
standard  to  observe  with,  for  example,  a  sling  psychrometer  the 
hygrometric  condition,  and  to  apply  a  correction  to  the  invariable 
electric  standard  to  make  it  agree  with  what  the  pentane  standard 
would  show  under  similar  conditions.  The  operation  involved  in 
this  correction  can  be  reduced  to  great  simplicity.12 

Total  Flux  Standards. — Standards  of  flux  or  of  mean  spherical 
candle-power  have  to  be  derived  from  ordinary  standards  of  candle- 
power.  In  the  primary  standardization  of  lamps  in  lumens,  there- 
fore, it  is  necessary  to  adopt  some  method  whereby  the  total  lumi- 
nous flux  can  be  computed  from  a  series  of  candle-power  measure- 
ments in  a  sufficient  number  of  directions.  In  the  practice  of  the 
Electrical  Testing  Laboratories  in  making  lumen  standards,  the 
lamp  is  first  standardized  carefully  as  an  ordinary  standard  for 
mean  horizontal  candle-power.  Then  its  candle-power  distribution 
curve  is  determined  by  measurements  at  various  angles  in  the  verti- 
cal plane  and  the  lamp's  spherical  reduction  factor,  or  the  relation 
between  its  spherical  candle-power  and  its  horizontal  candle-power 
is  computed.  The  known  horizontal  candle-power  multiplied  by  the 
spherical  reduction  factor  gives  then  the  spherical  candle-power. 
The  latter  multiplied  by  4?r  gives  the  total  lumens.  Having  estab- 
lished standards  by  this  procedure,  copies  sufficiently  accurate  for 
industrial  purposes  can  be  made  by  the  more  direct  method  of 
the  integrating  sphere. 

BAR  PHOTOMETER 

The  bar  photometer,  the  classic  apparatus  of  the  photometrist, 
is  being  used  mgre  and  more  according  -to  methods  which  were  but 
little  recognized  a  few  years  ago.  The  standard  method  in  the  use 
of  the  bar  photometer  was  for  many  years  to  fix  the  light  sources  at 
the  ends  of  the  bar  and  to  move  the  photometer  between  them  until 
the  point  of  balance  was  obtained.  It  is  becoming 'now  more  com- 
mon practice  to  allow  the  photometer  head  to  remain  stationary  and 
at  a  fixed  distance  from  the  light  source  to  be  photometered;  that  is, 
the  test  lamp,  while  the  photometric  balance  is  effected  by  moving 
the  comparison  lamp.  This  method  of  using  the  bar  is  an  almost 
necessary  one  in  the  photometry  of  large  sources  of  light,  particu- 
larly of  lamps  with  reflectors  having  concentrating  properties,  in  the 
photometry  of  projectors,  etc.,  where  it  is  important  to  measure  the 

12  Sharp  and  Schaaf.  American  Gas  Light  Journal,  Vol.  VIII,  p.  325,  1913. 


IIO  ILLUMINATING   ENGINEERING   PRACTICE 

apparent  candle-power  of  a  source  at  a  fixed  distance.  It  has  been 
found  feasible  and  desirable  from  the  point  of  view  of  convenience, 
to  diminish  the  length  of  the  bar  and  this  has  been  made  by  the  use 
of  small  low  voltage  tungsten  filament  lamps  for  comparison  lamps. 
A  low  voltage  tungsten  lamp  has  a  filament  so  small  that  it  can  be 
considered  as  a  point  source  of  light  when  very  much  closer  to  the 
photometer  disc  than  is  possible  with  the  ordinary  lamp.  On  this 
account  the  comparison  lamp  can  be  brought  up  much  closer  to  the 
disc  and  the  whole  bar  very  greatly  shortened  without  any  practical 
decrease  in  accuracy  of  the  apparatus.  In  doing  this  the  bar  photo- 
meter approaches  the  construction  which  is  well  known  in  the  case 
of  portable  photometers  intended  primarily  for  the  measurement  of 
illumination;  in  fact  it  is  found  that  in  a  great  deal  of  practical  work 
a  portable  photometer  may  be  substituted  for  much  more  elaborate 
and  cumbersome  photometer  bars.  In  precision  work  it  is  desirable 
that  the  brightness  of  the  disc  shall  have  a  known  and  constant 
value.  In  order  to  attain  this  condition  the  comparison  lamp  is 
fastened  to  the  carriage  on  which  the  photometer  head  is  mounted 
and  the  distance  of  the  two  from  the  test  lamp  is  varied  in  order  to 
get  the  photometric  balance.  In  this  case  again  the  use  of  the  small 
tungsten  lamp  as  a  comparison  lamp  enables  a  simplification  to  be 
made  in  that  the  lamp  can  be  mounted  on  a  short  arm  which  is 
rigidly  attached  to  the  photometer  carriage,  thereby  avoiding  the 
necessity  of  an  additional  carriage. 

INTEGRATING  SPHERE 

The  use  of  the  integrating  sphere  is  extending  very  rapidly.  The 
Committee  on  Nomenclature  and  Standards  of  the  Illuminating 
Engineering  Society  has  made  recommendations  as  follows: 

Illuminants  should  be  rated  upon  a  lumen  basis  instead  of  a  candle- 
power  basis. 

The  specific  output  of  electric  lamps  should  be  stated  in  terms  of  lumens 
per  watt  and  the  specific  output  of  illuminants  depending  upon  combustion 
should  be  stated  in  lumens  per  British  thermal  unit  per  hour. 

When  auxiliary  devices  are  necessarily  employed  in  circuit  with  a 
lamp,  the  input  should  be  taken  to  include  both  that  in  the  lamp  and 
that  in  the  auxiliary  devices.  For  example,  the  watts  lost  in  the  ballast 
resistance  of  an  arc  lamp  are  properly  chargeable  to  the  lamp. 

The  specific  consumption  of  an  electric  lamp  is  its  watt  consumption  per 
lumen.  "Watts  per  candle"  is  a  term  used  commercially  in  connection 
with  electric  incandescent  lamps,  and  denotes  watts  per  mean  horizontal 
candle. 


SHARP:  MODERN  PHOTOMETRY  in 

These  recommendations  have  been  adopted  by  the  American 
Institute  of  Electrical  Engineers  and  by  the  National  Electric  Light 
Association.  The  measurement  of  the  horizontal  candle-power  of 
gas-filled  lamps  has,  for  reasons  which  are  discussed  later,  been  found 
unsatisfactory.  All  of  these  facts  tend  to  bring  the  integrating 
sphere  into  a  position  of  greater  importance  in  practical  photometry. 
Little  has  been  added  to  the  theory  of  the  integrating  sphere  or 
to  the  principles  of  its  practice  since  Ulbricht's  treatment  of  the 
same,  but  there  has  been  a  considerable  development  of  the  sphere 
in  the  way  of  making  it  a  more  practical  apparatus  for  routine 
photometric  work.  Inasmuch  as  the  theory  of  the  sphere  was 
merely  hinted  at  in  the  1910  lectures,  it  may  be  well  here  to  say 
more  about  it. 

It  has  been  shown  that  a  diffusing  glass  window  on  the  surface 
of  the  sphere  is  illuminated  by  each  element  of  surface  of  the  sphere 
to  a  degree  dependent  only  on  the  brightness  of  that  element  and 
independent  of  its  position.  This  presupposes  that  the  direct  light 
of  the  lamp  in  the  sphere  is  not  allowed  to  shine  on  the  window. 
No  other  form  of  enclosure,  such  as  a  box,  conforms  to  this  theoretical 
law  and  hence  all  other  forms  are  imperfect  integrators  as  com- 
pared with  the  sphere. 

To  prevent  the  direct  light  of  the  lamp  from  falling  on  to  the 
window  a  white  diffusing  screen  is  interposed.  The  presence  of  the 
screen  is  a  disturbing  factor  in  the  sphere  for  two  reasons.  First, 
the  light  from  the  lamp  falling  directly  on  the  screen  must  be  re- 
flected from  the  latter  before  it  can  reach  the  sphere  and  hence  this 
part  of  the  flux  is  diminished  by  the  absorption  of  the  screen  before 
it  comes  to  the  sphere  surface  from  which  it  is  reflected  to  the 
window.  Second,  a  portion  of  the  sphere  surface  is  hidden  from 
the  window  by  the  screen,  and  the  light  falling  on  this  portion  must 
be  reflected  before  it  reaches  a  part  of  the  sphere  which  is  reflecting 
directly  on  the  window.  Hence  this  portion  of  the  total  flux  suffers 
a  diminution  due  to  the  absorption  of  the  sphere  coating.  As  a 
partial  compensation  for  these  two  losses  we  have  the  light  from  the 
sphere  reflected  by  the  side  of  the  screen  turned  toward  the  window. 

It  is  not  difl&cult  to  calculate  the  flux  falling  directly  on  the  screen 
and  the  flux  on  the  hidden  part  of  the  sphere.  The  position  of  the 
lamp  and  of  the  screen  should  be  such  as  to  make  the  sum  of  these 
two  elements  of  flux  a  minimum.  As  a  practical  matter  the  amount 
of  this  re-reflected  flux  depends  on  the  distribution  of  flux  from  the 
lamp  and  hence  the  position  of  the  screen  most  favorable  for  one 


112 


ILLUMINATING   ENGINEERING   PRACTICE 


lamp  would  not  be  best  for  another.  In  the  case  of  incandescent 
lamps  in  general  the  best  position  of  the  window  is  at  the  top  of 
the  sphere  with  the  lamp  vertical  on  the  vertical  diameter,  for  in 
this  position  the  lamp  base  which  casts  a  shadow  anyhow,  casts  it 
on  the  screen  and  so  the  flux  on  the  screen  is  less  than  it  would  be  in 
any  other  position.  This  arrangement  is  inconvenient  and  should 
be  resorted  to  only  when  the  highest  precision  is  desired.  In  any 
case  the  screen  error  can  be  made  a  very  small  one  by  using  a  sphere 
of  adequate  size.  Increase  in  the  size  of  the  sphere  reduces  the  error 

in  two  ways;  first,  by  reducing 
the  relative  area  of  the  screen 
as  compared  with  that  of  the 
sphere,  and  second  by  per- 
mitting the  screen  to  be  placed 
further  from  the  lamp  whereby 
the  flux  on  it  is  decreased.  If 
the  lamp  is  not  too  near  to 
the  wall  of  the  sphere  and  to 
the  screen  and  the  sphere  is  of 
sufficient  size,  no  danger  of  an 
excessive  screen  error  is  to  be 
apprehended.  In  any  case 
the  use  of  the  substitution 
method  when  lamps  of  more 
or  less  similar  candle-power  distribution  characteristics  are  being 
photometered,  ensures  the  partial  or  complete  elimination  of  the 
error. 

When  bulky  lamps,  such  as  arc  lamps,  or  lamps  with  shades  or 
reflectors,  are  to  be  photometered,  the  sphere  must  be  standardized 
or  its  constant  determined  with  the  test  lamp  in  place  in  the  sphere. 
This  requires  that  the  test  lamp  and  the  standard  lamp  have  separate 
locations  in  the  sphere.  The  standard  lamp  should  be  left  in  place 
in  the  sphere  while  the  test  lamp  is  being  measured.  There  must 
be  a  screen  between  the  standard  lamp  and  the  window  and  another 
between  the  test  lamp  and  the  window.  There  should  also  be  a 
screen  protecting  the  test  lamp  and  its  parts  from  the  direct  light 
of  the  standard  lamp.  A  scheme  of  the  arrangement  is  shown  in 
Fig.  8.  The  reason  for  the  latter  screen  is  as  follows:  The  test  lamp 
emits  a  certain  flux  of  light.  The  parts  of  the  lamp,  which  are  foreign 
bodies  or  obstacles  in  the  sphere,  interrupt  and  absorb  a  certain 
fraction  of  this  flux,  which,  therefore,  never  escapes  from  the  confines 


Fig.   8. — Integrating  sphere  with  large  lamp 
and  screen. 


SHARP:  MODERN  PHOTOMETRY  113 

of  the  lamp  as  useful  light,  and  should  not  be  measured.  The 
lamp  parts,  however,  interrupt  a  portion  of  the  reflected  flux  in  the 
sphere  which  otherwise  would  increase  the  brightness  of  the  window 
and  thus  needs  to  be  accounted  for.  If  the  direct  light  of  the 
standard  lamp  is  screened  off,  the  parts  or  appurtenances  of  the 
test  lamp  will  absorb  approximately  the  same  fraction  of  the  re- 
flected light  of  the  standard  lamp  as  that  of  the  test  lamp,  and  no 
great  error  is  incurred. 

To  determine  the  effect  of  the  added  screen  between  the  standard 
lamp  and  the  test  lamp,  a  photometer  setting  is  made  with  the  sphere 
containing  only  the  standard  lamp  with,  and  again  without,  the 
screen;  in  other  words  the  total  flux  of  the  lamp  with  the  screen  is 
measured  against  itself  without  the  screen  as  standard. 

The  improvements  in  the  integrating  sphere  have  been  in  the  direc- 
tion of  providing  for  easy  and  quick  introduction  and  removal  of 
lamps  and  in  the  adaptation  of  the  sphere  to  the  photometering 
of  gas  lamps.  Speaking  of  the  latter  point  first,  it  has  been  found 
that  with  a  sphere  of  ample  size,  from  2.0,  to  2.5  meters  in  diame- 
ter, provided  with  suitable  ventilating  openings  at  the  top  and 
bottom,  gas  lamps,  even  of  very  large  size,  can  be  photometered 
without  difficulty.  The  ventilating  openings  need  to  be  made  so 
as  to  rob  the  sphere  of  as  little  of  its  white  interior  surface  as  possible, 
and  at  the  same  time  to  prevent  the  escape  of  light  from  the  interior 
and  the  ingress  of  light  from  the  exterior.  This  is  quite  simply 
done  by  covering  the  opening  with  a  circular  disc  set  down  a  short 
distance  from  the  surface,  so  as  to  leave  a  sufficient  passage  for  the 
air,  while  cutting  off  the  light. 

To  facilitate  the  handling  of  incandescent  lamps,  a  number  of 
plans  have  been  employed.  One  quick  handling  device  for  the  sphere 
intended  for  the  photometering  incandescent  lamps  was  treated 
of  in  the  1910  lectures.  A  device  has  been  used  at  the  U.  S.  Bureau 
of  Standards  and  at  the  Physical  Laboratory  of  the  National  Lamp 
Works  of  the  General  Electric  Company,  whereby  the  act  of  opening 
the  door  of  the  sphere  swings  an  arm  carrying  a  lamp  socket  out  to 
the  opening  so  that  lamps  are  readily  changed.  When  the  door  is 
closed  this  arm  swings  back  again  and  places  the  lamp  at  the  center 
of  the  sphere.  A  complete  sphere  photometer  has  been  designed  and 
constructed  at  the  Electrical  Testing  Laboratories  which  seems  to 
meet  the  requirements  of  routine  photometry  of  incandescent  lamps 
of  all  sizes.  Inasmuch  as  no  other  device  of  this  kind  seems  to  have 
been  described  in  the  literature,  a  fairly  complete  description  may  be 

8 


114  ILLUMINATING  ENGINEERING   PRACTICE 

given  here  (Figs.  4  and  5)  .  The  sphere  is  of  one  meter  diameter  and 
has  been  variously  constructed  of  sheet  metal,  of  cast  aluminum  and 
cast  iron.  The  cast  aluminum  is  the  most  desirable  material,  but 
the  price  of  it  at  the  present  time  is  almost  prohibitive.  The  cast 
sphere  is  mounted  on  three  legs  of  two-inch  iron  pipe  with  floor 
flanges,  and  all  of  the  auxiliary  parts  are  screwed  or  bolted  to  the 
sphere  itself  which,  therefore,  forms  the  carcass  or  frame  of  the  in- 
strument. Referring  to  Fig.  5  an  opening  about  40  by  58  centi- 
meters is  cut  out  of  the  sphere  and  two  doors,  either  of  which  will 
fit  this  opening,  are  mounted  on  a  vertical  shaft.  To  each  of  these 
doors  is  fastened  a  bracket  carrying  a  lamp  socket.  Thus  when  the 
opening  of  the  sphere  is  filled  by  one  of  the  doors,  the  lamp  socket 
attached  to  it  is  in  the  sphere  carrying  the  lamp  to  be  photometered, 
while  the  other  lamp  socket  is  'outside  the  sphere  ready  to  have  its 
lamp  inserted.  When  the  photometering  of  the  lamp  in  the  sphere 
is  completed,  the  vertical  shaft  is  rotated  a  half  turn,  whereby  the 
already  photometered  lamp  is  withdrawn  from  the  sphere  and  the 
one  to  be  photometered  is  put  inside.  By  means  of  a  special 
switch  attached  to  the  vertical  shaft  the  lamp  inside  the  sphere  is 
automatically  connected  to  the  source  of  current.  Thus  while  the 
lamp  in  the  sphere  is  being  photometered,  the  lamp  which  has  been 
photometered  can  be  removed  and  a  fresh  one  substituted  for  it. 
Very  heavy  filament  lamps  require  the  current  to  be  flowing  through 
them  for  a  little  time  until  they  reach  their  ultimate  temperature, 
and  consequently  their  ultimate  candle-power.  For  instance,  gas- 
filled  lamps  of  20  amperes  rating  should  be  photometered  after  they 
have  been  heating  for  at  least  one  minute.  With  the  sphere  here 
described  an  auxiliary  preheating  circuit  can  be  attached  so  that 
the  lamp  which  is  outside  the  sphere  is  being  heated  during  part  of 
the  time  when  the  lamp  inside  the  sphere  is  being  photometered. 

The  photometric  arrangements  are  made  part  of  the  sphere  itself. 
The  photometer  bar  which  is  supported  by  a  cast-iron  bracket  per- 
mits the  travel  of  the  comparison  lamp  over  a  distance  of  46.3 
centimeters.  The  comparison  lamp  is  a  low  voltage  tungsten  lamp 
so  selected  that  it  will  have  the  requisite  candle-power  when  it  is 
operating  at  an  efficiency  which  will  cause  it  to  match  in  color  the 
regular  vacuum  tungsten  filament  lamps.  There  are  four  scales  to 
the  photometer,  giving  the  three  following  ranges:  30  to  240 
lumens,  200  to  1600  lumens  and  1200  to  9600  lumens.  These  scales 
are  directly  above  each  other  and  are  made  by  contact  printing  on  a 
photographic  plate.  The  scales  are  translucent  and  are  read  by  the 


SHARP:  MODERN  PHOTOMETRY  115 

photometer  operator  who  changes  the  lamp  and  who  sees  the  shadow 
of  a  stretched  wire  on  the  scale  thrown  by  the  test  lamp  itself.  The 
photometer  operator  who  makes  the  settings  is  ignorant  of  the  scale 
readings  and  is  thereby  protected  from  any  possible  bias.  Only 
one  scale  is  visible  at  a  time,  the  rest  being  covered  by  a  movable 
shutter.  In  order  to  enable  the  sphere  to  be  used  without  change 
of  calibration  of  the  standard  lamp,  the  following  arrangement  is 
employed. 

Over  the  diffusing  window  of  the  sphere,  which  has  a  diameter  of 
8  centimeters,  is  placed  a  hemisphere  of  the  same  diameter.  >  This 
hemisphere  contains  in  turn  a  small  diffusing  window  which  consti- 
,  tutes  one  side  of  the  photometer  disc.  In  a  narrow  slot  between  the 
hemisphere  and  the  diffusing  window  of  the  large  sphere  is  placed 
a  slide  with  four  openings.  The  largest  of  the  openings  has  the 
same  diameter  as  the  hemisphere.  The  next  opening  has  such  a 
diameter  that  when  it  is  introduced,  the  brightness  of  the  window 
of  the  small  hemisphere  is  cut  down  to  such  a  degree  that  the  pho- 
tometer is  direct  reading  on  the  second  of  its  scales  rather  than  on  its 
first.  Inasmuch  as  the  first  scale  reads  from  30  lumens  to  240  lumens 
and  the  second  scale  from  200  lumens  to  1600  lumens,  the  amount 
of  brightness  reduction  on  interposing  the  second  opening  is  in  the 
ratio  of  20  to  3.  The  first  scale  enables  vacuum  tungsten  lamps  of 
the  smaller  sizes  (7^  to  25  watt)  to  be  photometered.  For  larger 
lamps  it  is  necessary  to  go  to  the  second  scale  which  covers  the  range 
approximately  of  25  watts  to  200  watts.  Hence  by  the  use  of  these 
two  ranges  all  of  the  ordinary  sizes  of  vacuum  tungsten  lamps  are 
covered.  The  other  two  apertures  are  intended  for  the  photometry 
of  gas-filled  lamps  where  the  whiter  color  of  the  lamp  introduces  an 
additional  photometric  difficulty.  To  reduce  this  color  to  the  color 
of  the  comparison  lamp,  filters  of  pinkish  glass  are  placed  in  aper- 
tures 3  and  4.  These  apertures  again  are  so  dimensioned  that  the 
photometer  is  direct  reading  without  readjustment  of  the  comparison 
lamp.  The  scale  used  with  aperture  3  is  identical  with  the  scale 
used  with  aperture  2.  The  scale  used  with  aperture  4  has  a  range  of 
1 200  lumens  to  9600  lumens  and  covers  therefore  gas-filled  lamps 
up  to  500  watts.  For  still  larger  lamps  an  additional  diaphragm  is 
placed  in  aperture  4  and  the  range  thereby  extended  to  take  in  1000- 
watt  gas-filled  lamps.  Hence  with  one  and  the  same  setting  of  the 
comparison  lamps  any  ordinary  incandescent  lamp  may  be  read 
directly.  The  slide  containing  the  apertures  1,2,3  and  4  is  mechanic- 
ally connected  to  the  shutter  over  the  scales,  so  that  when  the 


n6 


ILLUMINATING    ENGINEERING   PRACTICE 


slide  is  moved,  the  shutter  is  also  moved  to  expose  the  proper 
scale. 

Photometric  measurements  are  made  by  the  aid  of  a  Lummer- 
Brodhun  prism.  The  comparison  lamp  is  held  at  its  standard  value 
by  means  of  a  Wheatstone  bridge  arrangement.  Such  is  described 
above  under  the  Portable  Electric  Standard.  With  this  apparatus 
it  is  found  that  incandescent  lamps  can  be  photometered  more 
rapidly  than  on  a  photometer  bar  with  a  rotator. 

APPLICATION  TO  THE  PHOTOMETERING  OF  STREET  LAMPS 

The  sphere  has  also  been  used  in  practice  in  the  determination  of 
the  candle-power  of  street  lamps.  The  photometering  of  lamps  in 
the  street  by  ordinary  methods  is  admittedly  unsatisfactory.  How- 
ever, by  the  use  of  the  arrangement  which  is  referred  to  in  the  first 
lecture  on  Street  Lighting,  where  a  i  meter  sphere  is  mounted  on 
a  truck  and  brought  directly  underneath  the  lamp  which  is  then 
lowered  into  the  sphere,  this  class  of  measurement  is  made  practically 
as  accurate  as  an  indoor  measurement. 

INSTRUMENTS  FOR  MEASUREMENT  OF  ILLUMINATION 

In  addition  to  the  instruments  described  in  the  1910  lectures 
certain  new  ones  have  entered  the  field  and  will  here  be  described. 


Fig.  9. — Sectional  view  of  Macbeth  illuminator. 

Macbeth  Illuminometer . — This  is  a  small,  light  weight  instrument 
differing  only  in  details  of  construction  but  not  in  principle  from  other 
well-known  instruments.  As  shown  in  Fig.  9  the  photometric 
device  is  a  Lummer-Brodhun  cube  which  is  looked  at  through  a  lens. 
A  lamp  is  carried  on  a  rod  projecting  from  the  end  of  the  tube  and 
can  be  moved  back  and  forth  by  means  of  a  rack  and  pinion.  The 


SHARP:  MODERN  PHOTOMETRY 


117 


scale  is  drawn  on  the  exposed  portion  of  this  rod.  The  light  from 
the  lamp  falls  on  a  small  translucent  screen  which  is  seen  on  one 
side  of  the  field.  The  other  side  of  the  field  is  a  reflecting  test- 
plate  located  at  the  point  where  the  illumination  is  to  be  measured. 
The  calibration  of  the  scale  is  in  accordance  with  the  inverse  square 
law,  the  theoretical  law  of  the  instrument.  A  small  housing  about 
the  lamp  provides  for  the  exclusion  of  stray  light  from  the  sides  of 
the  tube.  The  lamp  is  held  at  standard  condition  by  means  of  an 
ammeter  which  is  contained  in  a  separate  box  which  also  carries  the 
necessary  rheostats.  With 
this  instrument  is  provided  a 
so-called  reference  standard 
which  is  shown  in  cross-section 
in  Fig.  10.  This  reference 
standard  is  arranged  to  be 
placed  on  the  reflecting  test- 
plate,  and  the  tube  surrounding 
the  sighting  aperture  is  inserted 
at  D  so  that  this  test-plate  may 
be  viewed  under  the  light  of 
the  small  standardized  lamp 
contained  in  the  reference 
standard.  When  a  given  cur- 
rent is  passed  through  the 
standard  lamp,  known  illumina- 
tion is  produced  on  the  test- 
plate,  and  against  this  known 
illumination  the  photometer 
can  be  standardized.  It  is  to 
be  noted,  however,  that  any 
error  of  the  ammeter  is  involved 

in  the  use  of  the  reference  standard  and  hence  the  necessity  for 
maintaining  the  ammeter  in  correct  calibration.  The  range  of 
the  Macbeth  illuminometer  is  normally  from  i  to  25  foot-candles. 
It  is  increased  by  the  insertion  of  neutral  glass  screens  on  either  one 
side  or  the  other  of  the  Lummer-Brodhun  cube.  The  total  range 
of  the  instrument  with  the  two  screens  ordinarily  provided  with  it 
is  said  to  be  from  about  0.02  to  1200  foot-candles. 

Sharp-Millar  Photometer — Small  Model. — In  this  smaller  model  of 
the  photometer  described  in  the  1910  lectures  (see  Fig.  n),  the  size 
has  been  reduced  to  approximately  12}^  inches  in  length  and  2*/£ 


Fig.   10. — Section  view  of   Macbeth  reference 
standard. 


n8 


ILLUMINATING   ENGINEERING   PRACTICE 


by  2^  inches  in  cross-section.  The  box  is  of  metal  rather  than  of 
wood  and  the  scale  which  is  used  has  also  been  reduced  to  one-half. 
Like  the  previous  instrument,  this  is  adapted  to  the  measurement  of 
illumination,  candle-power  and  surface  brightness.  Instead  of 
using  an  ammeter  or  a  voltmeter  for  holding  the  comparison  lamp 
at  its  proper  candle-power,  a  Wheatstone  bridge  arrangement  such 
as  is  described  above  under  the  portable  electric  standard  is  used. 
Instead  of  a  galvanometer  with -the  bridge,  a  telephone  receiver  is 
used  as  a  detecting  instrument.  If  the  bridge  is  out  of  balance, 
making  and  breaking  the  circuit  through  the  telephone  gives  a 
series  of  audible  clicks.  When  the  current  is  reduced  to  zero,  as  is 


Fig.   ii. — Sharp- Millar  photometer,  small  model. 

indicated  by  a  state  of  silence  in  the  telephone,  the  proper  current 
is  flowing  through  the  comparison  lamp.  The  telephone  method, 
while  not  so  sensitive  as  the  galvanometer  method,  yet  is  sufficiently 
sensitive  so  that  a  careful  observer  can  set  the  current  to  its  correct 
value  within  closer  limits  than  is  possible  with  a  small  portable 
ammeter  or  voltmeter.  The  use  of  the  bridge  and  telephone  en- 
hances the  portability  of  the  instrument  very  greatly  inasmuch  as 
no  other  auxiliary  apparatus  is  required  than  two  dry  cells  or  a 
small  storage  cell  for  the  purpose  of  furnishing,  the  current.  The 
instrument  is  provided  with  the  usual  absorbing  screens  for  extend- 
ing its  range  and  can  easily  be  held  in  the  hand  when  used.  Either 
an  attached  transmitting  test-plate  or  a  detached  reflecting  test- 
plate  may  be  used. 

Calibrator. — For  use  with  the  above  instrument  and  also  with  the 
ordinary  model  of  the  Sharp-Millar  photometer,  a  calibrator  has 
been  devised  whereby  the  accuracy  of  the  calibration  of  the  instru- 
ment may  be  checked  at  one  point.  This  consists  of  a  short  tube 
(Fig.  6)  to  be  set  on  the  test-plate  of  the  photometer.  This  tube 
carries  near  its  upper  end  a  seasoned  incandescent  lamp  which  is  put 
in  a  bridge  connection  similar  to  that  described  above.  The  bridge 
is  non-adjustable.  In  connection  with  the  tube  is  also  a  rheostat  so 
that  the  current  through  the  lamp  may  be  varied  until  the  bridge  is 


SHARP:  MODERN  PHOTOMETRY 


119 


in  balance.  When  the  bridge  is  balanced  the  lamp  throws  a  known 
illumination  on  the  test-plate  and  the  photometer  may  be  adjusted 
to  give  the  corresponding  reading  on  its  scale.  Inasmuch  as  the 
scale  is  known  to  be  correct  throughout  its  length,  a  check  at 
one  point  insures  its  accuracy  at  all  points. 

Compensated  Test-plates. — All  illumination  photometers  measure 
illumination  on  the  assumption  that  the  light  reflected  or  trans- 


Fig.   12. — Principle  of  compensated  test-plate. 


Fig.   13. — Compensated  test-plate  on  a  photometer. 

mitted  from  the  test-plate  follows  Lambert's  cosine  law.  As  a 
matter  of  fact  no  substance  yet  found  will  diffuse  light  in  exact 
accordance  with  this  law.  The  light  received  at  high  angles  pro- 
duces a  brightness  of  the  test-plate  which  is  too  small  as  compared 
with  the  similar  flux  of  light  incident  at  small  angles.  The  failure  of 
the  test-plate  to  conform  to  this  law  is  therefore  reflected  in  an 
error  of  the  photometer  which  differs  according  to  the  conditions 


120 


ILLUMINATING   ENGINEERING  PRACTICE 


under  which  the  photometer  is  used.  In  an  attempt  to  obviate  this 
error,  a  so-called  compensated  test-plate  as  illustrated  in  Fig.  1 2  has 
been  produced.13  In  this  figure  the  light  incident  upon  the  upper 
surface  of  the  test-plate  P  is  reinforced  by  light  admitted  through 
a  translucent  glass  ring  A ,  so  that  at  all  angles  the  brightness  of  the 
under  surface  of  P  due  to  the  combined  action  of  the  light  trans- 


Fig.   14. — Principle  of  compensated  reflecting  test-plate. 

mitted  from  above  and  the  light  incident  upon  it  from  beneath, 
corresponds  to  the  theoretical  amount.  It  will  be  noted  that  the 
amount  of  compensation  increases  with  the  angle  of  incidence  and 
hence  can  be  made  practically  complete  for  all  angles  up  to  the  very 


-40 


0° 


10' 


20 


40°        50°        60°        70 v 
Angle  of  Incidence 


90°     100 l 


Fig.  15. — Errors  of  various  test-plates  viewed  normally.  A,  Depolished  transmitting 
plate;  B,  polished  transmitting  plate;  C,  depolished  glass  reflecting  plate;  D,  compensated 
transmitting  plate;  E,  compensated  transmitting  plate. 

high  ones.  The  intrusion  of  light  to  the  ring  A  when  the  rays  are 
parallel  to  P  is  prevented  by  the  screen  S.  The  test-plate  is  attached 
to  a  photometer  as  shown  in  Fig.  13.  The  same  principle  is  shown 
applied  to  a  reflecting  test-plate  in  Fig.  14.  In  this  case  the  reflect- 
ing test-plate  consists  of  a  sheet  of  depolished  white  glass.  This  is 
set  a  small  distance  from  another  diffusing  white  surface  C.  Com- 
pensating light  falling  upon  the  plate  C  is  reflected  on  the  lower  sur- 

13  Sharp  and  Little,  I.  E.  S.  Trans.,  Vol.  10,  p.  727,  191$- 


SHARP:  MODERN  PHOTOMETRY 


121 


face  of  P,  transmitted  to  the  upper  surface  of  P,  and  reinforces  the 
light  reflected  from  P  to  exactly  the  right  amount  to  insure  compen- 
sation for  the  deficiencies  of  P  at  high  angles.  The  behavior  of 
various  test  plates  is  summed  up  in  the  curves  of  Fig.  15.  It  is 
evident  that  where  high  precision  is  required  in  illumination  pho- 
tometry, and  where  test-plate  errors  cannot  be  computed  and 
allowed  for,  the  use  of  some  form  of  corrective  test-plate  is  of 
vital  importance. 

Static  Illumination  Tester.^ — This  instrument  provides  a  means  for 
the  ready  measurement  of  illumination  to  a  relatively  low  degree  of 


r 


Fig.   1 6. — Principle  of  simplified  illumination  tester. 

precision  by  an  instrument  of  extreme  simplicity  in  construction 
and  use.  The  principle  which  it  involves  may  be  described  as  fol- 
lows: The  field  of  uniformly  graded  brightness  produced  by  the 
comparison  lamp,  instead  of  being  formed  in  the  air  where  it  is 
invisible,  is  formed  on  a  sheet  of  diffusing  material,  which  thereby 
is  given  a  continuously  graded  brightness.  The  light  which  is  to 
be  measured  falls  on  a  diffusing  sheet  in  juxtaposition  to  this  field, 
so  that  the  point  can  be  readily  seen  where  the  brightness  of  the 
one  field  equals  that  of  the  other.  The  graded  field  being  cali- 
brated, the  brightness  of  the  unknown  field  is  determined  by  finding 
with  the  eye  the  point  where  the  brightness  of  the  unknown  field 
equals  the  brightness  of  the  known  graded  field. 

The  illumination  tester  embodying  this  principle  is  shown  in 
Fig.  1 6.  This  figure  shows  a  rectangular  box  B  approximately 
2.5X2.5  cm.  in  cross-section  by  20  cm.  long,  containing  at  one  end 
a  small  tungsten  filament  lamp  L  behind  an  opal  glass  screen.  The 
top  of  the  box  over  the  rest  of  its  length  is  made  up  of  a  sheet  of 

i«  Sharp,  Electrical  World,  September  16,  1916. 


122  ILLUMINATING   ENGINEERING   PRACTICE 

clear  glass  to  which  is  pasted  an  arrangement  of  papers  P  which 
constitutes  what  may  be  described  as  a  continuous  photometer 
disc  of  the  Leeson  type,  extending  from  one  end  of  the  glass  to  the 
other.  The  interior  of  the  box  is  painted  white,  except  for  the  far 
distant  end  which  is  black.  The  photometric  element  P  consists  of 
a  sheet  of  fairly  heavy  paper  with  a  slit  cut  out  of  it  having  saw 
tooth  edges.  Over  the  entire  arrangement  is  then  pasted  a  sheet  of 
thinner  translucent  paper  having  a  mat  surface.  When  the  lamp  is 
lighted,  the  end  of  the  slit  nearest  the  opal  glass  is  seen  to  be  very 
bright,  and  this  brightness  fades  away  gradually  toward  the  other 
end  of  the  slit.  When  an  exterior  illumination  falls  on  this  photo- 
metric device,  the  outer  portions  are  illuminated  almost  wholly 
by  this  exterior  illumination,  while  the  slit  is  illuminated  chiefly  by 
the  light  from  the  inside  of  the  box.  At  the  point  where  the  bright- 
ness of  the  exterior  portion  is  the  same  as  the  brightness  of  the  slit, 
the  saw  teeth  fade  away  and  are  hard  to  distinguish.  This  point 
which  can  be  recognized  without  difficulty  provided  the  papers  are 
properly  selected  for  the  purpose,  indicates  the  photometric  value  on 
the  scale. 

The  completed  apparatus  in  its  experimental  form  is  shown  in 
Fig.  17.  In  this  figure  the  photometric  box  is  seen  mounted  on  a 
larger  box  which  contains  a  single  dry  cell  serving  as  the  source  of 
current.  The  box  also  carries  a  small  precision  voltmeter  and  a 
rheostat.  The  photometric  box  is  so  arranged  that  it  can  be  re- 
moved from  the  rest  of  the  apparatus,  the  flexible  cord  conductor 
with  which  it  is  connected  being  stowed  away  in  the  larger  box  when 
the  two  are  used  as  a  unit.  Photometric  readings  are  taken  in  a 
direction  at  right  angles  to  the  axis  of  the  box.  The  exact  angle  to 
the  vertical  at  which  these  readings  are  made  seems  to  be  of  rela- 
tively little  importance  in  the  general  case. 

By  slight  structural  modifications  the  instrument  can  be  adapted 
to  the  measurement  of  the  brightness  of  surfaces  as  well  as  the 
illumination  incident  upon  them. 

Mr.  R.  ff.  Pierce15  has  also  produced  an  instrument  of  this  same 
eneral  class. 

ILLUMINATION  MEASUREMENTS 

Practice  in  illumination  measurements  is  so  varied  according  to 
conditions  governing  any  particular  test,  that  to  go  into  anything 
approaching  a  complete  discussion  would  be  beyond  the  scope  of 

»  American  Gas  Light  Journal,  page  67,  August  14,  1916. 


SHARP:  MODERN  PHOTOMETRY  123 

this  lecture.  Certain .  general  precautions,  however,  need  to  be 
taken  in  practically  every  case.  Among  the  most  important  ones 
are  the  following: 

To  be  sure  that  the  comparison  lamp  in  the  photometer  is  giving 
its  correct  candle-power.  This  involves  a  comparatively  recent 
standardization  of  the  same  taken  in  connection  with  its  electrical 
measuring  instrument,  and  an  assurance  that  the  electrical  measur- 
ing instrument  is  sufficiently  reliable  and  accurate  for  its  purpose. 

To  use  the  photometer  in  such  a  way  that  there  is  no  undue  loss 
of  light  on  the  test-plate  due  to  the  presence  of  the  operator  or  other 
person. 

To  select  the  test  stations  properly  according  to  the  design  of  the 
test. 

To  see  that  the  test-plate  is  level  and  in  the  proper  position. 

To  see  that  the  scale  readings  are  properly  recorded  and  any  con- 
dition such  as  the  introduction  of  a  neutral  glass  screen  is  noted. 

The  subject  of  precautions  to  be  taken  in  illumination  measure- 
ments has  been  quite  fully  treated  by  Little16  in  an  Illuminating 
Engineering  Society  paper. 

MEASUREMENT  OF  BRIGHTNESS 

For  many  purposes  it  is  desirable  to  know  the  brightness  values 
of  objects  or  of  walls  and  ceiling  in  a  room  or  of  a  shade  or  reflector 
of  a  lamp.  The  standard  forms  of  portable  photometers  designed 
to  measure  illumination  enable  these  measurements  to  be  made 
simply  by  removing  the  test-plate,  it  there  be  one,  and  sighting  the 
photometer  directly  on  the  object  in  question.  Photometric 
balance  is  then  secured  between  the  object  and  the  diffusing  plate 
in  the  photometer.  The  reading  of  the  scale  needs  to  be  multiplied 
by  a  constant  to  give  the  brightness  value  either  in  candle-power 
per  square  inch  or  in  rnillilamberts.17  The  determination  of  this 
constant  is  a  matter  for  the  standardizing  laboratory  and  is  not 
particularly  easy,  inasmuch  as  it  involves  illuminating  to  a  known 
degree  a  surface  of  known  area  which  is  then  photometered  as  a 
source  of  light.  It  should  be  noted  that  the  brightness  constant  of 
a  photometer  is  a  function  only  of  the  test-plate  which  is  used  with  it, 
and  that  with  changes  in  the  calibration  of  a  photometer  this  con- 
stant is  unaffected,  provided  the  test-plate  is  unchanged.  The 
relation  between  the  brightness  constant  expressed  in  apparent 

'•Little,  Transactions  I.  E.  S.,  Vol.  10.  page  766,  1915. 

17  The  millilambert  is  a  unit  of  brightness,  and  is  equal  to  the  brightness  of  a  perfectly 
reflecting  and  perfectly  diffusing  surface  on  which  one  millilumen  per  square  cenfimeter  falls. 


124  ILLUMINATING    ENGINEERING    PRACTICE 

lumens  emitted  per  square  foot,  and  the  illumination  which  is 
the  lumens  incident  per  square  foot,  is  evidently  the  transmitting 
or  reflecting  power  of  the  test-plate  according  as  the  test-plate  is  of 
the  transmitting  or  reflecting  type.  In  practice  in  the  standard- 
izing laboratory  it  is  convenient  to  have  carefully  preserved  a  stand- 
ard test-plate  of  known  brightness  constant  which  can  serve  as  a 
reference  plate  for  the  calibration  of  other  plates. 

DAYLIGHT  MEASUREMENTS 

For  measuring  daylight  the  use  of  a  light  filter  to  secure  an  ap- 
proximate color  match  is  indispensable.  Inasmuch  as  the  quality 
of  daylight  varies  greatly,  dependent  upon  the  character  of  the  sky, 
no  one  filter  will  enable  a  match  to  be  made,  but  a  single  filter  may 
in  practice  be  used  because  the  outstanding  differences  are  not  so 
excessive  as  to  prevent  fairly  good  measurements  being  made.  On 
account  of  the  very  high  values  of  illumination  usually  given  by 
daylight,  it  is  more  convenient  to  put  the  filter  on  the  daylight  side 
rather  than  on  the  side  of  the  comparison  lamp.  In  the  practice 
of  the  Electrical  Testing  Laboratories  a  sheet  of  suitably  colored 
gelatin  is  sometimes  utilized  such  as  is  employed  in  spot-lighting 
in  theatrical  work.  Daylight  foot-candle  values  alone  are  fre- 
quently, perhaps  usually,  of  subsidiary  importance  because 
illumination  values  vary  so  much  from  time  to  time  with  the 
outdoor  or  sky  conditions.  Rather  the  photometrist  must  give 
a  value  at  the  place  which  is  studied,  coupled  up  in  some  was 
with  a  value  representing  outdoor  conditions.  The  condition 
most  commonly  chosen  is  the  brightness  of  some  portion  or  of  all 
of  the  visible  sky,  or  the  illumination  produced  by  some  portion  of 
all  of  the  visible  sky.  For  this  purpose  various  types  of  apparatus 
have  been  produced  by  which  the  brightness  of  the  sky  can  be  com- 
pared with  the  brightness  of  a  test-plate  in  a  room,  for  instance. 
As  illustrative  of  this  class  of  problems,  the  methods  employed  by  the 
Electrical  Testing  Laboratories  in  studying  the  obstruction  of  day- 
light to  buildings  caused  by  alterations  in  a  structure  in  the  street 
will  be  instructive.  In  this  work  one  photometer  with  a  vertical 
test-plate  was  placed  close  to  the  window-line  of  the  building.  Along- 
side of  it  was  another  photometer  having  a  fan-shaped  arrangement 
placed  over  the  test-plate  whereby  the  test-plate  received  only  the 
light  from  the  unobstructed  portion  of  the  sky.  (See  Fig.  18.) 
Readings  were  made  simultaneously  with  these  photometers  and 
thereby  a  relation  was  obtained  between  the  illumination  produced 


Fig.  17. — Simplified  illumination  tester. 


Fig.   1 8. — Simultaneous  measurements  with  two  photometers  in  determining  daylight 

conditions. 

(Facing  page  124.) 


SHARP:  MODERN  PHOTOMETRY 


125 


by  the.sky  independently  of  all  structures,  and  the  light  entering  the 
building.  After  alterations  were  completed,  measurements  of  the 
same  kind  were  repeated  and  the  change  in  the  ratio  was  taken  to 
indicate  the  degree  of  light  obstruction  caused  by  the  alterations. 
It  was  found  that  even  with  these  precautions  it  was  necessary  to 
work  only  with  an  overcast  sky  of  practically  uniform  brightness. 
Otherwise  the  relations  did  not  hold.  As  an  aid  to  carrying  out 
measurements  of  this  kind  the  instrument  shown  in  Fig.  21  was 


Fig.  21. — Instrument  for  daylight  comparisons. 

devised  and  constructed.  In  this  instrument  a  direct  comparison 
is  obtained  between  the  light  falling  on  the  vertical  test-plate  on  one 
side  and  the  test-plate  turned  toward  the  sky  with  a  sky  limiting 
device  on  the  other. 

PHOTOMETRY  OF  GAS-FILLED  LAMPS 

It  was  found  at  the  Electrical  Testing  Laboratories  that  gas-filled 
lamps  when  rotated  for  the  purpose  of  determining  their  mean 
horizontal  candle-power,  changed  both  in  current  consumption  and 
in  candle-power,  and  that  this  change  varied  with  the  speed  of  rota- 
tion. With  high  speed  of  rotation,  centrifugal  force  causes  the 
cooler  portions  of  the  gas  in  the  interior  of  the  bulb  to  be  thrown  off 
to  the  periphery  of  the  bulb,  leaving  the  filament  surrounded  by 
hotter  gases  than  if  it  were  stationary.  Hence  the  temperature  of 
the  filament  with  the  same  watts  input  increases,  and  with  it  the 
candle-power  and  efficiency  of  the  lamp.17  This  effect  was  discovered 
about  the  same  time  also  by  Middlekauff  and  Skogland18  who  found 
further  that  with  very  low  speeds  of  rotation  the  candle-power  of 
these  lamps  decreased,  while  it  increased  with  higher  speeds,  so  that 
for  every  lamp  a  speed  can  be  found  at  which  the  candle-power  and 

11  Sharp  Photometry  of  gas-filled  incandescent  lamps,  Trans.  I.  E.  S.,  Vol.  9,  page  1021, 
1914- 

11  Photometry  of  the  gas-filled  lamp,  Bulletin  of  Bureau  of  Standards,  Vol.  12,  page  589. 


126  ILLUMINATING  ENGINEERING   PRACTICE 

watts  are  the  same  as  when  stationary.  The  determination  of  the 
horizontal  candle-power  of  gas-filled  lamps  is  at  the  present  time  a 
matter  of  little  importance,  but  it  can  best  be  accomplished  by 
rotating  the  lamp  quite  slowly  at  or  near  this  critical  speed  and  plac- 
ing behind  it  two  mirrors  120  degrees  apart  so  that  the  photometer 
disc  is  illuminated  not  only  by  the  lamp  itself  but  by  its  two  re- 
flected images  resulting  from  beams  equally  directed  about  the 
periphery  of  the  lamp.  The  two  mirrors  are  employed  to  obviate 
the  violent  flicker  on  the  photometer  disc  which  would  otherwise 
occur. 

PHOTOMETRY  OF  LAMPS  WITH  SHADES  AND  REFLECTORS 

The  measurement  of  distribution  of  light  about  illumination  ac- 
cessories is  carried  on  by  methods  which  are  well  known  and  which 
were  described  in  the  Johns-Hopkins  lectures.  A  new  apparatus 
for  the  purpose  designed  by  Little  is  shown  in  Fig.  19.  In  this 
apparatus  there  are  only  two  mirrors  and  the  record  of  the  photom- 
eter settings  is  made  on  a  paper  fastened  to  a  flat  board  in  front  of 
which  the  photometer  carriage  moves.  The  position  of  the  board  is 
changed  for  each  change  in  the  angle  of  the  movable  mirror.  This 
apparatus  is  equally  well  adapted  to  the  photometry  of  gas  mantle 
burners  and  of  incandescent  electric  lamps. 

The  determination  of  the  diffusing  power  and  of  the  absorption 
losses  in  lighting  glassware  is  receiving  more  of  the  attention  which 
it  deserves.  No  standard  method  for  the  measurement  of  diffusion 
has  yet  been  decided  on,  but  a  very  good  idea  of  the  diffusing  powers 
of  glassware  may  be  obtained  by  taking  two  measurements  of  the 
brightness  of  the  glassware  with  its  normal  lamp  inside  of  it;  one 
with  the  photometer  looking  directly  at  the  lamp  and  the  other 
looking  at  a  position  on  the  globe. about  45  degrees  distant  from 
the  first  position.  It  is  desirable  that  methods  of  measuring  diffu- 
sion should  be  further  investigated  and  finally  standardized. 

The  determination  of  absorption  of  globes  or  reflectors  may  be 
made  through  a  comparison  of  the  total  flux  of  the  light  from  a 
lamp  without  the  globe  and  with  it.  The  total  flux  may  be  found 
from  distribution  values  worked  up  by  means  of  the  Rousseau 
or  the  Kennelly  diagram,  or  by  direct  computation.  More  con- 
venient, however,  is  the  use  of  the  integrating  sphere.  A  good 
arrangement  of  the  standard  lamp  and  of  the  accessory  in  the 
sphere  for  determining  the  loss  of  light  in  the  accessory  is  shown  in 
Fig.  22.  It  will  be  noted  that  the  standard  lamp  is  placed  with  its 


SHARP:  MODERN  PHOTOMETRY 


127 


socket  turned  toward  the  globe  and  that  the  base  of  the  globe  is 
turned  toward  the  standard  lamp  so  that  the  sphere  losses  are  mini- 
mized. The  procedure  then  is  as  follows:  With  the  globe  removed 
from  the  sphere,  the  sphere  is  standardized  or  the  photometer  is 
adjusted  to  give  a  reading  corresponding  to  the  total  flux  of  light 
from  the  standard  lamp.  Then  the  standard  lamp  is  extinguished 
and  the  globe  lamp  is  lighted.  Reading  the  photometer  then  shows 
this  total  flux  of  light.  It  then  is  put  out  and  the  globe  is  placed 
over  it,  the  standard  lamp  is 
again  lighted,  and  a  reading  is 
taken.  This  reading  should  be 
equal  to  the  first  one,  except- 
ing for  the  reflected  flux  in  the 
sphere  which  is  intercepted  by 
the  globe.  Finally  the  stand- 
ard lamp  is  extinguished  and 
the  globe  lamp  is  lighted. 
This  reading  gives  by  compar- 
ison with  the  previous  one  the 
total  flux  of  light  issuing  from 
the  globe.  This  total  flux  of 
light  compared  with  the  total 
flux  of  light  of  the  lamp  without 
the  globe,  reading  No.  2,  shows  the  absorption  by  the  globe.  It  is 
very  necessary  to  notice  all  the  precautions  which  must  be  taken 
in  this  class  of  work  as  experimenters  have  been  led  into  error  by 
neglect  of  some  of  them. 


22. — Arrangment    for    measuring    globe 
absorption  in  integrating  sphere. 


REFLECTION  AND  TRANSMISSION  MEASUREMENTS 

Measurement  of  the  transmission  of  light  through  a  transparent 
medium  such  as  a  sheet  of  glass  is  most  simply  made  by  means  of  a 
bar  photometer  or  portable  photometer,  measuring  the  candle-power 
of  a  lamp  first  without  and  then  with  the  glass  interposed  in  the  beam. 
Similarly  the  reflecting  power  of  a  mirror  can  most  readily  be 
measured.  When  it  comes  to  the  measurement  of  diffusing  media, 
either  transmitting  or  reflecting,  the  measurement  is  more  difficult. 
Inasmuch  as  diffusing  media  not  only  diminish  the  light  but  also 
change  it  from  unidirectional  into  multidirectional  light,  some  in- 
tegrating device  is  in  this  case  required.  In  this  class  of  measure- 
ments the  integrating  sphere  may  very  suitably  be  used. 


128 


ILLUMINATING   ENGINEERING   PRACTICE 


In  Fig.  20  is  shown  a  view  of  a  1 2-inch  sphere  set  up  to  measure 
the  transmission  of  a  diffusing  glass.  There  is  an  opening  of  definite 
diameter  in  the  top  of  the  sphere,  limited  by  a  circular  metal  dia- 
phragm, and  the  light  from  the  lamp  outside  the  sphere  shines 
through  this  into  the  interior.  Photometric  measurement  gives 
the  value  of  this  luminous  flux.  Then  the  diffusing  glass  is  placed 
directly  beneath  the  limiting  diaphragm  and  another  measurement 
is  made  which  gives  the  amount  of  flux  traversing  the  glass.  In  the 
case  of  diffuse  reflectors  the  procedure  is  somewhat  different.  Fig. 
23  shows  the  arrangement.  The  diffuse  reflector  which,  as  will  be 


Fig.  23. — Measurement  of  coefficient  of     Fig.  24. — Nutting's  apparatus  for  measuring 
diffuse     reflection,    using      an     integrating  coefficients  of  reflection, 

sphere. 

seen,  is  placed  at  the  center  of  the  sphere  with  its  reflecting  side 
turned  at  an  angle  of  45  degrees  to  the  light  and  away  from  the 
photometer.  Thus  no  screen  is  needed  in  the  sphere.  The  amount 
of  flux  admitted  by  the  diaphragm  being  known,  and  the  amount 
reflected  from  the  diffusing  surface  being  measured,  the  reflection 
coefficient  at  this  angle  of  incidence  can  be  computed.  One  method 
by  which  the  amount  of  light  admitted  to  the  sphere  can  be  checked 
up  under  similar  conditions  to  those  of  the  measurement  of  the 
reflected  flux,  is  to  place  a  mirror  of  known  coefficient  of  reflection 
in  the  position  occupied  by  the  diffuse  reflector.  The  amount  of 
flux  then  measured  divided  by  the  known  coefficient  of  reflection 
of  the  mirror,  gives  the  amount  of  flux  incident  upon  the  diffuse 
reflector. 


SHARP:  MODERN  PHOTOMETRY  129 

A  singularly  ingenious  and  elegant  piece  of  apparatus  for  the 
measurement  of  coefficients  of  diffuse  reflection  has  been  devised 
by  Nutting.20  Thisinstrument  is  shown  in  plan  in  Fig.  24.  The 
ring  in  the  figure  is  covered  on  the  upper  surface  by  a  dense  milk 
glass.  On  the  under  surface  it  is  covered  by  the  diffuse  reflector 
which  is  to  be  tested.  A  special  photometric  device  is  supplied 
whereby  the  brightness  of  the  under  surface  of  the  milk  glass  may  be 
compared  with  the  brightness  of  the  diffuse  reflector.  If  a  diffuse 
reflector  had  100  per  cent,  reflecting  power,  its  brightness  would  be 
the  same  as  that  of  the  milk  glass.  Any  deficiency  is  due  to  its 
absorption.  The  photometer,  which  is  a  Martens-Konig  polariza- 
tion apparatus,  gives  a  comparison,  between  the  two  directly,  and 
hence  shows  the  reflecting  power  of  the  unknown  surface.  Direct 
and  reverse  readings  must  be  taken  in  order  to  eliminate  polarization 
errors. 

MEASUREMENTS  OF  PROJECTION  APPARATUS 

The  elementary  theory  of  a  projector  having  a  convex  lens  or 
a  parabolic  mirror  and  a  nearly  point  source  shows  that  when  the 
source  is  placed  at  the  principal  focus  of  the  mirror,  the  light  rays 
leave  the  surface  of  the  mirror  with  an  angle  of  divergence  which  is 
equal  to  the  angle  subtended  at  that  particular  point  of  the  mirror 
by  the  source  of  light.  Therefore  the  illumination  obtained  from  an* 
apparatus  of  this  kind  diminishes  with  the  distance,  and  if  the 
distance  is  great  enough,  the  exponent  of  the  distance  with  which  it 
diminishes  is  two.  In  other  words,  the  illumination  from  the  entire 
apparatus  follows  the  inverse  square  law.  With  a  small  projecting 
apparatus  accurately  focused  for  "parallel"  beam,  it  is  not  neces- 
sary to  take  any  very  great  distance  away  in  order  to  have  the  in- 
verse square  law  apply.  A  goodly  distance  is,  however,  in  all  cases 
advisable,  and  in  some  cases  imperative.  For  example,  in  the  case 
of  head-lamps  which  are  focused  so  as  to  throw  an  imperfect  image 
of  the  source  of  light  a  distance  of  200  or  300  feet  ahead,  it  is  evident 
that  the  inverse  square  law  could  not  be  assumed  without  taking  a 
distance  considerably  greater  than  200  or  300  feet.  Sometimes  the 
focusing  distance  is  shorter  than  this.  In  any  case,  in  the  photo- 
metric investigation  of  an  apparatus  which  is  to  be  used  approxi- 
mately at  a  certain  distance,  it  is  advisable  to  focus  it  for  that  dis- 
tance and  to  make  the  measurement  at  that  distance.  These 
measurements  can  then  be  expressed  as  apparent  candle-power  of  the 

20  Nutting,  Trans.  I.  E.  S.,  Vol.  8,  page  412,  1912. 
9 


130  ILLUMINATING    ENGINEERING    PRACTICE 

apparatus  at  that  distance,  and  in  so  doing  the  inverse  square  law 
is  not  assumed.  It  is  evident  that  measurements  of  this  character 
must  perforce  be  made  at  night  and  that  the  portable  photome- 
ter is  practically  the  only  apparatus  that  can  be  used  for  the  pur- 
pose. It  is  advantageous  to  set  the  projector  on  a  stand  which  can 
be  rotated  about  a  vertical  axis  and  which  has  a  divided  scale  whereby 
the  angle  at  which  the  measurement  is  taken  may  be  read  off.  By 
taking  a  series  of  measurements  covering  a  few  degrees  on  either  side 
of  the  axis  of  the  beam,  data  may  be  gathered  whereby  a  candle- 
power  distribution  curve  may  be  plotted.  In  view  of  the  narrow- 
ness of  such  a  beam  a  plot  in  polar  coordinates  is  as  a  general  thing 
of  little  use.  It  is  good  practice"  to  plot  these  measurements  in  rec- 
tangular coordinates,  putting  angles  in  the  axis  of  X  and  apparent 
candle-power  in  the  axis  of  F.  If  this  distribution  curve  is  carried 
far  enough,  it  may  be  integrated  according  to  the  Rousseau  method 
and  the  total  flux  of  light  emitted  by  the  apparatus  thereby  de- 
termined. This,  compared  with  the  total  flux  of  the  lamp  alone, 
gives  the  loss  of  light  in  the  apparatus.  A  plot  so  made  enables 
the  exact  position  of  the  maximum  candle-power,  which  should  be  the 
beam  candle-power,  to  be  determined.  In  the  case  of  lamps  for 
flood  lighting  the  efficiency  of  the  apparatus  is  of  great  importance 
and  hence  a  minimum  loss  of  light  in  it  should  be  striven  for,  more 
perhaps  than  is  the  case  with  projectors.  The  value  of  the  lost 
light  may  in  this  case  most  readily  be  determined  by  the  use  of  the 
integrating  sphere. 


RECENT  DEVELOPMENTS  IN  ELECTRIC  LAMPS 

BY   G.   H.   STICKNEY 

INTRODUCTION 

Lectures  by  Drs.  Steinmetz,1  Hyde2  and  Whitney,3  in  the  1910 
Course,  treated  of  the  physical  and  chemical  principles  of  light 
production,  and  described  the  electric  illuminants  from  the  scientific 
standpoint. 

On  this  foundation  it  is  the  purpose  of  this  lecture  to  trace  the 
more  important  of  the  recent  developments  and  describe  briefly  the 
principal  lamps  now  in  common  use. 

From  the  great  mass  of  available  data,  an  attempt  is  made  to 
present  such  information  as  will  be  of  most  practical  value  in  select- 
ing and  applying  electric  lamps. 

Since  arc  lamps  are  usually  furnished  as  complete  units  they  are 
so  treated.  Incandescent  lamps,  however,  are  equipped  with  a 
great  variety  of  reflectors  and  other  accessories,  which  are  furnished 
separately.  It  has,  therefore,  been  found  most  expedient  to  pro- 
vide a  separate  lecture  on  such  accessories4  and  give  but  slight  refer- 
ence to  them  in  this  lecture. 

LAMP  DEVELOPMENT 

The  basis  of  all  artificial  lighting  is  the  means  for  converting 
electrical  or  other  energy  into  light.  Advances  in  the  lighting  art 
have  followed  in  the  wake  of  the  improved,  practical  light  source, 
and  it  is  here  that  the  greatest  possibility  for  future  advance  lies. 
The  most  efficient  illuminants  are  still  very  far  below  the  ideals  of 
efficiency,  while  many  of  them  offer  much  opportunity  for  improve- 
ments, as  regards  reliability,  convenience  and  maintenance. 

Few,  if  any  of  the  recent  improvements  in  light  producers  have 
come  by  chance.  They  have  rather  been  the  result  of  arduous  and 
expensive  research  by  trained  physicists,  chemists  and  engineers  in 
well  organized  laboratories.  Even  when  an  improved  principle  of 
light  production  has  been  discovered,  practical  devices  have  had  to 
be  designed,  machinery  for  manufacturing  economically  and  in 
quantity  developed,  sizes  and  other  characteristics  determined 
upon,  in  order  that  the  improvement  could  be  utilized  to  advantage. 


132  ILLUMINATING   ENGINEERING   PRACTICE 

While  all  these  items  cannot  be  perfected  in  advance  of  the  prac- 
tical application  of  the  appliance,  it  is  remarkable  how  few  changes 
are  necessary.  It  is  a  tribute  to  Thomas  A.  Edison  that  so  many  of 
his  standards  still  hold. 

PROGRESS   SINCE   1910 

In  general,  the  progress  since  1910  may  be  summed  up  in  (a) 
improved  efficiency,  (b)  reliability  and  safety,  (c)  economy  of  main- 
tenance, (d)  adaptability,  (e)  simplicity  and  convenience. 

Accompanying  these  improvements  there  has  been  a  corre- 
sponding increase  in  intrinsic  brilliancy  of  light  sources,  which,  while 
advantageous  for  certain  applications,  has  in  general  been  undesir- 
able. Fortunately,  however,  diffusing  devices  can  be  readily  ap- 
plied, giving  an  over-all  result  much  in  favor  of  the  improved 
illuminants. 

TENDENCY  AS  TO  TYPES 

Among  the  incandescent  units  the  tungsten  filament  or  "Mazda" 
lamp  has  assumed  predominence.  The  tantalum  and  Nernst 
lamps  have  practically  disappeared  from  manufacture,  while  the  use 
of  metallized-carbon  filament  or  "Gem"  and  carbon  lamps  has 
decreased  very  rapidly  in  the  last  four  years. 

The  actual  percentages  reported  by  the  National  Electric  Light 
Association5  show  that  approximately  80  per  cent,  of  all  incandes- 
cent lamps  sold  during  1915  in  this  country  were  of  the  tungsten 
filament  type.  Incandescent  lamps,  as  a  whole,  have  increased  in 
importance,  encroaching  on  fields  of  lighting  formerly  assigned  to 
other  illuminants. 

While  many  enclosed  carbon  arc  lamps  are  still  in  use,  especially 
in  street  lighting,  their  manufacture  has  dwindled  to  a  very  small 
number,  giving  way  to  more  efficient  illuminants.  The  flaming 
arc  has  been  changed  from  an  open  to  an  enclosed  lamp,  and  has 
been  applied  to  street,  industrial  and  photographic  lighting  whereas 
formerly  its  principal  application  was  spectacular  lighting. 

The  "luminous,"  "magnetite"  or  "metallic  flame"  arc  lamp  has 
become  one  of  the  leading  street  illuminants,  especially  since  the 
ornamental  types  became  available,  while  the  multiple  lamp  used 
in  industrial  lighting  is  not  now  exploited. 

CLASSIFICATION 

Steinmetz1  classified  electric  illuminants  as  (a)  solid  conductor, 
(b)  gaseous  conductor,  (c)  arc  conductor,  and  (d)  vacuum  arc.  For 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        133 

the  present  lecture,  a  similar  classification,  with  the  more  common 
names,  is  used,  namely,  (a)  incandescent  lamp  (Mazda,  etc.),  (b) 
Moore  lamp  and  X-Ray  tube,  (c)  arc  lamp  (luminous  and  flame), 
(d)  mercury  vapor  lamp  (Cooper  Hewitt). 

INCANDESCENT  LAMPS 

Of  the  incandescent  lamps,  the  only  types  meriting  our  considera- 
tion are  the  tungsten-filament  lamps,  designated  by  the  principal 
American  manufacturers  as  " Mazda." 

The  principal  distinct  developments  since  1910  are  (a)  drawn 
tungsten  filaments,  (b)  coiled  filaments,  (c)  concentrated  filaments, 
(d)  chemical  "getters,"  (e)  gas-filled  construction. 

In  addition  to  these,  however,  there  have  been  innumerable 
minor  improvements,  which  have  resulted  in  large  aggregate  gains 
in  efficiency  and  have  tended  toward  uniformity,  increased  strength8 
and  reduced  cost.9 

For  example,  it  is  very  largely  due  to  the  minor  improvements 
that  the  60- watt  lamp  of  to-day  costs  one-third  less  than  in  1910 
and  gives  20  per  cent,  more  light. 

Drawn  Wire  Filaments. — Drawn  wire  filaments  superseded  the 
former  pressed  filaments  in  about  1911.  The  ductile  form  of  tung- 
sten6 was  finally  produced  in  the  Research  Laboratory  at  Schenec- 
tady,7  after  extended  experiments  and  many  discouraging  failures. 

It  revolutionized  lamp  manufacture,  simplifying  the  processes 
very  considerably  and  reducing  the  cost.  Further,  it  became  pos- 
sible to  draw  filaments  exactly  to  size,  which  in  turn,  increased  the 
practical  efficiency  by  eliminating  weak  points,  and  also  made  it 
possible  for  the  first  time  in  lamp  manufacture  to  produce  all  lamps 
of  a  lot  for  the  predetermined  voltage.  The  economic  influence  of 
this  last  factor  is  being  felt  to-day  in  the  demand  for  standardization 
of  circuit  voltages. 

With  the  development  of  the  drawn  wire  processes,  the  lamps  be- 
came much  more  rugged,  so  that  to-day  they  are  widely  used  in  steam 
and  trolley  cars,  automobiles,  on  moving  machinery  and  in  other 
relatively  rough  service. 

The  drawn  wire  could  also  be  made  more  slender,  so  that  the  10- 
watt  and  even  the  7.5-watt,  no-volt  lamps  became  practicable. 

Coiled  Filaments. — Another  result  of  the  use  of  ductile  tungsten 
was  the  possibility  of  winding  the  wire  around  a  mandrel,  thereby 
producing  the  helically  coiled  filament  (See  Fig,  i). 

The  first  application  of  this  was  in  the  so-called  " focus"  type 


134 


ILLUMINATING   ENGINEERING   PRACTICE 


lamp,  in  which  the  filament  was  concentrated  into  a  small  space, 
more  or  less  approximating  the  point  source.  The  automobile  and 
locomotive  headlight  lamps  and  a  much  more  effective  stereopticon 
lamp  became  practicable.10 

The  advantage  of  the  concentrated  light  source  in  connection  with 
lenses  and  reflectors  is  illustrated  in  Table  I,  which  shows  the  maxi- 
mum beam  candle-power  obtained  with  a  i6-in.  parabolic  reflector 
(G.  E.  Floodlighting  projector  Form  L-i)  with  lamps  of  approxi- 
mately 100  watts,  but  with  widely  varying  filament  dimensions.  In 
these  tests  the  lamps  were  focused  so  as  to  give  maximum  beam 
candle-power  and  operated  at  100  mean  spherical  candle-power. 

TABLE  I. — BEAM  CANDLE-POWERS 


Light  source 

Mazda  lamp  used 

dimensions 

Beam 

m.m. 

candle- 

Volt 

Watt 

Bulb 

Type 

Dia. 

Length 

6 

108 

G-3o 

C  headlight 

2.0 

6-5 

462,000 

32 

100 

0-30 

C  headlight 

5-0 

5-o 

223,000 

no 

IOO 

G-25 

C  stereopticon 

6-5 

6-5 

142,000 

no 

100 

0-30 

B  stereopticon 

8.0 

8.0 

32,600 

no 

IOO 

PS-25 

C  regular 

25 

o.S 

12,700 

no 

IOO 

G-35 

B  regular 

30 

68 

3,800 

The  most  important  effect  of  the  coiled  filament,  however,  is  in 
connection  with  the  gas-filled  construction. 

Chemical  "Getters" — This  refers  to  the  introduction  of  various 
chemicals,  sometimes  called  "getters,"  within  the  lamp.  Some  of 
these  chemicals  act  while  the  lamp  is  being  exhausted,  while  others 
continue  to  act  throughout  the  life  of  the  lamp.  Some  of  the  impor- 
tant effects  of  these  chemicals  are: 

1.  Regeneration,  that  is,  redepositing  evaporated  material  on  the  filament. 

2.  Combination  with  material  depositing  upon  the  bulb  to  form  more  trans- 
parent compounds. 

These  combined  actions  permit  increased  efficiency,  reduce  bulb 
blackening,11  and  help  maintain  the  candle-power  of  the  lamp 
throughout  its  rated  burning  life. 

GAS-FILLED  LAMPS 

With  the  elimination  of  several  weak  points,  it  had  been  possible 
to  raise  filament  temperatures  of  vacuum  lamps,  and  hence  efficien- 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        135 

cies,  to  a  point  corresponding  to  about  one  watt  per  candle-power, 
beyond  which  filament  evaporation  seemed  to  preclude  much  further 
advance. 

The  announcement  in  1913,  of  lamps  consuming  approximately 
one-half  watt  per  candle-power  was,  therefore,  rather  astounding  to 
the  lighting  world.  This  came  as  the  result  of  a  remarkable  re- 
search12 in  the  same  laboratory  that  produced  ductile  tungsten. 
The  new  principle  which  involved  the  gas-filled  construction,  de- 
pended upon  the  fact  that  when  operated  under  a  moderate  gas  pres- 
sure, the  tungsten  filament  could  be  maintained  at  a  higher  tempera- 
ture without  excessive  evaporation. 

The  introduction  of  gas  within  the  bulb,  however,  incurred  a  new 
loss;  namely,  convection,  that  is,  heat  carried  off  by  gas  currents. 
Such  losses  are  relatively  less  on  filaments  of  large  diameter,  so  that 
high  current  lamps  are  more  efficient  than  low.  By  using  the  heli- 
cally coiled  filament,  as  already  referred  to,  and  thereby  simulating 
large  diameter,  it  was  possible  to  apply  the  principle  to  practical 
lamps  (see  Fig.  i).  Later,  by  selection  of  gas  of  low  heat  conduc- 
tion, it  became  practicable  to  extend  it  to  lower  currents;  for  ex- 
ample, zoo-watt  and  75-watt  i  lo-volt  multiple  lamps.  The  gas-filled 
construction  is  most  advantageous  with  high  current  series  lamps 
and  high  wattage  multiple  lamps.  On  the  lower  wattage  multiple 
lamps,  it  as  yet  gives  lower  efficiency  than  the  vacuum  type  of  con- 
struction, and  hence  is  not  employed.  In  the  larger  sizes,  however, 
the  gas-filled  lamps,  which  are  designated  by  the  leading  American 
manufacturers  as  "Mazda  C,"  are  extensively  used,  their  high 
candle-power  and  efficiency  being  responsible  for  extending  the 
application  into  fields  not  formerly  occupied  by  incandescent  lamps. 

Owing  to  the  higher  filament  temperature  the  light  is  perceptibly 
whiter13  and  more  actinic,  than  that  of  the  vacuum  type  "  Mazda  B  " 
lamps.  Lamps  having  ratings  of  up  to  and  including  1000  watts 
(18,000  lumens)  are  in  regular  production.  Larger  lamps  have  been 
made  and  could  readily  be  provided  if  there  was  sufficient  commercial 
demand. 

Since  all  the  series  lamps  in  common  use  are  of  relatively  high  cur- 
rent, the  gas  filled  lamps  are  especially  advantageous,  and  have 
superseded  the  vacuum  lamps  all  along  the  line. 

A  still  further  gain  is  secured  for  the  higher  power  series  lamps  by 
providing  15-  and  20-amp.  lamps,  to  be  operated  irom  the  usual 
alternating  current  series  circuits;  namely,  6.6  and  7.5  amp.,  by 
means  of  individual  auto-transformers  or  series  transformers. 


136  ILLUMINATING   ENGINEERING    PRACTICE 

The  concentrated  arrangement  of  filament  permits  of  a  more 
effective  control  of  the  candle-power  distribution  with  refracting 
globes  and  small  diameter  reflectors. 

Candle-power  Distribution. — Formerly  all  clear  incandescent  lamps 
had  practically  the  same  distribution  of  candle-power,  so  that  the 
mean  horizontal  candle-power  bore  a  practically  fixed  relation  to 
the  mean  spherical  candle-power,  and  to  the  total  light  output. 
With  the  recent  development,  several  forms  of  filaments,  having  vari- 
ous candle-power  distributions  (see  Fig.  2)  are  used  in  the  different 
lamps.  Therefore,  the  mean  horizontal  candle-power  is  no  longer  a 
representative  measure  of  light  output. 

Position  of  Operation. — As  in  the  past,  the  smaller  lamps  can  be 
operated  in  any  position.  It  has  been  found  advantageous,  however, 
to  construct  some  of  the  larger  lamps  (for  example,  multiple  lamps 
of  200  or  more  watts)  without  bottom  anchors  on  the  filaments. 
Such  lamps  may  not  operate  satisfactorily  in  other  than  an  approxi- 
mately pendant  position. 

It  is  seldom  desirable  to  operate  these  larger  lamps  in  horizontal, 
tip-up,  or  inclined  positions,  but  where  such  is  the  case,  special 
lamps  can  be  obtained,  if  the  position  of  operation  is  specified. 

Some  of  the  high-power  focus  type  lamps,  on  the  other  hand, 
should  not  be  operated  within  45°  of  the  pendent  position.  Such 
lamps  are  usually  operated  tip  up  or  horizontally.  In  order  to 
economize  space  in  housings,  these  lamps  are  made  short  so  that 
if  used  pendant  it  is  not  practicable  to  protect  the  stems  from 
heated  gases  rising  from  the  filament. 

Accessories  for  focus  type  lamps  should  therefore  be  planned  for 
proper  lamp  position  according  to  information  given  by  the  lamp 
manufacturers. 

Circuits. — It  is  highly  desirable  to  operate  incandescent  lamps  at 
rated  voltage  or  current.  While  low  voltage  does  no  harm,  beyond 
lowering  the  light  output  and  efficiency,  and  also  changing  the  color 
of  the  light,  continued  low  voltage  is  often  a  source  of  complaint 
from  light  users.  Over-voltage  shortens  the  life  of  the  lamps  and 
if  excessive  may  destroy  the  filament. 

While  lamps  have  sufficient  leeway  to  permit  operation  at  a  reason- 
able over-voltage  and  so  operated  are  usually  more  economical,  the 
practice  of  running  lamps  at  labeled  voltage  is  generally  preferred 
and  is  recommended  by  the  manufacturers. 

Incandescent  lamps  operate  interchangeably  and  equally  well 
on  alternating-current  and  direct-current  circuits.  The  only  ex- 


Fig.  i. — Helically  coiled  filament  of  tungsten  wire.  (Magnified  to  show  turns.)  Illus- 
tration also  shows  concentrated  arrangement  of  filament  for  a  focus  type  lamp.  Note  the 
cooling  effect  of  supporting  anchors  on  the  heated  filament. 


Candlepower  Distribution  in  Vertical  Plane,  Multiple 
Mazda  Lamps,  with  Different  Forms  of  Filaments 
Clear  Bulbs,  no  Reflectors,  1000  Lumens. 


S.R.F.  =  Spherical  Reduction  Factor  =  ii.Horiz.C.P. 


Fig.  2. — Curves  of  candle-power  distribution  in  vertical  plane,  multiple  Mazda  lamps,  with 
different  forms  of  filament. 

(Facing  page  136.) 


< 0 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        137 

captions  to  this  are  the  non-vacuum  series  lamps  which  can  be 
operated  to  best  advantage  on  alternating-current  circuits. 

Although  the  lamps  give  satisfactory  life  on  series  direct  current, 
on  the  failure  of  the  lamp,  there  is  sometimes  maintained  a  lower 
voltage  arc,  which  may  burn  the  socket  contacts  before  the  protec- 
tive film  acts. 

On  account  of  the  low  heat  capacity  of  slender  filaments,  no  volts 
of  25  watts  or  less  (220  volt  lamps  of  60  watts  or  less)  show  percep- 
tible flicker  on  25-cycle  circuits,15  which  may  be  objectionable. 
Lamps  made  for  higher  amperage  avoid  this  effect.  In  general  no- 
volt  lamps  are  a  little  more  efficient  and  of  lower  cost  than  220- 


105-125  Volts 
(Lamps  14  Scale) 

Fig.  4. — Regular  (Mazda  B)  vacuum  lamps  for  no-volt  circuits. 


volt  lamps.  While  the  2  20- volt  lamps  are  made  for  the  same  oper- 
ating life,  no-volt  circuits  should  usually  be  preferred. 

Styles  and  Types. — Where  possible  lamps  as  listed  by  the  manu- 
facturer should  be  used.  Special  lamps  should  be  avoided.  Higher 
costs,  slower  deliveries  and  poorer  quality -may  be  expected  on  special 
lamps.  The  present  lists  include  lamps  to  cover  practically  all 
needs. 

Data  on  some  of  the  principal  types  of  Mazda  lamps  are  given  in 
Table  II.  These  data  are  subject  to  some  change  as  improvements 
become  available. 

The  variety  of  incandescent  lamps  is  so  great  that  it  is  imprac- 
ticable to  give  full  lists.  It  is  worth  while  to  call  attention  to  some 
of  the  more  special  types  which  come  into  common  use,  but  are  not 
so  well  known  as  the  regular  types. 


138 


ILLUMINATING   ENGINEERING    PRACTICE 


TABLE  II. — ENGINEERING  DATA  ON  MAZDA  LAMPS,  JULY,  1916 


.3 

1 

Input  Watts 
per  spherical 
c.p. 

1 

sis 
«f 

i 

9 

a 

3 

3 

'o 

h 

Reduction  facJ 
tor 

Bulb 

ill 
III 

Base 

Standard 
package 
quantity 

Position  of 
burning 

g 

**j 

•£  bo  y 
•£  C  C 

s-= 

& 

IK 
E- 

.j 

S'^ 

105-125  VOLT  "B"  STRAIGHT  SIDE  BULBS  (Fig.  4) 


IO 

.67 

7-SO 

75 

0.78 

S-I7 

2H 

4% 

Med.  screw 

IOO 

Any 

IS 

•47 

8.55 

128 

0.78 

S-i7 

2J,£ 

Med.  screw 

IOO 

Any 

20 
25 

•  41 
•  35 

8.90 
9-30 

178 
234 

0.78 
0.78 

S-I7 
S-IQ 

8 

45/i 

Med.  screw 
Med.  screw 

IOO 
IOO 

Any 
Any 

40 

•  32 

9-50 

380 

0.78 

S-I9 

2% 

In 

Med.  screw 

IOO 

Any 

SO 

•  31 

9.60 

480 

0.78 

8-19 

2% 

sK 

Med.  screw 

IOO 

Any 

60 

.28 

9.80 

590 

0.78 

S-2I 

2% 

Med.  screw 

IOO 

Any 

IOO 

.22 

10.3 

1030 

0.78 

S-30 

77/6 

Med.  sc.  sk. 

24 

Any 

105-125  VOLT  "C"  PEAR-SHAPE  BULBS  (Fig.  3) 


75 

1  .09 

ii.  5 

865 

PS-22 

2% 

6H 

Med.  screw 

50 

Any 

49« 

IOO 

1.  00 

12.6 

1260 

PS-2S 

3H 

7H 

Med.  screw 

24 

Any 

5M6 

200 

0.90 

14.0 

2800 

PS-30 

33A 

m 

Med.  sc.  sk. 

24 

Tip  down 

6 

300 

0.82 

15.3 

4600 

PS-35 

4% 

9H 

Mog.  screw 

24 

Tip  down 

7 

400 

0.82 

15.3 

6150 

PS-40 

5 

10 

Mog.  screw 

12 

Tip  down 

7 

500 

0.78 

16.! 

8050 

PS-40 

5 

10 

Mog.  screw 

12 

Tip  down 

7 

750 

IOOO 

0.74 
0.70 

17.0 
18.0 

12800 
18000 

.... 

PS-52 
PS-52 

6H 
6^ 

I3H 
13K 

Mog.  screw 
Mog.  screw 

8 
8 

Tip  down 
Tip  down 

£>H 
£>H 

220-250  VOLT  "B»  STRAIGHT  SIDE  BULBS 


25 

.65 

7.60 

191 

0.79 

S-I9 

2^ 

5« 

Med.  screw 

IOO 

Any 

40 

.42 

8.85 

354 

0.79 

S-I9 

2% 

$N 

Med.  screw 

IOO 

Any 

60 

-39 

9  05 

540 

0.79 

S-2I 

2% 

w 

Med.  screw 

IOO 

Any 

IOO 

.2? 

9-90 

990 

0.79 

S-30 

3* 

7% 

Med.  sc.  sk. 

24 

Any 

150 

.27 

9-90 

1480 

0.79 

S-35 

4% 

m 

Med.  sc.  sk. 

24 

Any 

250 

.20 

10.  S 

2620 

0.79 

8-40 

5 

10 

Med.  sc.  sk. 

12 

Any 

220-250  VOLT  "C"  PEAR-SHAPE  BULBS 


200 

I  .00 

12.6 

2520 

PS-30 

3% 

8% 

Med.  sc.  sk. 

24 

Tip  down 

6 

300 

o  .92 

13.7 

4100 

PS-35 

4% 

9;H 

Mog.  screw 

24 

Tip  down 

7 

400 

0.90 

14.0 

5600 

PS-40 

5 

10 

Mog.  screw 

12 

Tip  down 

7 

500 

0.85 

14.8 

7400 

PS-40 

5 

10 

Mog.  screw 

12 

Tip  down 

7 

750 

0.82 

15.3 

11500 

PS-52 

6^ 

139* 

Mog.  screw 

8 

Tip  down 

rii 

IOOO 

0.78 

16.1 

16100 

PS-52 

6}^ 

13% 

Mog.  screw 

8 

Tip  down 

S»H 

105-125  VOLT  "B"  ROUND  BULBS  (Fig.  5) 


15 

.53 

8.20 

123 

0.80 

G-i8^ 

aMe 

3% 

Med.  screw 

IOO 

Any 

15 

•43 

8.80 

132 

0.80 

G-25 

3tf 

4SA 

Med.  screw 

50 

Any 

25 

.41 

8.90 

222 

0.80 

G-i8^ 

2  Me 

3H 

Med.  screw 

IOO 

Any 

25 

•  31 

9.60 

240 

0.80 

G-25 

3Vi 

4% 

Med.  screw 

50 

Any 

40 

•  30 

9.65 

386 

0.80 

G-25 

3tf 

4% 

Med.  screw 

50 

Any 

60 

.20 

10.  s 

630 

0.80 

G-30 

3% 

SW 

Med.  screw 

24 

Any 

IOO 

•14 

II.  0 

I  IOO 

0.80 

G-35 

m 

7N 

Med.  sc.  sk. 

24 

Any 

220-250  VOLT  "B"  ROUND  BULBS 


25 

40 

1.50 
1.41 

8.40 
8.90 

2IO 
356 

O.80 
0.80 

G-25 
G-25 

3H 
3tt 

4H 
4X 

Med.  screw 
Med.  screw 

50 
50 

Any 
Any 

105-125  VOLT  "B"  TUBULAR  BULBS  (Fig.  5) 


25 

1.  35 

9-30 

232 

0.78 

T-io 

iH 

&t 

Med.  screw 

IOO 

Any 

25 

1.44 

8.75 

218 

T-8 

i 

12 

Med.  screw 

50 

Any 

40 

1.39 

9-OS 

362 

T-8 

i 

12 

Med.  screw 

50 

Any 

STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        139 

TABLE  II. — ENGINEERING  DATA  ON  MAZDA  LAMPS,  JULY,  1916. — (Continued) 
SIGN,  STEREOPTICON  AND   FLOODLIGHTING  LAMPS 


£ 

Watts  per 
spherical  c.p. 

Lumens  per 
watt 

Total  lumens 

Reduction  fac- 
tor 

Bulb 

Max.  over- 
all length 
(inches) 

Base 

Standard 
package 
quantity 

Position  of 
burning 

Light  center 
length 
(inches) 

| 

>, 
H 

ll 

VOLT  "B"  SIGN  STRAIGHT  SIDE  BULBS 


5 

1.  52 

1.46 

8.25 
8.60 

20.6 

43-0 

0.79 
0.79 

8-14 
8-14 

$; 

4 
4 

Med.  screw  1   100 
Med.  screw])   100 

Any 
Any 

50-65  VOLT  "B"  SIGN  STRAIGHT  SIDE  BULBS 

5 

1-73 

7.25   |36.2 

o.?8|  8-14 

*H 

4 

Med.  screw 

IOO 

Any 

105-125  VOLT  "B"  SIGN  STRAIGHT  SIDE  BULBS 

10 

1.92 
1.73 

6.55 
7-25 

49.0 
72.5 

0.78 
0.78 

S-I4 
8-14 

m 

4 

4 

Med.  screw 
Med.  screw 

IOO 
IOO 

Any 
Any 

105-125  VOLT  "C"  STEREOPTICON—  ROUND  BULBS 

IOO 

250 
500 

1.  00 

0.80 
0.67 

12.6 

15-7 
18.8 

1260 
3950 
9400 

.  .  .  . 

G-2S 
G-30 
0-40 

3% 
5 

7H 

Med.  screw 
Med.  screw 
Mog.  screw 

50 

24 

12 

* 
* 

4M 

105-125  VOLT  FLOOD  LIGHTING  "C" 

200 

400 

0.95 

0.85 

13.2 
14.8 

2640 
5920 

.... 

G-3O 
G-40 

3% 
5 

Stt 

Med.  screw 
Mog.  screw 

24 
12 

• 

3H 

Can  be  operated  in  any  position  except  within  45  degrees  of  vertical,  base  up. 

MAZDA  STREET  LIGHTING  LAMPS 


Nominal  rated 

c.p. 

Total  lumens 

Average  volts 

Average  watts 

Input  watts 
per  spherical 
c.p. 

Output  lumens 
per  watt 

Bulb 

Max.  over- 
all length 
(inches) 

Base 

Standard 
package 
quantity 

Position  of 
burning 

Light  center 
length 
(inches)  1 

1 

B! 

si 

5.5-AMP.  "C"  STREET  SERIES  STRAIGHT  SIDE  AND  PEAR-SHAPE  BULBS 

(Fig.  7) 


60 
80 

600 

800 

8.5 
10.8 

46.8 
59-5 

0.98 
0.93 

12.8 

13.5 

£38 

$; 

7V4 

Mog  screw 
Mog.  screw 

50 
50 

Any 
Any 

SH 
sH 

IOO 

1000 

13.0 

71-5 

0.90 

14.0 

S  —  24^ 

3  Vifl 

7^4 

Mog.  screw 

50 

Any 

s** 

250 

2500 

29.7 

163.0 

0.82 

IS.  3 

PS-35 

4H 

934 

Mog.  screw 

24 

Tip  down 

7 

400 

4000 

47-4 

260.0 

0.82 

15-3 

PS-40 

5        !IO 

Mog.  screw 

12 

Tip  down 

7 

6.6-AMP.  "C"  STREET  SERIES  STRAIGHT  SIDE  AND  PEAR-SHAPE   BULBS 

(Fig.  7) 


60 
80 

IOO 

250 

400 

000 

600 

800 

1000 

2500 
4000 
6000 

7.1 
9-1 
10.9 

23-5 
37.1 
55-7 

46.8 
60.0 
72.0 

155.0 
244.0 

368.0 

0.99 
0-94 
0.90 
0.78 
0.77 
0.77 

12.7 
13-4 
14-0 
16.1 
16.3 
16.3 

S-24h 
S-24h 
S-24h 
PS-35 
PS-40 
PS-40 

3H« 
3Me 
3Hfl 
4H 
5 
5 

7H 
7W 
7H 
9% 
10 

IO 

Mog.  screw 
Mog.  screw 
Mog.  screw 
Mog.  screw 
Mog.  screw 
Mog.  screw 

50 
50 

50 

24 

12 
12 

Any 
Any 
Any 
Tip  down 
Tip  down 
Tip  down 

5H 
5H 
sH 
7 
7 
7 

7.5-AMP.  "C"  STREET  SERIES  STRAIGHT  SIDE  AND  PEAR-SHAPE  BULBS 

(Fig.  7) 


60 
80 

IOO 

250 

400 

000 

600 
800 

1000 

2500 
4000 
6000 

6-4 
8.0 
9-6 
19.6 
30.5 
45-8 

48.0 
60.0 
72.0 
147-0 
228.0 
344-0 

I  .00 
0.94 
0.90 
0.74 
0.72 
0.72 

12.6 

13-4 
14.0 
17.0 
17-5 
17.5 

S-24H 

PS?3S 
PS-40 
PS-40 

i 

4V 
5 
5 

10 
IO 

Mog.  screw 
Mog.  screw 
Mog.  screw 
Mog.  screw 
Mog.  screw 
Mog.  screw 

50 
50 
50 
24 

12 
12 

Any 
Any 
Any 
Tip  down 
Tip  down 
Tip  down 

lit 

7 
7 

7 

15-AMP.  "C"  STREET  SERIES  PEAR-SHAPE  BULBS  (Fig.  7) 

400 1  4000 j  14.4! 216     |  o.68|  18.5]  PS-40  |  5     |i2H    | Mog.  screw!      12  I  Tip  down | 
20-AMP.  "C"  STREET  SERIES  PEAR-SHAPE  BULBS  (Fig.  7) 


600 

1000 

6000!  15  .5 
10000  25.9 

310 
520 

0.65 
0.65 

19.3 

19  3 

PS-40 

PS-40 

5 

5 

I2# 
12^ 

Mog.  screw 
Mog.  screw 

12 

12 

Tip  down 
Tip  down 

9V* 
9H 

140  ILLUMINATING   ENGINEERING   PRACTICE 

TABLE  II. — ENGINEERING  DATA  ON  MAZDA  LAMPS,  JULY,  1916. — (Continued) 
MAZDA  TRAIN  LIGHTING  LAMPS 


£ 
£ 

Input  (Output 

al  lumens 

d 

o 

Is 

Bulb 

% 

s-s 
sf3- 

Base 

Standard 
package 
quantity 

Position  of 
burning 

fc 

y 

1!I 

>J~ 

I's 

w'C 

-S  o    . 

S    •** 

Type 

Diam. 
(Inches) 

££« 

_£** 

e 

o>  rt 
(£*** 

25-34  VOLT  AND  50-65  VOLT  -"B"  TRAIN  LIGHTING  ROUND  BULBS 


IO 

•  44 

8.75 

8? 

0.81 

G-i8^ 

2M« 

3* 

Med.  screw 

100 

Any 

is 

.38 

9.10 

137 

0.81 

G-i8^ 

2Me 

3* 

Med.  screw 

IOO 

Any 

20 

.36 

9-25 

185 

0.81 

G-i8^ 

2Me 

3% 

Med.  screw 

100 

Any 

25 

•  36 

9-25 

232 

0.81 

G-i8^ 

2Mfl 

3% 

Med.  screw 

IOO 

Any 

40 

.22 

10.3 

412 

0.82 

G-30 

M 

6V4 

Med.  sc.  sk. 

24 

Any 

*75 

.16 

10.8 

810 

0.82 

G-30 

3% 

6H 

Med.  sc.  sk. 

24 

Any 

25-34  VOLT  AND  50-65  VOLT  TRAIN  LIGHTING  STRAIGHT  SIDE  BULBS 


1 

10 

•  50 

8.40 

84!0.78 

S-I7 

2H 

4*6 

Med.  screw 

IOO 

Any 

15 

•  44 

8.  75 

131  0.78 

S-I7 

2H 

4H 

Med.  screw 

IOO 

Any 

20 

•41 

8.90 

I78J0.78 

S-I7 

2^ 

\% 

Med.  screw 

IOO 

Any 

25 

•  41 

8.90 

222  0.78 

8-19 

2H 

sH 

Med.  screw 

IOO 

Any 

40 

.28 

9.80 

392  0.78 

S-I9 

2% 

5M 

Med.  screw 

IOO 

Any 

AND  6  VOLT  "C"  LOCOMOTIVE  HEADLIGHT  ROUND  BULBS 


36 

0.85 

14.8 

*530 

G-i&M 

2Me 

3% 

Med.  screw 

IOO 

Any 

23f6 

72 

0.80 

iS-7 

*H30 

G-25 

3H 

fo 

Med.  screw 

So 

Any 

234 

108 

0.75 

16.8 

*i8io 

G-30 

3% 

57/i 

Mog.  screw 

24 

Any 

zVi 

30-34  VOLT  "C"  LOCOMOTIVE  HEADLIGHT  ROUND  BULBS 


IOO 

I  .00 

12.6 

1260 

G-25 

3W 

4% 

Med.  screw 

SO 

Any 

2% 

ISO 

0.90 

14.0 

2100 

G-25 

JM 

4$* 

Med.  screw 

50 

Any 

2% 

250 

0.80 

15.7 

3920 



G-30 

3K 

SX 

Med.  screw 

24 

t 

3W 

*  6  volt  lamp  only;  5^  volt  lamp,  6^  per  cent,  less.'1 

t  Can  be  operated  in  any  position  except  within  45  degrees  of  vertical,  base  up. 

t  30-34  and  60-65  volts. 

MAZDA  STREET  RAILWAY  LAMPS 


Input 

Output 

Bulb 

d 

*-je 

•£» 

g 

"rt 

Watts  pei 
spherica 
c.p. 

I 

JM 

ll 

Reductio 
factor 

Type 

Diam. 
(Inches 

SSj 
^•g 

«~£ 
S*~ 

Base 

Standard 
package 
quantity 

Position  o 
burning 

III 

105,  110,  115,  120, 125  AND  130  VOLT  "B"  STREET  RAILWAY  STRAIGHT 
SIDE  BULBS 


t23 
t36 

t56' 
t94 

1.42 
1.40 

1.31 
1.24 

8.85 
9.00 

9.60 

10.  I 

*2l8 

*354 
*S70 

*IOOO 

0.78 
0.78 

0.78 
0.78 

S-I9 
S-I9 

S-2I 

8-24^ 

2% 
2% 

2ft 

sH 

5/4 

Med.  screw 
Med.  screw 

Med.  screw 
Med.  sc.  sk. 

IOO 
IOO 

IOO 

50 

Any 
Any 

Any 
Any 

*  115  volt  lamps  only,  other  lamps  in  proportion  to  their  volts. 
t  Nominal  watts. 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS 


141 


(Lamps  U  Scale) 


G-25  T-10 

15,  25  and  40  25  Watts 

Watts  105-125  Volts 
105-125  Volts 


T-8 

25  and  40  Watts 
105-125  Volts 

Fig.  5. — Round  bulb  and  tubular  (Mazda  B)  vacuum  lamps  for  no- volt  circuits. 


(Lamps  l/4  Scale) 


LJr 


G-25 

100  Watt 
105-125  Volts 
Stereopticon 


G-30 

250  Watt  Stereopticon  & 

200  Watt  Flood  Lighting 

105-125  Volts 


G-40 

500  Watts  Stereopticon  & 

400  Watt  Flood  Lighting 

105-125  Volts 


Fig.  6. — Floodlighting  and  Stereopticon  (Mazda  C)  gas-filled  lamps  for  no-volt  circuits* 


142 


ILLUMINATING   ENGINEERING   PRACTICE 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        143 

Focus  Type  Lamps. — These  lamps  are  especially  designed  for 
use  with  lenses  and  parabolic  reflectors.  They  are  used  with 
stereopticons,  small  moving-picture  machines,  signals,  and  for 
spotlighting,  floodlighting,  headlighting,  etc.10  The  essential  fea- 
ture of  the  lamps  is  the  concentration  of  the  filament  to  approxi- 
mate the  "point  source." 

Miniature  Lamps. — This  term  is  applied  to  a  wide  variety  of 
small  lamps  used  for  many  special  purposes.  Such  lamps  are  usu- 


(All  Lamps  V2  Scale) 


B-9H  D-10  T-8  G-W2  S-12l/2 

Candelabra  Candelabra          Candelabra  Candelabra  Decorative 

Style  B  Style  D  Style  E  Style  G  Style  F 

Fig.  8. — Candelabra  (Mazda  B)  vacuum  lamps  for  multiple  circuits. 

ally  for  low  voltage  and  provided  with  small  bases,  such  as  the  can- 
delabra or  bayonet  types.  Among  these  lamps  are  those  for  auto- 
mobile and  electric  vehicle  service.  The  no-volt  candelabra  lamps 
shown  in  Fig.  8  are  becoming  popular  for  decorative  purposes,  as, 
for  example,  electric  candles.  Frosted  lamps  are  generally  preferred. 

The  Christmas  tree  lamps,  which  were  designed  originally  to 
eliminate  the  fire  risk  in  Christmas  tree  lighting,  are  now  being  em- 
ployed extensively  for  producing  special  decorative  effects,  where  the 
lamps  are  used  as  ornaments  rather  than  to  produce  any  consider- 
able illumination.  Many  special  forms  of  bulbs,  such  as  fruits, 
flowers,  etc.,  are  made.  These  lamps,  which  usually  operate  eight 
in  series  on  100  volts,  are  now  made  with  tungsten  rather  than 
carbon  filaments. 

Important  among  the  battery  types  of  miniature  lamps  are  those 
for  small  "  flashlights ";  while  among  the  recent  developments  are 
the  miner's  lamps,  specified  by  the  U.  S.  Bureau  of  Mines. 

Colored  Lamps. — For  color  matching,  photography,  theatrical  and 


144 


ILLUMINATING   ENGINEERING   PRACTICE 


decorative  purposes,  various  colored  lamps  are  obtainable.  The 
color  is  introduced  either  by  means  of  a  dip  or  by  the  use  of  colored 
glass  bulbs.  The  former  is  less  expensive,  but  the  latter  is  more 
permanent.  Some  of  the  lamps  are  special  and  not  usually  obtain- 
able on  short  notice.  Bowl-frosted  or  all-frosted  lamps  are  more 


commonly  used  in  the  small  sizes.     All-frosted  lamps  are  not  recom- 
mended in  the  high  wattage  lamps. 

Complete  Equipment. — The  incandescent  lamp  is  not  generally  to 
be  regarded  as  a  complete  lighting  unit.  For  most  purposes  it  is 
desirable  to  provide  suitable  reflectors,  shades  or  globes,  for  direct- 
ing and  diffusing  the  light  in  accordance  with  particular  require- 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        145 

ments.  The  most  effective  illumination  is  secured  when  the  proper 
accessory  is  selected. 

Previous  to  the  advent  of  the  high-power  lamps,  little  attention 
was  necessary,  from  the  lamp  standpoint,  in  the  design  of  the  fixture, 
beyond  assuring  general  suitability.  Now,  however,  owing  to  the 
large  amount  of  light  and  heat  emitted  in  a  small  space,  certain  pre- 
cautions are  necessary  to  insure  proper  performance  of  lamps.14 
This  problem  is  similar  to  that  encountered  with  other  high-power 
illuminants.  While  the  majority  of  fixtures  take  care  of  these  re- 
quirements, there  are  some  fixtures  in  which  suitable  provision  has 
not  been  made. 

High-candle-power  filaments  are  too  brilliant  to  be  viewed  with 
comfort,  and  fixtures  should  have  provision  for  shielding  the  eyes 
and  diffusing  the  light,  olepending  upon  the  application  of  the  equip- 
ment. 

Ventilation  must  be  provided  to  carry  off  the  heat  and  avoid  ex- 
cessive temperature  at  the  top  of  the  lamp.  Suitable  sockets  and 
leading-in  wires  should  be  provided.  For  high  wattage  lamps,  used 
out  of  doors,  it  is  highly  important  that  the  fixtures  be  weatherproof 
so  as  to  exclude  moisture;  otherwise  during  rain  and  snow  storms, 
water  will  enter.  A  drop  of  water  falling  on  the  heated  glass,  near 
the  top  of  the  lamp,  is  liable  to  produce  a  crack,  which  will  result  in 
failure  of  the  lamp. 

Fig.  9  shows  the  candle-power  performance  of  the  5oo-watt  gas- 
filled  lamp  with  a  few  of  the  most  commonly  used  equipments.  For 
larger  or  smaller  lamps  the  candle-power  can  be  approximated  by 
proportioning  the  values  to  the  respective  total  lumens  of  the  lamps. 

MOORE  COLOR-MATCHING  LAMP 

The  principle  of  producing  light  by  electrical  discharge,  through 
a  gas  of  very  low  pressure,  enclosed  in  a  glass  tube,  was  applied  by 
D.  McFarlan  Moore.  Both  long  and  short  tube  lamps  were  devel- 
oped. The  color  of  the  light  from  such  a  lamp  depends  upon  the 
gas  used.  For  example,  nitrogen  produces  a  pinkish  light,  carbon 
dioxide  a  white  light,  neon17  a  reddish  light. 

The  short  carbon  dioxide  tube  is  the  only  type  in  active  commer- 
cial manufacture  in  this  country  at  the  present  time.  While  this 
lamp  is  not  widely  used  it  is  notable  because  of  the  superiority  of 
its  light  where  very  accurate  color  matching  is  required,  as,  for  ex- 
ample, in  dying  silk,  wool,  etc.  An  entirely  new  form,16  eliminating 


146  ILLUMINATING   ENGINEERING   PRACTICE 

the  gas  valves  and  other  complicating  features,  has  been  developed. 
This  lamp,  which  is  shown  in  Fig.  10,  consumes  about  250  watts  and 
operates  on  alternating-current  circuits.  While  the  overall  efficiency 
is  relatively  low,  the  light  is  distributed  according  to  the  require- 


Fig.   10. — Moore  color  matching  lamp. 

ments  of  the  accurate  color  matchers,  and  intensities  up  to  about 
200  foot-candles  can  be  secured  over  a  small  area. 

X-RAY  TUBES 

Illuminants  of  this  class  do  not  generally  interest  illuminating 
engineers  directly,  though  they  play  an  important  part  in  surgery 
and  various  physical  and  chemical  researches,  in  which  the  peculiar 
quality  of  these  radiations  reveal  what  cannot  otherwise  be  observed. 

The  recent  development  by  Dr.  Coolidge,  which  has  been  char- 
acterized as  the  most  important  advance  since  the  original  discovery, 
has  been  summed  up  as  follows:18 

"Briefly,  the  device  consists  of  a  tube  exhausted  of  all  gases  to  the  ex- 
treme possible  limit,  in  which  is  supported  the  cathode,  so  arranged  that 
it  may  be  heated  electrically;  an  electrically  conducting  cylinder  or  ring 
connected  to  the  heated  cathode,  and  so  located  with  reference  to  it  as  to 
focus  the  cathode  rays  on  the  target;  and  the  anti-cathode,  or  target.  The 
advantages  of  the  tube  are  complete  and  immediate  control,  of  the  inten- 
sity and  the  penetrating  power  of  the  Rontgen  rays,  continuous  operation 
without  change  in  the  intensity  or  character  of  the  rays;  absence  of  fluores- 
cence of  the  glass;  and  the  realization  of  homogeneous  primary  Rontgen 
rays  of  any  desired  penetrating  power." 

ARC  LAMPS 

The  common  forms  of  arc  lamp  include  the  open  and  enclosed 
carbon  electrode  lamp,  the  open  and  enclosed  flaming  carbon  elec- 
trode lamp  and  the  luminous,  magnetite  or  metallic  arc  lamps. 

The  large  variety  of  arc  lamps  now  in  active  use  is  indicated  by 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        147 

Fig.  ii,  which  shows  the  types  of  electrodes  regularly  furnished  by 
the  National  Carbon  Company.  The  engineering  data  of  the  prin- 
cipal forms  of  arc  lamps  ior  general  lighting  service  are  given  in 
Table  III. 

Open  and  Enclosed  Carbon  Arc  Lamps. — For  general  lighting  pur- 
poses these  lamps  are  generally  considered  to  be  superseded,  although 
there  are  a  considerable  number  still  in  use,  especially  for  street 
lighting. 

The  open  arc  is  the  standard  illuminant  for  high  power  projection 
lighting,10  as,  for  example,  with  large  stereopticons,  moving  picture 
machines,  and  for  search  lighting  and  spotlighting.  On  direct  cur- 
rent the  brilliant  homogeneous  crater  of  the  positive  is  the  most 
effective  approximation  of  the  "point  source."  A  heavily  impreg- 
nated flame  carbon  electrode  is  used  in  the  most  powerful  search- 
lighting  equipments. 

Recent  developments  have  done  much  to  increase  the  effective- 
ness of  the  open  arc,  especially  for  searchlight  work,  by  surrounding 
the  crater  with  a  cooling  atmosphere.19 

Introduction  of  chemicals  has  steadied  the  arc  and  the  use  of 
small  diameter  copper  or  duplex  coated  negative  electrodes  has 
served  to  reduce  electrode  shadows. 

Flaming  Arc  Lamps. — The  flaming  arc  lamp  has  the  lowest  specific 
consumption  of  all  the  common  illuminants.  It  differs  from  the 
ordinary  carbon  arc  in  that  the  addition  of  certain  metallic  salts 
changes  the  process  of  light  production,  the  light  emanating  from 
the  arc  steam  rather  than  from  the  craters.  The  composition  of  the 
electrodes  determines  the  color  of  the  light  and  to  a  considerable 
extent  the  efficiency.  Both  yellow  and  white  light  electrodes  are 
in  common  use.  Red,  blue  and  green  electrodes  are  used  for  special 
medical  purposes. 

Both  the  open  and  enclosed  (white)  flame  arcs  are  used  extensively 
in  photo-engraving  and  other  photographic  purposes,  including 
moving  picture  studios,  as  well  as  for  fading  tests  of  dyes  and  paints. 
Some  commercial  forms  for  photo-engraving  and  similar  purposes 
are  illustrated  in  Fig.  12. 

The  inclined  electrode  type  of  lamp,  formerly  used  for  spectacular 
lighting,  has  in  general  given  way  to  the  enclosed  lamp,  while  the 
field  has  extended  to  street  and  industrial  lighting.  White  electrodes, 
are  usually  employed  on  street  and  photographic  lighting,  and  yellow 
electrodes  for  industrial  lighting.25 

While  enclosed  flame  arc  lamps  had  been  produced  in  1910,  they 


148 


ILLUMINATING    ENGINEERING    PRACTICE 


did  not  come  into  common  use  in  this  country  until  191 1.20  The 
early  lamps  gave  an  unsteady  light,  and  the  solid  residue  from  the 
electrodes  formed  an  absorbing  coating  on  the  enclosing  globe. 
Many  improvements  have  been  made  in  the  past  few  years.  Im- 
proved condensing  chambers  have  minimized  the  accumulation  on 
the  globes.21  Probably  the  greatest  recent  improvement  has  been 


A  =  Clear  Inner,  Alba  Outer  Globe    >  White  Flame 
=  Clear  Inner,  Clear  Outer  Globe  )        Carbons 
-  Clear  Inner,  Alba  Outer  Globe    )  Yellow  Flame 
=  Clear  Inner,  Clear  Outer  Globe  )         Carbons 


Fig.   13. — Direct  current  multiple  6.5  ampere  no  volt,  enclosed  flame  arc  lamps. 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 

with  regard  to  the  composition  of  electrodes,23  tending  to  steady 
the  arc  and  increase  the  efficiency.  The  effectiveness  of  the  photo- 
graphic arcs  has  been  especially  increased. 

An  ornamental  type  of  enclosed  flame  lamp  for  street  lighting 
has  been  developed  and  is  exploited  by  a  leading  manufacturer.24 

The  principal  types  of  enclosed  flame  arc  lamps  now  on  the  market 
for  general  illumination,  are  listed  below.  Their  photometric 
curves  are  shown  in  the  Figs.  13  to  17  as  indicated: 

Luminous,  Magnetite,  or  Metallic  Flame  Arc  Lamp. — While  no 
radical  changes  have  been  made  in  these  lamps  since  1910,  the  effi- 
ciency, steadiness  and  light  control  have  been  much  improved.27  An 
ornamental  form  has  been  developed  which  is  receiving  quite  ex- 


II 


5   jj 

*o    g 

Is 
f 


Fig.  12. — Typical  open  and  enclosed  flame  arc  lamps  floor  types,  for  photo-engraving  and 
other  photographic  purposes. 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS 


149 


A- Clear  Inner,  Alba  Outer  Globe  \  White  Flame 
-  Clear  Inner,  Clear  Outer  Globe  f  Carbons 

C- Clear  Inner,  Alba  Outer  Globe  )  Yellow  Flame 
— Clear  Inner,  Clear  Outer  Globe  )  Carbons 


14. — Alternating  current  multiple  7.5  ampere,  no  volt  enclosed  flame  arc  lamp. 

(Internal  auto-transformer  gives  10.5  amperes  at  arc). 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 


A  -  Clear  Inner.  Alba  Outer  Globe  I 

=  Clear  Inner.  Clear  Outer  Globe  ) 

C- Clear  Inner,  Alba  Outer  Globe  \ 

=  Clear  Inner,  Clear  Outer  Globe  f 


White  Flame 
Carbons 

Yellow  Flame 
Carbons 


Pig.   15. — Alternating  current  series  6.6  (or  7.5)  ampere  enclosed  flame  arc  lamp.     (Internal 

auto-transformer  gives  10  amperes  at  arc.) 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 


ILLUMINATING   ENGINEERING    PRACTICE 


10  Amp.  A.  C.  Series  Enclosed  Flame  Carbon 
Arc  Lamp  with  Clear  Inner,  Clear 
Outer  Globe  and  White  Flame  Carbons 


Fig.  1 6. — Alternating  current  series  enclosed  flame  arc  lamp  (9.5  amperes  at  arc). 
(Data  furnished  by  Westinghouse  Elec.  &  Mfg.  Co.) 


10  Amp.,  A.  C.  Series  Enclosed  Flame 
Arc  Lamp  with.  Clear  Inner 
Alba  Outer  Globe  and  White 
Tlame  Carbons 


Fig.   17. — Alternating  current  series  ornamental^enclosed  flame  arc  lamp. 
(Data  furnished  by  Westinghouse  Elec.  &  Mfg.  Co.) 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS 


tensive  use.26     Changes  in  the  composition  and  form  of  electrodes 
have  been  responsible  for  the  increased  efficiency  and  steadiness. 

This  type  of  lamp  can  be  operated  only  on  direct-current  circuits. 
The  copper  or  positive  electrode  consumes  slowly  by  erosion;  the 
negative  or  magnetite  electrode  furnishes  the  arc  stream  material. 


Pendent  Type 


Ornamental  Type 


I  4  Amp.  D.  C.  Series  Luminous  Arc  Lamp 
/  with  High  Efficiency  Electrode 

A  —  Ornamental  Type  Equipped  with 

Light  Alba  Globe 
B  —    Pendent  Type  Equipped  with 

Carrara  Globe  and  Internal 

Concentric  Reflector 
C  —   Pendent  Type  Equipped  with 

Clear  Globe  and  Internal  Concentric 

Reflector 
D  -    Pendent  Type  Equipped  with 

Clear  Globe  and  Prismatic  Glass 

Reflector 


Fig.  1 8. — Candle-power  distribution  obtained  with  different  equipments,  4  ampere  luminous 

arc  lamp. 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 

In  the  lamp  as  made  by  the  General  Electric  Company,  a  large 
massive  positive  electrode  (which  is  not  replaced  at  each  trimming) 
is  above  the  arc.  On  4  amperes  its  operating  life  is  from  6000  to 
8000  hours;  and  on  6.6  amperes  from  2000  to  4000  hours. 

The  magnetite  electrodes  are  made  in  two  types,  designated  as 
" long-life"  and  "high-efficiency."     The  operating  life  of  the  former 


152 


ILLUMINATING   ENGINEERING   PRACTICE 


is  nearly  double  that  of  the  latter,  both  varying  inversely  with  the 
amperage.  The  high-efficiency  type  is  usually  used  on  4-ampere 
lamps,  giving  about  175  hours;  while  the  long-life  type  is  used  on  the 
6.6  ampere  lamp,  giving  100  hours  or  over. 

The  lamp  made  by  the  Westinghouse  Electric  &  Manufacturing 
Company  employs  what  is  known  as  the  "down-draft"  principle:29 
A  small  inexpensive  positive  electrode  is  located  below  the  arc  and  is 
renewed  at  each  trimming. 

While  lamps  for  multiple  operation  have  been  made  and  used  for 


4  Amp.  D.  C.  Series  Metallic  Flame  Arc  Lamp 
with.  Clear  Globe 


Fig.  19. — Four  ampere  metallic  flame  arc  lamp. 
(Data  furnished  by  Westinghouse  Electric  &  Mfg.  Co.) 

industrial  lighting,  the  large  amount  of  ballast  resistance  necessary 
to  insure  steady  operation,  makes  them  relatively  inefficient  and  they 
are  no  longer  exploited.  The  series  lamp,  on  the  other  hand,  is  quite 
economical,  having  a  low  maintenance  cost.  The  light  approxi- 
mates daylight  in  color  and  the  operation  is  quite  reliable.  The 
series  direct  current  is  usually  secured  from  combination  constant- 
current  transformers  and  mercury  arc  rectifiers,  which  in  turn  are 
supplied  with  power  from  alternating-current  multiple  circuits. 
One  of  the  most  interesting  developments  in  connection  with  the 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS 


153 


magnetite  lamp  is  the  variety  of  reflectors  and  of  diffusing  and  re- 
fracting globes  by  which  the  light  distribution  is  modified  to  meet 
the  various  requirements  of  street  lighting. 


A  "•  4  Amp.,  Long  Life  Electrode 

B  -  4  Amp.,  Higrh  Efficiency  Electrode 

C  —  6  Amp.,  Long  Life  Electrode 

D  —  5  Amp.,  High  Efficiency  Electrode 

E  —  6.6  Amp.,  Long  Life  Electrode 


Fig.  20. — Luminous  arc  lamp,  clear  globe,  concentric  reflector. 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 


A  =  4  Amp.,  Long  Life  Electrode 
5-4  Amp.,  High  Efficiency  Electrode 
C  —  5  Amp.,  Long  Life  Electrode 
D  —  5  Amp.,  High  Efficiency  Electrode 
JE7—  6.6  Amp.,  Long  Life  Electrode 


Fig.  21. — Luminous  arc  lamp,  clear  globe,  refractor. 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 

The  photometric  characteristics  of  the  4-amp.  luminous  lamps, 
with  the  principal  types  of  equipments  are  shown  in  Fig.  18.     Those 


154 


ILLUMINATING   ENGINEERING   PRACTICE 


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STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS 


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ILLUMINATING   ENGINEERING    PRACTICE 


A-*  4  Amp.,  Long  Life  Electrode 

B  —  4  Amp.,  High  Efficiency  Electrode 

C  —  5  Amp.,  Long  Life  Electrode 

D—  5  Amp.,  High  Efficiency  Electrode  ( Calculated ) 

E  =-  6.6  Amp.,  Long  Life  Electrode 


Fig.  22. — Luminous  arc  lamp,  opal  globe. 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 


A  =4  Amp.,  Light  Density  Alba  Globe  and 

Long  Life  Electrode 
.B  =  4  Amp.,  Light  Alba  Globe  and  High 

Efficiency  Electrode 
C  —  5  Amp.,  Light  Alba  Globe  and  Long 

Life  Electrode 
D  —  5  Amp.,  Medium  Density  Diffusing  Globe 

and  High  Efficiency  Electrode 
E  -  6.6  Amp.,  Medium  Density  Diffusing  Globe 

and  Long  Life  Electrode 

Fig.  23. — Ornamental  luminous  arc  lamp,  opal  globe. 
(Data  furnished  by  Ilium.  Eng.  Laboratory,  G.  E.  Co.,  Schenectady,  N.  Y.) 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        157 

of  the  5  and  6.6-amp.  lamps  correspond  approximately  in  form.  The 
actual  candle-power  performance  of  the  various  standard-equipments 
is  shown  in  Figs.  19,  20,  21,  22,  and  23.  These  give  average  initial 
values  taken  from  several  tests  on  separate  lamps  and  electrodes. 
For  general  data  see  Table  III. 

The  ornamental  type  of  lamp  represents  one  of  the  important 
developments,  which  is  receiving  wide  use  in  "white  way"  lighting.28 
It  is  an  inverted  lamp  only  in  the  sense  that  the  regulating  mechan- 
ism is  located  below  the  arc,  so  as  to  be  concealed  in  the  pole.  Sev- 
eral special  types  of  globes  have  been  furnished  to  conform  with  par- 
ticular artistic  requirements.  Such  equipments  have  different 
candle-power  characteristics  due  to  variations  in  shape,  light  ab- 
sorption and  diffusion. 


MERCURY  VAPOR  LAMPS 

Two  principal  types  of  mercury  vapor  lamps  are  made  in  this 
country;  namely,  low  (vapor)  pressure  glass  tube  lamps  and  high 
pressure,  quartz  tube  lamps. 

Glass  Tube  Lamps. — There  has  been  relatively  little  change  in 
this  type  of  lamp  since  1910.  Some  improvements  have  been  intro- 
duced in  the  alternating-current  lamp,  making  it  a  little  more  efficient 
and  reliable  in  operation. 

A  fluorescent  reflector30  has  been  developed  with  a  view  to  color  cor- 
rection, supplying  some  of  the  missing  red  rays.  While  considerable 
color  modification  is  obtained  by  this  means,  it  is  at  some  sacrifice  in 
efficiency,  and  the  fluorescent  reflector  is  not  used  to  any  consider- 
able extent. 

In  order  to  provide  a  more  convenient  arrangement  for  photo- 
graphic lighting,  where  a  large  flood  of  light  is  necessary,  as  in  a  mov- 
ing picture  studio,  special  supporting  frames  have  been  devised  for 
banking  tubes  from  high  power  units.  These  are  arranged  to  pro- 
ject the  light  in  one  general  direction  (Fig.  26). 

The  usual  line  of  lamps  for  industrial  lighting,31  is  illustrated  in 
Fig.  24,  which  gives  candle-power  distribution  curves.  The  curves 
show  the  initial  candle-power.  Fig.  25  shows  the  variety  of  standard 
tubes.  The  operating  life  of  tubes  is  stated  by  the  manufacturer  as 
4000  hours.  Published  data  indicates  that  the  candle-power  falls 
to  80  per  cent,  of  the  initial  in  about  2000  hours.  General  data 
are  given  in  Table  IV. 


158 


ILLUMINATING   ENGINEERING   PRACTICE 


TABLE  IV. — CANDLE-POWER  CHARACTERISTICS  OF  COOPER  HEWITT 
MERCURY  VAPOR  LAMPS 

Lamps  for  Alternating-current  Circuits 


Rating 
of  lamp 
in  aver- 
age watts 

Voltage 

Type 

Length 
of  tube 
in  inches 

Mean 
lower 
hemi- 
spherical 
c.p. 

Watts  per 
mean  lower 
hemi- 
spherical 
c.p. 

Total 
lumens 

Lumens 
per  watt 

2IO 
380 

IQ2 
385 
385 
22O 
385 

726 

100-125 
100-125 

E 
F 

35 

50 

400 
800 

0-53 
0.48 

3,179 
6,283 

I5-I4 
16.53 

For  Direct-current  Circuits 

Series    on 
100-125 
100-125 
100-125 
100-125 
100-125 

H 
HH 
K 
L 
P 

21 
21 

45 
35 
50 

300 
600 
700 
4OO 
800 

0.64 
0.64 
0-55 
0-55 
0.48 

2,388 
4,712 
5,529 
3,142 
6,283 

12.43 
12.23 

I4-36 
14.28 
16.31 

Quartz  Lamps  for  Direct-current  Circuits 

200-240 

Z 

4 

2400 

°  3 

18,839 

25.96 

Data  furnished  by  Cooper  Hewitt  Electric  Co. 

Quartz  Tube  Mercury  Arc. — The  high  pressure  mercury  arc  was 
developed  in  Europe.  Its  commercial  exploitation  in  this  country, 
dates  from  about  1913. 32  The  tube  is  much  shorter  than  that  of  the 
low  pressure  arc  and  the  current  density  greater.  The  high  tem- 
perature of  operation  necessitates  the  use  of  quartz  glass  as  a  tube 
material.  Most  of  its  characteristics  are  similar  to  those  of  the  low 
pressure  arc,  but  the  appearance  of  the  unit  is  more  like  that  of  a 
flame  arc  lamp.  In  starting,  an  electro-magnet  mechanism  tilts  the 
tube  to  draw  the  arc.  In  starting  with  the  lamp  cold,  about  five 
minutes  is  required  to  attain  the  operating  condition.  During  this 
period  the  current  and  watts  are  above  normal  and  the  candle-power 
below  normal. 

Practically  all  the  lamps  in  use  are  operated  on  2  20- volt  direct 
current  circuits.  Lamps  for  other  wattages  and  alternating  current 
have  been  made. 

The  light  is  essentially  similar  in  color  to  that  of  the  low  pressure 
mercury  arc.  Use  is  made  of  an  outer  globe  of  glass  which  cuts  off 


D.  C.  55  Volt  3.5  Amp.  Two  in  Series  on 
100-125  Volt  Circuit 


A.  C.  100-125  Volt  4.1  Amp. 

2  &  3 
Curve  A     Represents  the  Candle-Power  Distribution  in  a 

Plane  Perpendicular  to  the  Axis  of  the  Tube 
Curve  B     Represents  the  Candle-Power  Distribution  in  a 

Plane  Parallel  to  the  Axis  of  the  Tube 
Curve   C     Represents  the  Mean  of  Curves    A  &  B 

Fig.  24. — Glass  tube  mercury  vapor  lamps — Cooper  Hewitt. 
(Data  furnished  by  Cooper  Hewitt  Co.) 

(Facing  page  158.) 


Fig.  25. — Standard  tubes  now  manufactured  by  Cooper  Hewitt  El.  Co. 


Fig.  26. — Cooper  Hewitt  mercury  vapor  lamps  banked  for  moving-picture  photography. 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS 


the  ultra-violet  light  and  tends  to  diffuse  the  light.  A  metal  reflector 
confines  practically  all  the  light  to  the  lower  hemisphere. 

The  unit  is  essentially  one  of  high  power,  so  that  in  industrial 
plants  it  is  usually  installed  where  it  can  be  hung  20  ft.  or  more  from 
the  floor. 

This  lamp  is  illustrated  in  Fig.  27,  which  also  shows  candle-power 
distribution,  while  the  general  data  are  given  in  Table  IV. 


Hemispherical  Candle- 

Power  Distribution  with 

Reflector  and  Clear 

Glass  Globe 


Hemispherical  Candle- 

Power  Distribution  with 

Reflector  and  Diffusing 

Globe 


Curve  A  Represents  the  Candle -Power  Distribution  in  a 
Plane  Perpendicular  to  the  Axis  of  the  Burner 

Curve  B  Represents  the  Candle  -  Power  Distribution  in  a 
Plane  Parallel  to  the  Axis  of  the  Burner 

Curve    C   Represents    the    Mean    of  Curves    A   &  B 

Fig.  27. — Quartz  tube  mercury  vapor  lamp — Cooper  Hewitt. 
(Data  furnished  by  Cooper  Hewitt  Co.) 


The  light  from  both  forms  of  mercury  arc  lamp  is  steady  and  fairly 
diffuse.  Its  most  prominent  characteristic  is  its  blue-green  color, 
there  being  no  red  rays.  This  precludes  its  use  for  decorative  light- 
ing, except  for  special  effects.  The  appearance  of  faces  under  the 
light  is  not  at  all  pleasing.  On  the  other  hand,  the  light  is  highly 
actinic  and  of  such  character  as  to  reveal  detail  to  advantage,  where 
visual  acuity  is  an  important  factor. 

The  light  from  the  quartz  tube  lamp  (without  glass  globe)  is 
destructive  to  certain  forms  of  germ  life,  and  hence,  valuable  for 
sterilization. 

Both  the  low-pressure  and  the  high-pressure  lamps  are  used  exten- 


l6o  ILLUMINATING   ENGINEERING    PRACTICE 

sively  for  photographic  lighting,  for  which  the  actinicity  of  the  light 
renders  them  quite  effective. 

These  lamps  also  find  considerable  application  in  industrial 
lighting.31 

CARE  OF  LAMPS 

The  performance  of  any  lamp  depends  upon  its  receiving  a  reason- 
able amount  of  care.  While  some  types  of  lamps  require  more  at- 
tention than  others,  no  lamp  will  give  its  best  service  if  entirely 
neglected. 

The  glassware,  whether  of  lamps  or  windows,  will,  if  allowed  to 
become  coated  with  dirt  or  dust,  absorb  an  excessive  amount  of 
light.  The  same  is  true,  though  usually  in  a  lesser  degree,  of  reflect- 
ing surfaces.  It  is  economical  to  keep  globes  and  reflectors  clean, 
especially  those  which  are  so  turned  as  to  facilitate  the  accumulation 
of  dust.  Moreover,  the  good  appearance,  both  lighted  and  un- 
lighted,  often  depends  very  much  on  cleanliness. 

Beside  the  depreciation  due  to  external  accumulation,  all  illumi- 
nants  are  subject  to  what  is  sometimes  designated  as  inherent  de- 
preciation; that  is,  decrease  in  light  due  to  accumulation  or  changes 
inherent  in  the  light  source  itself.  For  example,  incandescent  lamps 
are  subject  during  their  operating  life  to  a  gradual  decrease  in  can- 
dle-power, due  to  bulb  blackening  and  the  filament  shrinking.  At 
the  end  of  the  rated  life,  this  depreciation  amounts  to  from  10  to  20 
per  cent.  With  nearly  all  other  illuminants  the  losses  are  fully  as 
great  or  greater.  With  arc  lamps  losses  are  principally  due  to  the 
accumulation  of  electrode  material  on  the  globes.  With  some  arc 
lamps,  the  washing  of  the  globe  at  the  time  of  trimming  returns  the 
lamp  to  initial  efficiency,  while  with  others  the  material  fuses  into  the 
globes  so  that  it  cannot  be  readily  removed.  In  the  case  of  the 
incandescent  lamp  and  mercury-vapor  lamp,  the  lamp  should  be 
replaced  when  the  loss  exceeds  certain  economic  limits,  20  per  cent, 
loss  having  been  generally  assumed  as  the  " smashing  point"  for 
the  incandescent  lamp.  For  arc  lamps,  the  cleaning  of  the  globes  at 
each  trimming  and  the  replacing  of  the  globes  when  badly  pitted 
are  the  recommended  practices. 

Unfortunately  for  best  economy  the  above-mentioned  losses 
accumulate  so  slowly  that  their  magnitude  is  not  generally  recog- 
nized, and  many  lamps  are  operated  at  unnecessarily  low  economies. 
In  a  large  installation,  it  is  profitable  to  provide  for  regular  periodic 
inspection,  cleaning  and  replacement. 

In  trimming  arc  lamps  it  is  important  to  use  electrodes  of  the  cor- 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        161 

rect  length  and  proper  diameter,  and  to  make  sure  that  they  are 
in  alignment,  making  good  electrical  contact  with  holders. 

Mechanisms  should  be  kept  clean  and  in  adjustment.  Care 
should  be  taken  on  installing  to  insure  that  the  adjustment  is  proper 
for  the  line  current  and  frequency.  Certain  forms  of  arc  lamps  have 
suffered  more  in  popularity  from  careless  maintenance  than  through 
inherent  inferiority. 

Incandescent  lamps  should  be  specified  to  correspond  to  the  actual 
socket  voltage  (series  lamps  to  amperage).  All  lamps  operate  best 
on  steady  voltage.  Excessive  change  in  voltage  means  unsteadiness 
of  light,  and  in  some  cases  objectionable  jumping  and  flickering. 

SELECTION  OF  LAMPS 

For  most  classes  of  lighting,  practice  has  indicated  some  one  type 
of  lamp  which  is  better  suited  than  others,  so  that  there  is  not  so 
keen  a  competition  between  types  as  formerly.  The  problem  now  is 
more  the  selection  of  lamps  of  proper  power.  This  subject  is  so 
broad  and  involves  spacing  and  height,  as  well  as  candle-power 
distribution  characteristics  to  such  an  extent  as  to  render  a  full 
discussion  at  this  point  impracticable.  It  does  seem  desirable, 
however,  to  warn  against  giving  too  much  weight  to  abstract  com- 
parisons of  candle-power  or  lumen  output,  or  efficiency,  or  even  of 
operating  cost,  especially  where  the  differentials  are  relatively  small. 

Reliable  and  accurate  comparisons  can  only  be  made  by  taking 
into  account  many  factors  with  reference  to  the  conditions  to  be  met 
in  installation.  It  often  happens  that  the  higher  efficiency  of  a 
high  power  lamp  is  counteracted  by  waste  of  light,  or  objectionable 
shadows,  accompanying  wide  spacing.  Again,  an  efficient  lamp 
may  have  a  high  investment  or  maintenance  cost. 

Cost  comparisons  are  of  value  and  should  be  made  where  large 
numbers  of  lamps  are  involved.  Such  an  estimate  should  include 
the  following  items: 

1 .  Cost  of  energy. 

2.  Material  of  maintenance. 

3.  Labor  of  maintenance. 

4.  Depreciation  (which  will  refund  the  investment  when  the  lamps  are  worn 
out  or  become  obsolete,  but  not  include  material  of  maintenance). 

5.  Interest  on  investment  (including  installation  cost). 

6.  Any  other  overhead  charges,  such  as  insurance. 

On  the  other  hand,  there  are  important  factors  which  do  not  lend 
themselves  to  expression  in  figures. 


1 62  ILLUMINATING   ENGINEERING   PRACTICE 

The  following  are  some  of  the  desirable  qualities  which  should  be 
considered  in  lamp  selection:33 

(a)  Intensity  or  light  flux — suited  for  condition,  allowance  being  made  for 
depreciation. 

(6)  Diffusion — of  a  degree  depending  upon  requirements. 

(c)  Distribution  characteristic — such  as  to  insure  economical  utilization. 

(d)  Color — to  meet  the  demands  of  utility  and  pleasing  appearance. 

(e)  Steadiness — slight  animation  not  being  necessarily  objectionable;    per- 
ceptible flicker  almost  invariably  objectionable. 

(/)  Reliability — insuring  continuity  of  illumination,  also  safety. 

(g)  Economy — as  previously  noted,  should  be  judged  by  concrete  rather  than 
abstract  estimate  of  costs. 

(A)  Artistic  features — involves  the  appearance  of  lamp  fixtures,  lighted  and 
unlighted,  as  well  as  the  lighting  effect  itself.  Deserves  more  attention  in 
ordinary  installations  than  it  usually  receives. 

(j)  Adaptability — this  is  important  in  large  installations  where  it  is  desirable 
to  meet  a  variety  of  conditions  with  a  minimum  number  of  types  and  renewal 
parts  to  be  kept  in  stock. 

(K)  Construction — quality.  Practically  all  the  established  illuminants  are 
well  made.  For  severe  service,  special  constructions  are  sometimes  necessary. 

(/)   Convenience — ease  of  handling  by  unskilled  persons. 

(m)  Congruity.  This  applies  to  the  general  suitability  of  the  illuminant  to 
its  surroundings. 

While  no  accurate  method  of  applying  these  considerations  is 
here  suggested,  a  common-sense  consideration  of  these  points  will 
facilitate  forming  a  true  evaluation  of  a  lighting  unit  for  particular 
service. 

CONCLUSION 

Much  of  the  foregoing  is  necessarily  suggestive,  but  definite  in- 
formation is  given  where  practicable.  It  must  be  remembered 
that,  with  the  rapid  advance  in  the  art  of  lamp  manufacture,  the 
performance  of  illuminants  is  likely  to  be  bettered  in  the  near  future. 

In  conclusion,  the  writer  desires  to  express  appreciation  for  the 
data  and  information  furnished  by  lamp  and  electrode  manufacturers. 

References    . 

1 C.  P.  STEINMETZ. — "Electric  Illuminants."  Lectures  on  Illuminating 
Engineering,  Johns  Hopkins  University,  1910,  page  109. 

2  E.  P.  HYDE. — "The  Physical  Characteristics  of  Light  Sources."  Lectures  on 
Illuminating  Engineering,  Johns  Hopkins  University,  1910,  page  25. 

3W.  R.  WHITNEY. — "The  Chemistry  of  Luminous  Sources."  Lectures  on 
Illuminating  Engineering,  Johns  Hopkins  University,  1910,  page  93. 

4  W.  F.  LITTLE . — "Lighting  Accessories"  (See  lecture  in  this  series). 

6  F.  W.  SMITH,  Chairman,  Lamp  Committee  N.  E.  L.  A.— "Report  of  Lamp 
Committee."  Proceedings  of  National  Elec.  Light  Assn.,  1916. 


STICKNEY:  DEVELOPMENTS  IN  ELECTRIC  LAMPS        163 

6  C.  G.  FINK. — "Ductile  Tungsten  and  Molybdenum."  Trans.  American 
Electro- Chemical  Society,  Vol.  XVII  (1910),  page  229.  General  Electric  Review, 
Vol.  XII  (1910)  page  323. 

7W.  D.  COOLIDGE. — "Wrought  Tungsten."  Trans.  American  Inst.  of 
Electrical  Engineers.  Vol.  XXIX  (1910),  page  961. 

8J.  W.  HOWELL  —  "The  Manufacture  of  Drawn  Wire  Tungsten  Lamps." 
G.  E.  Review,  Vol.  XVII  (1914),  page  276. 

'WARD  HARRISON  and  E.  J.  EDWARDS. — "Recent  Improvements  in  Incan- 
descent Lamp  Manufacture."  Trans.  111.  Eng.  Society.  Vol.  VIII  (1913), 
page  533- 

10  E.  J.  EDWARDS  and  H.  H.  MAGDSICK.— "Light  Projection"  (See  lecture  in 
this  series). 

11  IRVING  LANGMUIR. — "The  Blackening  of  Tungsten  Lamps  and  Methods  of 
Preventing  It."     Trans.  American  Inst.  of  Electrical  Engrs.,  Vol.  XXXII  (1913), 
page  1913. 

12  IRVING  LANGMUIR  and  J.  A.  ORANGE. — "Nitrogen  Filled  Lamps."     Trans. 
Amer.  Inst.  of  Elect.  Engrs.,  Vol.  XXXII  (1913),  page  1935. 

13  G.  M.  J.  MACKAY. — "The  Characteristics  of  Gas-filled  Lamps."     Trans. 
111.  Eng.  Society,  Vol.  IX  (1914),  page  775. 

14  F.  W.  SMITH  (Chairman,  Lamp  Committee  N.  E.  L.  A.). — "Report  of  Lamp 
Committee."     Proceedings  National  Elec.  Light  Assn.,  1915. 

G.  F.  MORRISON. — Review  of  Lamp  Committee  Report.     G.  E.  Review, 
Vol.  XVIII  (1915),  page  925. 

16  D.  B.  RUSHMORE. — "Frequency."  Trans.  American  Inst.  of  Electrical 
Engrs.,  Vol.  XXXI  (1912),  pages  970  and  978. 

16  D.  MCFARLAN  MOORE. — "Gaseous  Conductor  Lamps  for  Color  Matching." 
Trans.  111.  Eng.  Society,  Vol.  XI  (1916),  page  162. 

17  GEORGES   CLAUDE. — "Neon  Tube  Lighting."     Trans.   111.   Eng.   Society, 
Vol.  VIII  (1913),  page  371. 

18 W.  D.  COOLIDGE. — "A  Powerful  Rontgen  Ray  Tube  with  Pure  Electron 
Discharge."  Physical  Review,  Dec.,  1913.  G.  E.  Review,  Vol.  XVII  (1914), 
page  104. 

19  C.  S.  MCDOWELL.— "Illumination  in  the  Navy."     Trans.  111.  Eng.  Society, 
Vol.  XI  (1916),  page  574. 

20  S.  H.  BLAKE.— "Flame  Arc  Lamps."     G.  E.  Review,  Vol.  XIV  (1911), 
page  595- 

21  G.  N.  CHAMBERLIN.—"  Enclosed  Flame  Arc  Lamp."     G.  E.  Review,  Vol. 
XV  (1912),  page  706. 

22  R.  B.  CHILLAS.— "The  Development  of  the  Flame  Carbon."     Trans.  111. 
Eng.  Society,  Vol.  IX  (1914),  page  710. 

"  V.  A.  CLARK.— "  Present  Status  of  Arc  Lamp  Carbons."  Electrical  Review 
and  Western  Electrician,  Vol.  LXVII  (1915),  page  406. 

24  C.  E.  STEPHENS. — "Modern  Arc  Lamps."     Electrical  Review  and  Western 
Electrician,  Vol.  LXVII  (1915),  page  409. 

25  A.  T.  BALDWIN. — "The  Flaming  Arc  in  the  Iron  and  Steel  Industry." 
Proceedings  Assn.  Iron  &  Steel  Elect.  Engrs.  (1914),  page  491. 

26  C.  A.  B.  HELVORSON,  JR. — "New  Types  of  Ornamental  Luminous  Arc 
Lamps."     G.  E.  Review,  Vol.  XV  (1912),  page  710. 


164  ILLUMINATING   ENGINEERING   PRACTICE 

27  C.  A.   B.   HALVORSON,  JR. — "Improvements  in  the   Magnetite  Lamp." 
G.  E.  Review,  Vol.  XVII  (1914),  page  283. 

28  C.  A.  B.  HALVORSON,  S.  C.  ROGERS  and  R.  B.  HUSSEY.— "Arc  Lamps  for 
Street  Lighting."     Trans.  111.  Eng.  Society,  Vol.  XI  (1916),  page  251. 

29  F.  CONRAD  and  W.  A.  D ARRAH.— "  The  History  of  the  Arc  Lamp."     Electric 
Journal,  1916,  pages  103  and  140. 

30 H.  E.  IVES.— "Study  of  the  Light  from  the  Mercury  Arc."     Electrical 
World,  Vol.  LX  (1912),  page  304. 

31  W.  A.  D.  EVANS.— "Industrial  Lighting  with  Mercury  Vapor  Lamps." 
Trans.  111.  Eng.  Society,  Vol.  X  (1915),  page  883. 

32  W.  A.  D.  EVANS.— "The  Mercury  Vapor  Quartz  Lamp."     Trans.  111.  Eng. 
Society,  Vol.  IX  (1914),  page  i. 

33  P.  S.  MILLAR.— "The  Status  of  the  Lighting  Art."    Trans.  111.  Eng.  Society, 
Vol.  VIII  (1913),  page  652  (See  "Categories  of  Illumination,"  page  654). 


RECENT  DEVELOPMENTS  IN  GAS  LIGHTING 

BY  ROBERT  FFRENCH  PIERCE 

For  the  purpose  of  this  lecture  the  term  "recent  developments," 
will  be  applied  to  changes  and  improvements  in  gas  lighting  appli- 
ances effected  and  reduced  to  commercial  practice  since  1910, 
progress  prior  to  that  year  having  been  set  forth  in  the  lectures  at 
Johns  Hopkins  University. 

The  economic  position  of  the  gas  industry  has  tended  to  restrict 
development  to  the  refinement  and  elaboration  of  existing  types 
rather  than  to  encourage  increasing  diversity  in  the  application  of 
gas  to  lighting. 

Gas  was  the  first  central  station  illuminant  and  until  1880  the 
only  one.  At  the  present  time,  in  the  older  communities  of  the 
East  there  are  from  four  to  seven  times  as  many  gas  meters  as  elec- 
tric meters  in  use,  while  even  in  the  newer  communities  of  the 
West,  where  cheap  hydro-electric  power  and  dear  coal  place  the 
gas  industry  under  a  severe  handicap,  the  number  of  gas  meters 
usually  exceeds  that  of  electric  meters  in  use.  Following  the  line 
of  least  resistance  the  gas  industry  has  directed  such  of  its  energies 
as  have  been  devoted  to  lighting  toward  those  improvements  which 
would  best  protect  its  existing  lighting  business,  while  the  commercial 
exigencies  of  electrical  development  have  favored  the  creation  of 
new  uses  and  excursions  into  new  fields. 

During  the  past  five  years  the  principal  developments  in  gas 
lighting  have  had  for  their  objects  increased  economy  in  light  pro- 
duction through  more  efficient  utilization  of  the  gas  and  decreased 
maintenance  expense,  and  the  elimination  of  inconvenience  in  the 
use  and  maintenance  of  gas  lighting  units,  with  the  purpose  of  fore- 
stalling, overcoming  or  reducing  the  users'  inclination  toward 
providing  facilities  for  the  use  of  competing  illuminants. 

The  gas  lamp  is  composed  of  two  essential  parts — the  burner  and 
the  mantle,  the  former  usually  being  fitted  with  a  glass  chimney  to 
secure  satisfactory  and  efficient  operation. 

Possibilities  of  increased  economy  of  light  production  lie  in  ob- 
taining higher  temperatures  through  improved  burner  design;  in 

165 


1 66  ILLUMINATING   ENGINEERING   PRACTICE 

securing  a  larger  proportion  of  luminous  radiation  through  the  selec- 
tion of  mantle  materials  having  a  more  favorable  selective  radia- 
tion characteristics;  in  prolonging  the  useful  life  of  the  mantle  by 
the  utilization  of  less  fragile  base  fabrics;  and  in  eliminating  such 
accessories  as  chimneys  the  maintenance  of  which  is  an  item  of 
expense. 

Opportunities  for  securing  added  convenience  in  the  use  of  gas 
lamps  lie  in  such  of  the  above  developments  as  reduce  the  number 
of  parts  requiring  attention  and  the  frequency  with  which  essential 
parts  need  replacement,  and  in  the  provision  of  simple,  inexpensive 
and  reliable  means  of  ignition  and  distance  control. 

THE  MANTLE 

The  physical  character  of  the  mantle  is  determined  by  the  two 
essential  substances  which  enter  into  its  manufacture,  (i)  the  organic 
fabric  which  is  impregnated  with  solutions  of  salts  of  the  (2)  rare 
earths  (ceria  and  thoria)  that  form  the  ultimate  mantle  structure, 
the  organic  matter  being  burned  out  in  the  process  of  manufacture. 
The  character  of  the  fabric  used  determines  the  mechanical  strength 
of  the  mantle,  its  shrinkage  under  the  continued  heat  of  the  flame, 
and  to  a  small  extent  the  luminosity  of  the  mantle.  The  rare  earths 
employed  determine  the  radiant  efficiency  of  the  mantle,  and  the 
color  of  the  light  emitted. 

No  significant  change  in  the  proportions  of  ceria  and  thoria  em- 
ployed has  taken  place  in  the  past  twenty  years,  and  although  a 
theoretical  consideration  of  the  physics  of  rare  earths  radiation 
indicates  the  possibility  of  greatly  increased  efficiency  through  the 
employment  of  hitherto  unused  elements,  no  promising  experimental 
results  have  as  yet  been  recorded. 

The  utilization  of  " artificial  silk"  as  a  base  fabric  was  noted  by 
Whittaker  in  his  Johns  Hopkins  lecture,  but  this  material,  had  not 
at  that  time  been  brought  to  such  a  commercial  stage  as  would  war- 
rant specific  quantitative  statements  as  to  its  performance,  and  the 
employment  of  this  substance  may  for  the  purposes  of  this  lecture 
be  regarded  as  a  subsequent  development.  Mantles  made  upon  this 
base  have  been  used  in  large  quantities  during  the  past  three  years 
and  exhibit  a  great  superiority  over  previous  types  in  tensile 
strength,  flexibility,  permanence  of  form  and  maintenance  of 
luminosity.  The  artificial  silk  mantle  of  the  upright  type  after 
several  hundred  hours  service  will  support  a  suspended  weight  of 


•s 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING 


i67 


one  ounce,  may  with  care  and  skill  be  folded  and  crumpled  upon  itself 
and  restored  to  its  original  form  without  apparent  damage  and  will 
maintain  its  initial  candle-power  practically  unimpaired  for  an 
indefinite  period — 5000  hours  actual  service  producing  a  deprecia- 
tion of  less  than  10  per  cent.  These  facts  while  exemplifying  no 
practical  condition,  are  highly  significant  as  indicating  most  desir- 
able and  important  physical  properties.  It  should  be  understood, 
of  course,  that  the  rather  theatrical  demonstrations  of  desirable 
physical  qualities  referred  to  are  not  to  be  attempted  by  the  user 
unless  he  wishes  to  purchase  a  new  mantle. 


New 


Old 


\ 


2345678 
%  Ceria 

Fig.  6. — Influence  of  ceria  content  on  candle-power  of  mantle. 


10 


The  desirable  qualities  of  artificial  silk  are  due  to  the  fact  that  the 
fibers  are  solid  and  continuous,  instead  of  cellular  and  comparatively 
short. 

Figs.  3,  4  and  5  showing  magnified  sections  of  different  mantle 
fabrics  illustrate  the  steel-cable-like  structure  of  the  artificial  silk 
mantles  compared  to  that  of  mantles  based  upon  vegetable  fibers 
more  resembling  a  hempen  rope.  The  cellular  structure  is  largely 
responsible  for  the  shrinkage  during  burning  which  characterizes 
cotton  mantles. 

Due  to  causes  not  altogether  apparent  the  luminosity  of  a  mantle 
is  considerably  influenced  by  proportioning  of  the  rare  earth  contents 
with  relation  to  the  physical  structure  of  the  mantle  fabric,  and  re- 
finements in  manufacturing  processes  have  resulted  not  only  in 


1 68  ILLUMINATING   ENGINEERING   PRACTICE 

increasing  efficiencies  with  the  same  fabrics,  but  in  altering  the  re- 
lation between  ceria  content  and  luminosity.  Fig.  6  shows  two 
curves  of  mantles,  made  upon  the  same  fabric,  the  one  designated 
"old"  being  that  reproduced  by  Whittaker  in  the  Johns  Hopkins 
lectures.  Since  the  yellowness  of  the  light  emitted  varies  with  the 
ceria  content,  it  is  apparent  that  the  later  mantles  appreciably  widen 
the  range  of  color-values  which  may  be  economically  obtained  in 
the  gas  mantle. 

Other  interesting  developments  involving  departures  from 
previous  methods  of  mantle  construction  have  occurred,  but,  since 
they  are  more  directly  related  to  modifications  in  the  burner,  they 
will  be  introduced  later. 

BURNERS 

Since  the  efficiency  of  an  incandescent  gas  lamp  is  directly  related 
to  the  flame  temperature,  and  the  latter  depends  largely  upon  the 
proportion  of  primary  air  entrained,  it  is  desirable  that  the  latter 
be  as  large  as  practicable.  But  since  the  speed  of  flame  propaga- 
tion is  also  increased  with  the  proportion  of  primary  air,  the  latter 
is  practically  limited  by  the  velocity  of  the  outflowing  mixture  at 
the  nozzle,  because  the  speed  of  flame  propagation  and  velocity  of 
outflow  must  be  equal  in  order  to  avoid  " flashing  back"  of  the 
flame  on  the  one  hand,  or,  "blowing  off"  on  the  other — the  latter 
difficulty,  however,  never  being  experienced  at  ordinary  pressures. 

The  highest  velocity  of  outflow  is  secured  by  means  of  proper 
design  of  the  bunsen  tube  and  freedom  from  bends  or  obstructions 
in  the  burner.  Such  a  burner,  however,  fails  to  secure  thorough 
mixture  of  the  gas  and  air  with  the  result  that  the  more  highly 
aerated  "streaks"  permit  the  flashing  back  of  the  flame  even  though 
the  average  speed  of  flame  propagation  is  far  below  that  in  the  more 
highly  aerated  portions.  Since  thorough  mixture  of  the  gas  with 
the  entrained  air  involves  some  loss  in  the  velocity  of  outflow, 
burner  design  is  resolved  into  the  elimination  of  all  obstructing  and 
retarding  influences  except  those  required  for  mixing  the  gas  and 
air  in  the  most  efficient  manner. 

The  sole  source  of  energy  for  the  entrainment  of  air,  mixing  it 
with  the  gas  and  the  propulsion  of  the  mixture  into  the  flame  is  the 
kinetic  energy  of  the  gas  issuing  from  the  orifice  under  a  pressure  of 
(ordinarily)  less  than  2  ounces  per  square  inch,  and  it  is  the  conserva- 
tion of  this  small  amount  of  energy  that  presents  the  greatest  problem 
to  the  designer  of  incandescent  gas  lamps. 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING  169 

Within  the  last  three  years  a  greater  appreciation  of  the  im- 
portance of  this  feature  has  led  to  the  development  of  a  type  of 
burner  having  not  only  improved  efficiency,  but  simpler  construc- 
tion and  fewer  parts  than  have  characterized  previous  types.  These 
results  are  direct  consequences  of  greater  air  entrainment,  more 
thorough  mixing  of  the  gas  and  air,  and  higher  nozzle  velocities. 
In  the  previous  types  larger  proportions  of  secondary  air  were 
required.  To  bring  this  secondary  air  into  the  flame  with  sufficient 
speed  to  localize  the  combustion  area  most  effectively  in  the  mantle 
surface  and  secure  satisfactory  efficiencies,  various  devices  were 
employed — notably  air-hole  cylinders  and  " stacks"  to  produce 
strong  upward  drafts.  These  accessories  complicated  design,  in- 
creased maintenance  expense  and  often  interfered  with  adaptation 
of  the  lamps  in  fixture  design.  In  the  recent  lamps  it  has  been 
found  practical  to  eliminate  chimneys,  lamp  housings,  stacks,  etc., 
with  no  loss  of  efficiency.  The  elimination  of  the  chimney  Or 
cylinder  removes  one  of  the  most  troublesome  sources  of  candle- 
power  depreciation  in  gas  lamps.  Reduction  of  illumination  of 
from  10  to  20  per  cent,  in  1000  hours'  active  service  commonly 
results  from  the  dust  deposits  on  chimneys. 

Relieved  from  the  necessity  of  accommodating  these  accessories, 
the  designer  has  employed  greater  freedom  in  the  development  of  a 
range  of  sizes,  and  in  their  application  and  these  lamps  are  now  made 
in  sizes  from  one  to  six  mantles  and  in  upright,  inverted  and  hori- 
zontal forms.  The  mantle  generally  used  with  these  burners  is 
ij£  in.  in  diameter  by  ij^  in.  long,  mounted  on  the  common  open 
top  ring.  It  has  been  found  however  that  with  this  type  of  burner 
closed  top  mantles  5^  X  i.in.,  consuming  about  i  cu.  ft.  of  gas  per 
hour  may  be  used,  there  being  no  necessity  for  leaving  a  space  at 
the  top  of  the  mantle  for  the  egress  of  combustion  products  in  excess 
of  those  which  pass  through  the  mantle  mesh.  This  permits 
the  use  of  the  so-called  rag  or  soft  mantle — a  mantle  from  which 
the  organic  fabric  has  not  been  burned  out,  this  operation,  which 
is  usually  performed  in  the  factory,  being  done  by  the  purchaser. 
In  order  for  the  mantle  to  fill  out  properly  an  appreciable  pressure 
inside  the  mantle  is  necessary.  This  is  obtained  by  the  use  of  com- 
pressed ah-  at  the  factory,  but  on  the  customers'  premises  only  the 
ordinary  pressure  within  the  mantle  is  available,  and  in  order 
for  this  to  be  effective,  the  top  of  the  mantle  must  be  closed,  forcing 
all  the  products  through  the  meshes  of  the  fabric.  With  the 
existing  pressures  on  the  customers  premises,  it  is  not  practicable  to 


170  ILLUMINATING   ENGINEERING   PRACTICE 

burn  off  and  properly  harden  a  mantle  larger  than  %  X  i  in.  on 
the  customers'  burners. 

The  rag  mantle  has  many  advantages.  It  is  as  soft  and  pliable 
as  any  other  knitted  fabric.  It  cannot  be  injured  by  handling  and 
may  be  packed  in  a  small  space  and  transported  with  impunity. 
The  lamp  shown  in  Figs.  7  and  8  is  equipped  with  three  of  these 
small  size  rag  mantles  and  is  particularly  adapted  to  fixtures  with 
upright  outlets,  as  for  example,  those  ordinarily  fitted  with  open 
flame  tips. 

The  inverted  lamp  (that  is,  that  in  which  the  bunsen  type  projects 
downward  from  the  gas  orifice)  requires  a  housing  of  some  sort  to 
which  the  shade  may  be  attached  and  in  which  means  for  conducting 
the  combustion  products  away  from  the  air  ports  may  be  provided. 


Fig.  7. — New  upright  burner  with  Fig.  8. — Installation  of  lamp  shown  in 

inverted  mantles.     (Cut    about  one-  Fig.  7. 

third  actual    size.) 

Until  recently,  the  discoloration  of  the  lamp  housing  and  support- 
ing fixture  arm  or  pendant  by  heat  and  combustion  products  was  a 
serious  drawback  in  the  use  of  inverted  gas  lamps,  particularly  in 
residences  and  in  mercantile  establishments  of  the  better  class. 
In  the  latest  designs  this  trouble  has  been  eliminated  by  providing 
an  air  space  between  on  the  inner  and  the  outer  shell,  and  a  deflector 
which  ejects  the  combustion  products  with  sufficient  velocity  to 
carry  them  several  inches  out  from  the  top  of  the  lamp.  Figs,  ga 
and  gb  show  distributions  of  temperatures  about  two  lamps  of  this 
type,  the  center  of  the  uprising  column  of  products  being  shown  by 
the  heavy  line  connecting  the  points  of  maximum  temperature  at 
each  level.  Protracted  tests  indicate  that  the  elimination  of  fixture 
discoloration  by  this  method  is  complete. 

An  interesting  development  in  the  design  of  inverted  burners  is 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING 


171 


TEST  3761 

Welsbach  Testing  Laboratories 

3-3-'/6  J.RA. 


Fig.  pa. — Distribution  of  temperature  about  side-vent    lamp  consuming  2%  cubic  feet  of 

gas  per  hour. 


172 


ILLUMINATING   ENGINEERING   PRACTICE 


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Bf   Of     9f    iqp   ty    If 3  I4S   Iff  324  X3    lf/>  l&  9J 

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TEMPERATURE  I 
72*  F 


TEST  2341 

ing  £aboratorifs 


Fig.  96. — Distribution  of  temperature  about  side-vent  lamp  consuming  9}-^  cubic  feet  of 

gas  per  hour. 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING  173 

shown  in  Fig.  10.  In  this  design  both  the  air  intakes  and  the  vents 
are  concealed  from  ordinary  view,  and  the  parts  are  so  arranged  as 
to  permit  the  design  of  a  burner  exterior  of  unobtrusive  form  and 
attractive  lines.  This  design  has  been  applied  to  sizes  ranging  from 
85  to  250  candle-power,  arid  thus  accomplishes  a  standardization  of 
appearance  approaching  that  obtained  by  means  of  the  standardized 
socket  construction  in  incandescent  electric  lamps.  The  gas  cock 
is  operated  by  a  single  pull  chain,  and  the  complete  unit  possesses 
many  features  which  appeal  to  the  fixture  designer  as  well  as  to  the 
customer  who  wishes  to  avoid  the  use  of  lamps  which  too  strongly 
announce  by  their  appearance  the  nature  of  the  illuminant  supplied 
to  them. 

These  lamps  together  with  the  small  upright  lamp  shown  in  Fig. 
7  comprise  the  leading  products  of  the  most  important  American 
manufacturers  of  gas  lamps,  and  it  is  interesting  to  note  that  in 
both  types  the  tendency  has  been  to  eliminate  features  which 
emphasize  to  the  eye  the  burner  itself. 

IGNITION 

During  the  past  five  years  several  means  of  ignition  have  been 
attempted,  the  most  general  being  electrical — in  the  form  of  either 
a  jump  spark  or  an  electrically  heated  platinum  wire.  The  accessory 
apparatus  required — dry  batteries,  accumulators,  etc.,  and  the 
comparatively  high  cost  of  the  ignition  devices,  have  limited  the 
application  of  even  the  most  satisfactory  of  electrical  systems  to 
special  conditions  in  which  ignition  of  this  character  is  particularly 
desirable.  For  several  years  the  jump-spark  system  of  ignition  has 
been  utilized  for  gas  ignition.  This  system  usually  consists  of  a 
dry  battery,  induction  coil  and  spark  gaps,  one  for  each  lamp,  ar- 
ranged in  series.  The  drawbacks  to  this  system  have  been  the 
difficulty  of  securing  proper  insulation  for  the  secondary  or  high- 
tension  circuit,  the  high  first  cost  of  the  installation,  and  the  neces- 
sity for  providing  a  separate  system  for  distant  control  when  the 
latter  is  required — which  is  usually  the  case.  A  recent  development, 
originating  in  and  at  present  confined  to  Germany,  but  of  sufficient 
interest  to  warrant  description  here,  involves  the  use  of  a  special 
form  of  switch  which,  when  operated,  sets  in  motion  a  vibrating 
contractor  in  the  primary  circuit,  the  vibrations  persisting  for  a 
period  sufficient  to  permit  the  gas,  which  is  turned  in  by  a  magnet 
valve  in  the  same  circuit,  to  reach  the  lamp  before  the  high-tension 
spark  induced  by  the  making  and  breaking  of  the  primary  circuit, 


174  ILLUMINATING   ENGINEERING   PRACTICE 

dies  out.  The  induction  coil  is  placed  in  a  canopy  above  the  lamp 
which  also  contains  the  magnet  valve.  In  this  system  the  high- 
tension  circuit  is  confined  to  the  lamp  fixture.  This  device  is 
absolutely  positive  and  reliable  in  action,  its  only  drawback  being  the 
high  cost,  a  separate  induction  coil  and  magnet  valve  being  required 
for  each  fixture  (see  Figs,  n  and  12). 

Many  attempts  have  been  made  to  utilize  the  catalytic  action  of 
platinum  for  gas  ignition.  In  the  finely  divided  form  known  as 
platinum  black  this  element  possesses  the  property  of  condensing 
oxygen  upon  its  surface  and  initiating  combination  with  hydrogen 
in  the  presence  of  the  latter.  The  self-lighting  mantles  which 
sporadically  appear  upon  the  market  rely  upon  a  "pill"  of  platinum 
black  upon  the  mantle  surface  to  secure  ignition.  •  The  catalytic 
action  is,  however,  so  rapidly  decreased  by  the  agglomeration  of  the 
particles  of  heat  and  other  unavoidable  influences,  and  the  conse- 
quent reduction  of  catalyzing  surface  presented,  that  this  expedient 
has  never  come  into  extended  commercial  application. 

It  has  been  found,  however,  that  platinum  wire  heated  to  about 
5oo°C.  is  capable  of  initiating  the  combination  of  hydrogen  and 
oxygen  and  this  fact  has  been  utilized  in  the  "hot-wire"  ignition 
system  (Fig.  13),  in  which  electric  current  from  a  small  dry  battery 
or  accumulator  provides  the  heating  energy.  When  this  system 
was  first  applied  a  dry  battery  was  placed  in  the  shell,  a  switch  being 
actuated  by  the  operation  of  turning  the  gas  cock.  As  long  as  the 
battery  voltage  is  regulated  within  narrow  limits  the  results  are  very 
satisfactory.  A  device  of  similar  principle  in  which  the  heated 
platinum  filament  is  used  to  ignite  a  pilot  flame  which  in  turn  ignites 
the  gas  at  the  lamp  burner,  has  been  on  the  market  for  sometime  but 
apparently  without  radically  affecting  the  current  practice  in  gas 
ignition,  which  is  by  means  of  a  continuously  burning  pilot  flame. 

The  pilot-flame  method  is  too  commonly  used  and  known  to  re- 
quire explanation.  The  greatest  drawbacks  of  the  earlier  and  in 
fact  all  but  the  most  recent  types  were  the  cost  of  the  gas  consumed, 
which,  though  negligible  in  a  frequently  used  installation,  is  com- 
paratively great  in  the  case  of  lamps  in  active  service  for  only  a 
few  hours  per  week;  and  the  liability  to  outage  from  draughts,  de- 
posits of  pipe-scale,  tar,  etc.  In  well-operated  gas  works  the  gas 
is  freed  from  the  tar  at  the  works.  Where  practice  is  poor  in  this 
particular  a  small  filter-box  is  placed  in  the  gas  supply  to  the  pilot. 
The  asbestos  packing  in  this  filter-box  which  retains  dust,  scale,  tar, 
etc.,  and  can  easily  be  removed  and  renewed  when  fouled. 


Fig.  10. — Recent  types  of  inverted  lamps. 
To  Switch-On         .ds*.    To  Switch-Off 


Magnet  Valve 


Canopy 


From  Induction  Coil 
To  Spark-Plug 


Fig.   II. — Installation  of  magnet-valve  and  induction  coil  for  distant  control  and  jump- 
spark  ignition. 

(Facing  page  174.) 


\      Primary 


.Secondary     / 


i 

On  Magnet    / 


Off  Magnet 


.Ground 
N    "Armature 

[JF^   Gas  Cock 


Ground 


'Lamp 


Around  /Spark  Gap 

Fig.   12. — Wiring  connections  for  electro-magnetic  distance  control  and  ignition  of  gas 

lamps. 


Fig.  13. — Self-contained  fixture  operating  by  "hot-wire"  ignition. 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING 


Pilot  flames  may  be  protected  against  draughts  to  some  extent  by 
a  shield  (Fig.  14)  but  this  expedient  is  not  sufficiently  effective  to 
render  the  ordinary  pilot  an  altogether  reliable  means 
of  ignition. 

During  the  past  four  or  five  years  the  pilot  flame  has 
been  utilized  to  some  extent  as  a  low-intensity  illumi- 
nant.  When  a  small  Bunsen  flame  is  directed  against 
the  outside  of  a  gas  mantle  the  mantle  area  affected 
becomes  to  all  intents  and  purposes  a  small  incandescent 
mantle.  A  pilot  flame  of  the  Bunsen  type  consuming 
Lg  cu.  ft.  per  hour  will  if  directed  against  a  mantle,  pro- 
duce about  %  horizontal  candle-power,  as  against  J£ 
candle-power  for  a  luminous  or  open  flame  consuming 
gas  at  an  equal  rate.  This  is  sufficient  to  enable  the 
occupant  of  a  room  to  see  his  way  about,  to  find  keys 
or  pull-chains  controlling  the  lamps,  and  measurably  to  Fig-  14-— Pro- 
discourage  those  adventurers  into  high  finance  who 
operate  at  night  and  specialize  in  second-story  operations.  A  lamp 
equipped  with  such  a  pilot  becomes  a  " high-low" 
unit  operating  at  "low"  continuously.  Such  a  unit 
has  a  considerable  field  of  application  but  the  con- 
sumption of  90  cu.  ft.  of  gas  per  month  per  lamp 
costing  9  cents  per  lamp  per  month  with  gas  at  $1.00 
per  1000  cu.  ft.  constitutes  in  many  cases  an  obstacle 
to  the  general  use  of  this  system. 

In  1916  a  radical  development  appeared  in  the  form 
of  a  pilot  (Fig.  15)  consuming  but  J^6  cu.  ft.  per 
hour  and  of  a  very  simple  and  inexpensive  construc- 
tion.    This  pilot  consists  of  a  tip  surrounded  by  a 
small  bundle  of  mantle  fabric  saturated  with  salts 
of  rare  earths  which  have  been  found  effective  in  re- 
taining the  flame.     This  device  possesses  the  remark- 
able property  of  being  unaffected  by  breezes  of  12 
miles  per  hour,  sufficient  to  blow  a  mantle  of  ordi- 
Fig.  15.— Section  nary  size  from  its  supporting  ring.     A  unit  contain- 
wit^Tame-retaS  ing  tnis  device,  combines  an  unfailing  means  of  igni- 
ing  fabric  of  rare  tion  and  a  continuous  small  intensity  of  illumination, 
24  hours  per  day,  at  a  cost  of  only  4.5  cents  per  month 
per  lamp  with  gas  at  $1.00  per  1000  cu.  ft.  or  45  cents  per  month  for 
10  lamps — about  the  number  usually  required  in  a  y-room  dwelling. 
A  tabulation  of  pilot  consumptions  follows: 


i76 


ILLUMINATING  ENGINEERING  PRACTICE 


Lamp 

Normal 
pilot 
cons,  per 
hour 
(cu.  ft.) 

Approx. 
length 
of  flame 
(inches) 

Pilot 
cons,  per 
year 
(cu.  ft.) 

Normal 
lamp 
cons,  per 
hour, 
(cu.  ft.) 

Lamp 
cons,  per 
year  4 
hrs.  daily 
(cu.  ft.) 

Pilot 
cons,  per 
cent,  of 
total 
cons. 

I  Burner    inv.    indoor    Bunsen 
pilot  

O    I2O 

y. 

•t     en 

i  Burner  upr.  indoor  luminous 

VA 

A    fie 

6  780 

3  Burner  inv.  indoor  arc,  semi- 

*A 

8   i 

5  Burner  inv.  outdoor  arc,  semi- 
Bunsen  pilot 

o  213 

y~ 

l86e   o 

6  8 

i  Burner  inv.  indoor  luminous 
pilot 

o  152 

H 

1331  s 

•I     AC 

5  037 

20  9 

"Glower"  pilot 

o  04 

547   5 

3   o—  10* 

Depending  on  the  size  of  lamp. 


It  may  be  frankly  stated  that  prior  to  the  development  of  this 
device  the  use  of  gas  lighting  imposed  a  certain  unavoidable  sacrifice 
of  convenience  due  mainly  to  the  faultiness  of  existing  ignition  sys- 
tems, which  may  now  be  regarded  as  eliminated. 


DISTANT  CONTROL 

The  difficulty  of  controlling  gas  lamps  from  a  distant  point  lies 
mainly  in  the  necessity  for  controlling  the  flow  of  gas  at  a  point  near 
the  lamp.  If  considerable  pipe  capacity  is  placed  between  the  gas 
cock  and  the  lamp  the  admission  of  the  air  in  the  pipe  with  the  gas 
entering  when  the  cock  is  turned  on,  may  be  sufficient  to  cause  the 
flame  to  "flash  back"  to  the  orifice,  and  in  any  case  the  nearer  the 
cock  to  the  lamp  the  less  violent  the  ignition  of  the  gas.  Distant 
control  therefore  necessitates  means  of  operating  a  gas  cock  at  or 
very  near  to  the  lamp.  Usually  a  very  small  amount  of  energy 
must  suffice  for  the  actuating  of  the  gas  cock.  Unfortunately,  the 
most  satisfactory  type  of  cock  is  the  "plug"  type  in  which  a  tapered 
plug  containing  a  gas-way  is  ground  into  a  tapered  seat,  in  which  it 
turns.  On  account  of  the  large  bearing  surfaces  the  friction  is  con- 
siderable, and  though  it  may  be  much  reduced  by  proper  lubrica- 
tion, the  grease  used  is  soluble  in  some  of  the  gas  constituents 
(notably  benzol),  which  liquefy  at  the  temperatures  occasionally 
met  in  practice  and  dissolve  the  lubricant,  thereby  making  a  con- 
siderable increase  in  the  energy  required  to  actuate  the  cock. 
Another  form  of  valve  consists  of  an  annular  knife  edge  making 
contact  with  a  flat  seat.  Such  a  valve  is  easily  actuated  and  requires 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING  177 

no  lubricant,  but  may  be  kept  from  operating  by  small  particles  of 
scale  falling  between  the  knife  edge  and  its  seat,  thereby  preventing 
the  closing  of  the  valve  and  resulting  in  leakage.  The  probability 
of  failure  through  this  cause  may  be  greatly  reduced  by  proper  design, 
and  many  very  satisfactory  valves  have  been  constructed  upon  this 
principle. 

Gas  valves  for  remote  or  distant  control  may  be  actuated  by  air 
pressure,  by  gas  pressure  or  by  electricity. 

One  of  the 'simplest  examples  of  the  application  of  the  former 
method  is  the  pneumatic  cock,  consisting  of  a  cylindrical  plug  with 
gas- way  which  moving  axially  in  a  cylindrical  seat  controls  the  flow 
of  gas  to  the  lamp. 

A  small  hand  pump  having  a  bore  of  about  %  in.  and  a  stroke  of 
from  i  to  3  in.  furnishes  the  impulse,  transmitted  through  a  small 
tube  of  Jf  2  m-  inside  diameter,  which  moves  the  cock,  a  single  im- 
pulse of  compression  or  rarefaction  sufficing  to  open  and  close  the 
gas  way  respectively.  This  device  is  simple,  inexpensive  and  when 
carefully  designed,  constructed  and  installed,  reliable.  Unfor- 
tunately most  of  the  commercial  types  which  have  been  offered, 
lacked  the  first  two  qualifications,  and  were  so  designed  as  to  render 
the  accomplishment  of  the  third  difficult. 

Another  form  of  gas-pressure-actuated  valve  consists  of  an 
inverted  bell  over  mercury,  the  bell  serving  as  the  valve  proper 
and  the  mercury  as  the  "seat."  The  bell  is  weighted  so  as  to  be 
lifted  and  sustained  clear  of  the  mercury  by  the  gas-pressure  re- 
quired to  operate  the  lamp,  sinking  and  cutting  off  the  gas  supply 
when  the  pressure  is  reduced  below  a  predetermined  point.  The 
controlling  valve  is  fitted  with  a  by-pass  which  admits  enough  gas 
to  supply  the  pilot  flame  at  the  lower  pressure  when  the  main  gas 
supply  is  turned  off.  Valves  of  this  type  must  be  located  at  a 
sufficient  distance  from  the  lamp  to  avoid  evaporation  of  the  mercury 
by  heat. 

A  simple  and  reliable  automatic  shut-off  for  extinguishing  the 
lamp-flame  at  a  predetermined  time  consists  of  a  clock  incorporated 
into  the  gas-cock  arm,  the  latter  being  in  a  horizontal  position  for 
turning  the  gas  on.  At  the  predetermined  time  the  clock  disen- 
gages the  chain  which  maintains  the  arm  horizontal,  the  weight  of 
the  clock  and  arm  then  closing  the  cock. 

In  another  type  of  gas-pressure-actuated  valve  the  valve  proper 
is  a  flexible  metal  diaphragm  seating  against  an  annular  knife- 
edge.  The  space  opposite  the  seat  is  connected  with  the  main  gas 


178  ILLUMINATING   ENGINEERING   PRACTICE 

supply  pipe  by  a  small  controlling  pipe.  At  any  convenient  point 
in  the  small  controlling  pipe  a  three-way  cock  is  installed,  which  in 
one  position,  connects  the  main  gas  supply  with  the  diaphragm  cham- 
ber opposite  the  valve-seat,  and  in  another  connects  the  diaphragm 
chamber  with  the  outer  air.  In  the  first  position  the  pressures  on 
either  side  of  the  diaphragm  are  equalized  and  the  valve  is  closed. 
In  the  second  position  the  pressure  in  the  chamber  opposite  the  seat 
is  reduced  to  that  of  the  atmosphere  and  the  gas  pressure  on  the  seat 
side  of  the  diaphragm  opens  the  valve. 

ELECTROMAGNETIC  VALVES 

Two  forms  of  electrically  operated  valves  are  in  commercial  use 
in  this  country.  In  one  the  armatures  of  two  electromagnets 
actuate  a  tapered  plug  gas  cock  of  the  ordinary  type,  one  turning  the 
gas  on  and  the  other  off.  On  account  of  the  energy  required  to 
operate  a  cock  of  this  type,  it  is  desirable  that  the  magnet  be  of 
efficient  design  in  order  satisfactorily  to  utilize  the  limited  amount 
of  energy  available  from  small  dry  batteries.  Most  of  the  com- 
mercial types  fail  to  realize  the  possibilities  of  this  system  in  this 
direction  and  these  valves  are  principally  used  in  interior  installa- 
tions. They  are  comparatively  expensive  and  do  not  enjoy  exten- 
sive commercial  use.  Four  ordinary  dry  batteries  are  required  for 
one  valve. 

In  a  recent  valve  of  the  electromagnet  type  use  is  made  of  a  polar- 
ized core  in  a  solenoid  controlled  by  a  reversing  switch.  The  valve 
itself  consists  of  a  diaphragm  seating  upon  an  annular  knife  edge. 
The  normal  position  of  the  diaphragm  is  in  the  open  position,  seating 
being  accomplished  by  the  weight  of  the  solenoid  core,  assisted  by 
a  spring;  current  in  one  direction  lifts  the  solenoid  core  and  the 
diaphragm,  the  residual  magnetism  retaining  the  core  in  its  upper 
position  after  the  current  is  turned  off.  Current  in  the  opposite 
direction  overcomes  the  influence  of  the  residual  magnetism  and 
permits  the  core  to  fall,  closing  the  diaphragm  against  the  seat. 
One  dry  cell  is  sufficient  to  operate  this  valve  and  extremely  satis- 
factory operation  has  followed  its  commercial  application.  It  is 
somewhat  more  expensive  than  the  previously  described  type  of 
electromagnet  valve,  and  is  limited  in  commercial  application  to  the 
larger  units  with  which  the  cost  of  the  valve  is  a  relatively  unim- 
portant feature. 

A  recently  developed  magnet  valve  is  shown  in  Figs.  i6a  and  i6b. 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING 


179 


The  valve  proper  consists  of  a  disc  secured  to  the  solenoid  plunger 
by  means  of  a  ball  and  socket  joint,  ensuring  accurate  seating  against 
the  annular  knife  edge  seat.  A  small  spring  secured  in  the  plunger 
and  bearing  against  the  bore  of  the  magnet  spool  prevents  the  un- 


Fig.   i6a. — Electro-magnetic  gas  valve,  off. 

seating  of  the  valve  from  shock  or  vibration.  This  device  is  always 
installed  with  the  annular  knife-edge  in  a  vertical  plane  so  as  to 
eliminate  fouling  from  particles  of  pipe  scale,  and  the  seat  is  further 
protected  by  a  small  drip  in  the  upper  part  of  the  valve. 


Fig.   i6&. — Electro-magnetic  gas  valve,  on. 

GENERAL  DESIGN  OF  LAMP  AND  FIXTURES 

Store  Lighting. — The  development  of  means  for  securing  highly 
efficient  incandescence  of  the  gas  mantle  without  chimneys,  cylinders 
or  stacks  has  made  possible  a  freedom  and  variety  in  design  unattain- 
able with  the  older  types.  As  long  as  each  mantle  required  enclosure 


l8o  ILLUMINATING   ENGINEERING    PRACTICE 

in  a  chimney  or  cylinder,  or  each  small  group  of  mantles  in  a  globe, 
the  output  of  individual  lighting  units  was  limited  to  about  300 
c.p.  and  the  design  of  attractive  fixtures  was  difficult  on  ac- 
count of  the  obtrusion  of  awkward  mechanical  features  and  the 
limitations  imposed  thereby.  Figs.  17  and  18  show  the  burner 
arrangement  and  general  appearance  of  new  types  of  fixtures  exem- 
plifying the  importance  of  recent  developments  in  modifying  and 
improving  semi-indirect  fixture  design.  Figs.  19  and  20  show  other 
semi-indirect  feature  designs  recently  produced  by  leading  Ameri- 
can manufacturers. 

Fig.  22  shows  the  plan  of  a  20oo-c.p.  semi-indirect  fixture — the 
largest  modern  low-pressure  unit  thus  far  constructed — a  type  of 


Fig.   17. — Arrangement  of  horizontal  burners  in  semi-indirect  fixtures. 

design  altogether  impossible  with  the  older  lamps.  Several  of 
these  units  are  in  commercial  service  and  have  given  excellent 
satisfaction.  The  fixtures  as  installed  are  shown  in  Fig.  21. 

RESIDENCE-LIGHTING  FIXTURES 

It  is  generally  recognized  that  the  upright  mounting  of  lighting 
units  is  preferable  for  the  illumination  of  the  more  conventional 
interiors.  The  inherited  sense  of  appropriateness  by  the  satisfaction 
of  which  aesthetic  requirements  are  governed,  is  based  upon  the 
almost  universal  use  of  the  flame  as  a  light  source  in  the  past.  The 
tendency  toward  the  inversion  of  the  lighting  unit — notable  of  recent 
years — had  its  impulse  in  consideration  of  economy,  which  have  in  a 
large  measure  been  counterbalanced  by  improved  efficiency  in  the 
use  of  illuminants,  reduced  cost  of  energy,  and  by  the  increasing 


Fig.  1 8. — Recent  type  of  semi-indirect  gas  fixture. 


Fig.  19. — Recent  type  of  semi-indirect  gas  fixtures. 

(Facing  page  180.) 


Fig.  20. — A  novel  design  in  semi-indirect  fixture. 


Fig.  21. — 2000-c.p.  fixtures  installed.     (The  small  fixtures  belong  to  the  previous 

installation.) 


3  * 

fe    £ 


Fig.  24. — Incandescent  gas-lamps  arranged  for  lighting  a  photographic  studio.  Each 
lamp  consumes  4^  cubic  feet  of  gas  per  hour,  and  is  fitted  with  a  special  mantle  giving 
increased  radiation  at  the  shorter  wave-lengths. 


PIERCE:  DEVELOPMENTS  IN  GAS  LIGHTING 


181 


appreciation  of  the  semi-indirect  system  of  illumination.  In  the 
older  upright  gas  lamps  use  was  made  of  a  mantle  suspended  from 
the  top  and  open  at  the  bottom.  The  mantle,  freely  swinging  from 
its  support  suffered  mechanically  from  the  repeated  striking  of  the 
lower  portion  against  the  burner  head  and  the  life  was  much  shorter 
than  that  of  the.  inverted  mantle.  Furthermore,  the  chimney  re- 
quired was  an  item  of  expense,  and  a  source  of  annoyance  by  reason 
of  the  cleaning  required.  The  development  of  the  inverted  mantle 
upright  burner  before  mentioned  (Fig.  7)  has  made  possible  the  satis- 


Fig.  22. — Arrangement  of  six  s-mantle  burners  with  total  candle-power  of  2000  in 
42-inch  bowl. 

factory  and  convenient  use  of  gas  in  fixtures  of  the  type  shown  in 
Figs.  230  and  236. 

The  foregoing  have  been  cited  merely  to  emphasize  the  important 
influence  upon  fixture  design  of  the  mechanical  simplification  of  the 
lamp.  Though  there  is  nothing  unusual  about  any  of  these  designs 
they  really  exemplify  considerable  progress  for  they  represent  the 
removal  of  great  handicaps. 

SPECIAL  APPLICATIONS 

Although  the  economic  position  of  gas  and  the  traditional  con- 
servatism of  the  industry  have  directed  the  principal  developments 
13 


1 82  ILLUMINATING   ENGINEERING   PRACTICE 

along  the  line  of  enhancing  the  value  of  gas  lighting  in  existing  uses 
and  to  existing  customers,  some  interesting  excursions  into  new  fields 
have  been  conducted. 

A  great  deal  of  shop-window  lighting  has  been  creditably  done. 
In  one  city  where  some  efforts  has  been  directed  toward  developing 
this  use,  many  of  the  leading  exclusive  stores  in  this  city  use  the 
incandescent  gas  lamp  for  show-window  illumination.  The  greatest 
obstacle  to  the  use  of  gas  for  show-window  lighting  has  been  the 
expense  of  and  the  space  occupied  by  the  installation,  the  mainte- 
nance of  clean  glassware  and  the  liability  to  pilot  outage,  since  in 
most  cases  a  distributed  system  requiring  a  large  number  of  units  is 
preferred. 

PHOTOGRAPHY 

Through  the  development  of  a  mantle  of  low  ceria  content  having 
a  large  energy  radiation  in  the  violet  end  of  the  spectrum,  very 
creditable  studio-lighting  has  been  accomplished.  Fig.  24  shows  a 
studio  lighting  fixture  consuming  about  100  cu.  ft.  per  hour  by  which 
portraits  may  be  taken  with  shutter-drop  (J^  second)  exposures  on 
Orthonon  plates.  Although  no  effort  has  been  made  to  exploit  this 
system  a  considerable  commercial  demand  has  spontaneously  arisen. 

The  foregoing  are  indicative  of  the  fact  that  gas  lighting  in  its 
most  recent  development  is  susceptible  of  a  much  more  extended  and 
diversified  use  than  it  has  enjoyed  in  the  past,  and  waits  only  upon 
the  expenditure  of  energy  on  commercial  activity  in  the  less  fre- 
quently exploited  fields. 


MODERN  LIGHTING  ACCESSORIES 

BY    W.    F.    LITTLE 

The  term  "accessories"  is  here  used  not  in  its  general  sense,  but 
rather  according  to  the  definition  of  the  Committee  on  Nomen- 
clature and  Standards  as  employed  to  designate  reflectors,  shades, 
globes  and  other  devices  for  modifying  and  controlling  the  light 
produced  by  lamps.  The  usual  functions  of  such  devices  are  to  re- 
direct the  light;  to  diffuse  the  light;  to  interrupt  the  light  in  certain 
directions;  to  modify  the  hue  of  the  light,  or  to  protect  the  light 
source.  Accessories  in  this  sense  are  the  tools  with  which  the  illumi- 
nating engineer  works. 

Since  1910  development  in  accessories  has  followed  principally 
the  lines  of  reduced  brightness  and  improved  appearance.  The  in- 
creased brightness  of  light  sources  has  led  to  the  development  of 
accessories  which  partially  or  entirely  conceal  the  source.  These 
form  in  themselves  a  secondary  light  source  and  are  capable  of  decora- 
tive treatment  to  a  degree  not  offered  by  earlier  forms  of  accessories. 
Prior  to  1910  control  of  light  flux  was  considered  to  mean  very  largely 
the  re-direction  of  the  light  as  desired.  The  progress  of  the  past 
six  years  lies  in  the  broader  definition  of  control  which  now  is  con- 
sidered to  include  not  only  control  of  the  direction  of  light  but  also 
control  of  brightness  and  control  of  color.  This  extension  of  the 
functions  of  lighting  accessories  has  not  involved  the  abandonment 
of  the  most  effective  and  flexible  means  of  controlling  direction  of 
light,  such  as  prismatic  glass  and  mirror  glass,  but  has  brought  about 
a  more  subtle  and  pleasing  use  of  these  means  in  combination  with 
other  means  for  softening  and  tinting  the  light. 

The  development  of  more  efficient  illuminants  in  this  interim  has 
brought  not  only  the  necessity  for  concealment  of  sources  by  light- 
ing accessories,  but  also  the  opportunity  to  apply  more  effectively 
the  less  expensive  illumination  which  they  make  possible.  Acces- 
sories development  is  thus  definitely  involved  with  the  improvement 
in  light  sources,  without  which  such  development  would  probably 
have  been  neither  essential  nor  possible. 

The  increased  brightness  of  the  lamps,  particularly  in  the  smaller 

183 


184  ILLUMINATING   ENGINEERING   PRACTICE 

sizes,  makes  some  protecting  substance  advisable.  Central  station 
interests,  as  well  as  lamp  manufacturers,  are  experimenting  with 
certain  bulb  coatings  which  diffuse,  and  others  which  both  diffuse 
and  tint  the  light.  A  demand  is  beginning  to  manifest  itself  for  a 
glass  bulb  to  accomplish  this  same  purpose. 

Lamp  accessories  as  above  described  may  be  made  of  a  variety  of 
materials  and  of  innumerable  shapes,  and  may  still  fulfil  the  require- 
ments of  the  foregoing. 

The  material  from  which  accessories  are  made  may  be  classified 
in  general  as:  Metal,  enameled  metal,  glass,  fabric,  stone  and 
pottery;  and  the  shapes  as:  Flat,  cone,  bowl  and  miscellaneous. 

From  the  standpoint  of  tabulation  it  is  rather  unfortunate  that 
so  many  of  the  accessories  fall  in  the  miscellaneous  class. 

MATERIALS 

The  material  should  be  selected  with  the  proper  weighting  of  the 
several  optical  properties  of  lighting  accessories,  namely  reflection, 
transmission,  and  diffusion. 

Metal. — Metal  accessories  applied  as  reflecting  media  have  many 
advantages,  such  as  durability  and  rigidity.  However,  few  metal 
surfaces  retain  their  high  reflecting  power  unless  the  reflecting 
surface  is  protected.  A  few  surfaces,  such  as  polished  or  matte 
satin  finish  aluminum,  have  been  used  with  some  success.  Alumi- 
num bronze  lacquer  on  metal  is  largely  used,  and  has  been  found  very 
satisfactory,  particularly  when  properly  protected  from  dust  and 
moisture  by  a  transparent  coating.  Aluminum  finished  reflectors 
without  a  protective  surface  have  been  known  to  depreciate  15  to 
20  per  cent,  within  a  very  short  time,  and  once  the  surface  lustre  is 
gone  the  reflection  coefficient  is  permanently  impaired.  The  metal 
reflector  with  a  porcelain  or  glass  enameled  surface  has  more  than 
held  its  own  during  recent  years  for  purely  utilitarian  purposes. 
The  metal  gives  durability  and  rigidity  and  the  enamel  gives  per- 
manency of  surface.  The  enamel  surface  is  made  so  tough  that  it 
will  withstand  much  abuse  without  cracking.  Metal  reflectors 
coated  with  paint,  or  baked  enamel  surfaces,  make  a  reasonably 
satisfactory  substitute,  where  they  are  not  subjected  to  too  much 
moisture  or  great  changes  in  temperature.  However,  the  reflecting 
power  deteriorates  rapidly  and  the  surface  becomes  yellow  with  age. 

Glass. — The  best  all  round  material  for  lighting  accessories  is 
glass,  which  although  brittle  is  beyond  question  the  most  permanent 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  185 

available  for  this  purpose.  On  account  of  its  reflection,  transmission 
and  diffusion  it  is  far  in  the  lead.  Clear  glass,  with  mirror  backing, 
furnishes  a  combination  of  excellent  qualities,  such  as  permanency 
and  efficiency. 

Fabrics. — Silks,  satins,  chintz,  etc.,  are  much  used  for  decorative 
effects  where  efficiency  and  permanency  are  not  of  importance  or 
where  they  can  be  protected  against  depreciation. 

Stone. — Marble,  alabaster  and  several  other  minerals  have  been 
used  to  some  extent  where  richness  and  distinction  are  sought. 

Pottery. — Pottery  is  used  where  decorative  effects  and  not 
efficiency  are  desired.  Mirror  reflectors  are  sometimes  used  in  such 
accessories,  thus  greatly  increasing  their  efficiency. 

USES 

The  uses  to  which  lighting  accessories  are  put  may  be  divided  into 
four  main  classes:  (i)  utilitarian,  (2)  utilitarian  and  semi-decorative, 
(3)  semi-utilitarian  and  decorative  and  (4)  purely  decorative. 

Utilitarian. — The  utilitarian  accessory  may  be  designated  as  one 
whose  main  functions  are  efficiency,  light  control,  and  in  some 
cases  color  value,  without  serious  regard  to  the  appearance  of  the 
unit.  Usually  the  accessory  is  a  reflector,  and  in  a  few  cases  a  shade 
or  globe.  Under  this  classification  will  be  found  a  wide  variation  of 
materials  and  types  such  as:  Enamel,  aluminum,  aluminum  bronze, 
white  glass,  mirror  glass  and  clear  glass.  Among  the  most  practical 
is  the  white  enameled  steel  reflector.  Its  permanency  and  durability 
of  surface  and  practical  indestructibility,  coupled  with  its  high 
coefficient  of  reflection  and  diffusion,  have  caused  it  to  be  very  widely 
used. 

Aluminum  and  aluminum  bronze  reflectors  fulfill  most  of  the 
functions  of  the  enameled  reflector,  with  slightly  better  light  control, 
though  the  permanency  of  surface  even  when  protected  is  not  so 
good.  White  diffusing  glass,  where  protected  from  breakage,  is 
efficient  and  durable.  Mirror  and  prismatic  reflectors,  by  reason  of 
their  flexibility  of  light  control  have  a  field  of  usefulness.  Clear 
blue  glass  units  for  color  matching  also  fall  in  this  class. 

Utilitarian  and  Semi-Decorative. — The  utilitarian  and  semi-decora- 
tive lighting  accessories  must  be  reasonably  efficient,  accurate  in 
light  control,  and  present  an  appearance  which  is  unobjectionable. 
Of  this  class  the  majority  of  accessories  are  reflectors,  a  few  are  bowls, 
and  a  few  globes,  and  as  a  rule  these  are  made  of  white,  clear  and  mir- 
rored glass.  For  this  purpose  the  white  diffusing  glass  is  perhaps 


1 86  ILLUMINATING   ENGINEERING   PRACTICE 

most, used,  though  prismatic  and  mirrored  glass  are  also  employed 
and  in  some  cases  the  "crystal  roughed  inside"  globe  is  still  retained. 

Decorative  and  Semi- Utilitarian. — In  the  decorative  and  semi-utili- 
tarian class  the  principal  functions  necessary  are,  a  thoroughly  satis- 
factory appearance  and  a  reasonable  degree  of  efficiency  and  effec- 
tiveness. The  effectiveness  must  not  only  be  measured  by  the  ratio 
of  output  to  input,  but  also  in  terms  of  light  control  and  satis- 
faction. Light  control  in  this  connection  is  not  necessarily  light  re- 
direction, but  is  the  securing  of  the  proper  balance  or  weighting  of 
reflection,  transmission,  and  diffusion.  This  class  embraces  the 
reflector,  the  transparent,  translucent  and  opaque  bowl,  and  the 
transparent  and  translucent  globe.  It  is  therefore  essential  that  a 
wide  range  in  these  qualities  be  available.  The  materials  from 
which  these  are  usually  made  are:  white,  clear  and  mirrored  glass, 
tinted  or  colored  glass  and  fabrics.  In  this  class  may  be  placed  the 
white  diffusing  glass  where  transmission  and  diffusion  are  important; 
the  prismatic  and  mirrored  glass  where  control  and  efficiency  are  im- 
portant; tinted  or  colored  glass,  and  fabrics  where  colored  light  and 
decorations  are  required. 

Decorative. — The  decorative  accessory  may  be  of  such  varied  con- 
struction, design  or  material  that  it  may  include  anything  from  the 
bare  light  source  to  the  most  inefficient  and  highly  absorbing  media. 
It  includes  the  reflector,  the  shade,  the  bowl,  the  globe  and  other 
forms  which  cannot  be  classified.  In  many  cases  the  decorative 
feature  is  all-important  and  the  illuminating  value  is  a  secondary 
consideration.  The  materials  from  which  these  accessories  are 
made  are:  white,  tinted  or  colored  glass,  iridescent  and  art  glass, 
fabrics,  stone  and  pottery.  The  white  glass,  tinted  with  a  superficial 
coating  of  enamel,  paint,  or  iridescent  glass,  is  much  used.  This 
superficial  coating  may  be  etched  away,  making  innumerable  possi- 
bilities for  ornamentation;  or  the  white  glass  may  be  employed  in  its 
usual  form  with  the  walls  of  the  accessory  varied  in  thickness,  in 
order  to  bring  out  the  decoration  in  relief.  The  use  of  colored  glass, 
iridescent  glass,  art  glass,  fabrics  and  pottery  is  extensive  in  this 
type  of  accessory,  and  glass  has  supplanted  to  a  considerable  extent 
the  metal  work  formerly  utilized. 

STRUCTURAL  CHARACTERISTICS 

The  glass  used  for  lighting  accessories  may  be  divided  into  four 
structural  types:  Clear  glass,  opal  glass,  cased  glass  and  suspension 
glass.  The  clear  or  crystal  glass  is  used  in  the  manufacture  of  prism, 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  187 

" ground"  and  "daylight"  accessories;  the  white,  "opal"  type  of 
glass  for  accessories  in  which  the  complete  mix  is  homogeneous;  the 
cased  glass,  in  which  is  a  combination  of  the  crystal  or  colored  glass 
and  refined  opal,  and  "diffusing"  glass  which  may  be  described  as 
crystal  glass  with  small  reflecting  particles  held  in  suspension  (ala- 
baster type). 

With  these  four  types  of  glassware  the  manufacturers  make  prac- 
tically all  of  the  more  popular  grades  of  accessories.  To  be  sure  each 
manufacturer  has  his  own  way  of  treating  his  product,  and  his  own 
slight  variation  of  the  mix  and  firing  in  order  to  give  some  character- 
istic finish. 

"The  crystal  glass  is  the  ordinary  clear  glass  when  applied  to  illuminat- 
ing glassware.  This  must  not  be  confused  with  other  crystal  glass  which 
is  used  for  cut  glass,  tableware,  etc.  The  former  is  a  common  flint  glass 
with  no  particular  brilliancy,  and  is  of  a  more  or  less  inferior  quality  in  so 
far  as  the  glass  itself  is  concerned.  In  the  latter  case,  the  glass  is  a  highly 
refined  decolored  prismatic  glass,  having  unusual  brilliancy.  As  the 
commoner  type  of  crystal  glass  meets  all  the  requirements  for  illuminating 
purposes,  it  is  generally  adopted  for  this  class  of  work."1 

The  opal  type  of  glass  is  the  basis  of  a  large  portion  of  all  diffusing 
glass,  and  is  made  in  a  number  of  degrees  of  refinement,  from  the 
cheapest  muddy  white  glass,  as  used  in  the  earliest  type  of  flat- 
shades,  to  the  refined  opal  used  for  casing  purposes.  Very  different 
and  varied  effects  can  be  secured  by  surface  treatment,  and  by  vary- 
ing the  thickness  of  different  portions  of  the  glass.  The  thin  por- 
tions show  in  many  cases  a  fiery  red;  the  thicker  ones  a  pure  white 
transmission.  This  characteristic  is  frequently  taken  advantage  of 
in  working  the  design  in  the  glassware.  On  the  other  hand  opal  glass 
may  be  so  made  that  it  gives  almost  a  pure  white  light  transmission. 
With  the  refinement  comes  a  more  nearly  perfect  diffusion,  and  the 
glass  usually  becomes  more  dense.  With  the  increased  density  the 
flashing  or  cased  process  is  usually  employed.  The  casing  may  be 
either  on  the  inside,  outside,  or  on  both  sides  of  the  crystal  glass,  and 
the  layer  of  casing  may  be  as  thin  or  thick  as  desired,  thus  giving  a 
large  range  in  transmission  and  diffusion. 

The  surface  treatment  may  be  an  acid  etch  (wax  or  satin  finish), 
a  sand  blast,  or  a  superficial  tinting  applied  with  an  air  brush  or  other 
means,  and  fired  in,  making  a  fairly  permanent  surface.  The  tint- 
ing may  be  shaded  off  by  spraying  the  surface  at  an  angle  so  that 
shades  and  shadows  are  produced.  The  glass  may  also  be  covered 

i  Contributed  by  Mr.  A.  Douglas  Nash. 


1 88  ILLUMINATING   ENGINEERING    PRACTICE 

with  an  enamelled  tinted  surface  which  is  frequently  etched  away  in 
patterns.  Other  methods  of  treating  the  surface,  such  as  chipped 
glass,  make  very  effective  finishes.  The  chipping  process  is  accom- 
plished by  covering  the  surface  of  the  roughed  glass  with  a  specially 
prepared  mucilage  or  glue,  and  then  placing  the  glass  in  a  furnace  and 
allowing  the  paste  to  shrink  away,  pulling  small  particles  of  the 
glass  with  it.  The  opal  glass  is  suitable  for  all  classes  of  manufac- 
ture, while  the  cased  glass  is  made  only  by  the  blown  process. 
Tinted  glass  may  be  made  having  the  same  structure  and  char- 
acteristics as  the  white  opal,  the  tinting  being  in  the  glass  and  making 
a  homogeneous  mass.  This  glass  is  of  course  selective  in  reflection 
and  transmission,  and  therefore  not  highly  efficient.  However,  it 
has  possibilities  as  a  practical  lighting  glassware,  where  color  effects 
are  desirable. 

The  alabaster  type  (sometimes  called  phosphate  or  alumina  glass), 
or  crystal  glass  holding  small  reflecting  particles  in  suspension,  forms 
the  basis  of  many  diffusing  accessories.  This  glass  may  be  made 
in  varying  degrees  of  density,  from  almost  transparent  to  almost 
opaque.  It  may  be  either  blown  or  pressed.  The  formulas  and 
methods  of.  working  this  glass  are  varied  so  that  each  manufacturer 
may  secure  his  characteristic  types.  Glass  is  produced  with  particles 
so  fine  that  the  mass  appears  homogeneous,  or  the  particles  may  be 
sufficiently  large  to  be  readily  seen.  The  texture  may  present  a 
pure  white  appearance,  or  a  watery  appearance.  It  may  be  left 
as  it  is  taken  from  the  iron  mold,  or  it  may  be  finished  with  a  high 
gloss  or  fire  polish.  When  blown  this  glass  is  usually  thin,  highly 
translucent,  and  in  many  cases  poor  in  diffusing  qualities.  When 
pressed  it  is  usually  more  dense,  and  better  in  diffusing  qualities. 
A  tinted  glass  of  this  same  character  has  been  produced,  but  up  to 
the  present  time  there  has  been  but  little  on  the  market. 

Some  effort  has  been  made  to  secure  perfect  diffusion  from  clear 
crystal  glass.  This,  of  course,  would  show  a  very  high  transmission 
value  with  very  little  absorption.  However,  it  will  probably  have 
the  disadvantage  of  appearing  as  bright  as  the  light  source  in  numer- 
ous small  spots.  This  same  phenomenon  manifests  itself  to  a  con- 
siderable extent  in  prismatic  glass,  particularly  where  the  prisms  are 
not  sharply  cut. 

MANUFACTURE 

Metal. — The  processes  of  manufacture  of  metal  accessories  need 
but  little  explanation.  However,  they  may  be  classified  as  follows : 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  189 

Process. — Spinning  and  pressing. 

Finishing. — Polishing,  etching,  spraying,  and  enameling. 

The  metal  reflectors  are  either  spun  on  a  form  or  pressed  in  a 
die.  The  finishes  consist  of  polishing  the  metal,  scratch  brushing 
or  acid  etching  the  aluminum  surfaces,  or  spraying  aluminum  bronze 
on  the  surface  and  enameling  with  porcelain  or  paint.  A  consider- 
able increase  in  the  life  of  the  aluminum  surface  is  secured  by  the 
use  of  a  transparent  coating  which  prevents  the  removal  of  the  alum- 
inum when  cleaning  and  there  are  no  roughened  surfaces  to  accu- 
mulate dust.  The  porcelain  enamel  is  applied  as  a  liquid,  and  fired 
in  the  furnace,  each  reflecting  surface  receiving  at  least  five  coats, 
each  coat  individually  fired. 

Glass. — The  glass  accessories  are  manufactured  from  the  different 
types  of  glass  already  described,  by  the  following  processes:  blown, 
pressed,  pressed-blown,  cased,  bent  and  offhand. 

Blown  Process. — The  blown  process  consists  of  blowing  a  bubble 
of  the  glass  in  an  iron  or  paste  mould.  The  paste  mould  process  is 
used  whenever  the  accessory  may  be  rotated  in  the  mould;  namely 
when  smooth  and  without  design.  This  mould  is  made  of  iron  lined 
with  paste.  The  rotating  not  only  eliminates  the  seam  in  the  glass 
but  produces  a  highly  polished  surface.  Where  a  pattern  or  design 
is  traced  on  the  accessory,  an  iron  mould  without  a  paste  lining  is 
used.  In  this  case  a  seam  corresponding  to  the  parting  of  the  mould 
will  usually  be  found  on  the  glass,  and  where  a  high  polish  is  desired 
the  accessory  must  be  fire  polished.  The  fire  polishing  is  accom- 
plished by  re-heating  the  glass  almost  to  the  point  of  fusion  and  cool- 
ing it  slowly. 

The  pressed  process  consists  of  placing  the  glass  in  the  mould  and 
pressing  it  by  means  of  various  shapes  and  types  of  plungers.  The 
blown  process  is  one  of  expansion  or  stretching,  while  the  pressed 
process  is  one  of  compression.  The  blown  accessory  has  its  two  sur- 
faces, inside  and  out,  parallel,  and  the  glass  is  of  approximately  the 
same  thickness  throughout,  while  the  inside  surface  of  a  pressed 
accessory  does  not  necessarily  conform  to  the  outside  surface,  thus 
giving  a  wall  of  varied  thickness. 

Casing  or  flashing  of  glass  consists  of  superimposing  upon  a  core 
two  or  more  layers  of  glass  of  different  kind  or  structure.  The  cas- 
ing is  done  while  the  glass  is  on  the  blow  tube. 

The  very  nature  of  this  glass  means  that  each  layer  has  its  own 
coefficient  of  expansion,  which  may  differ  from  the  adjacent  layers. 
Therefore,  the  annealing  process  is  more  difficult,  and  after  installa- 


1 90  ILLUMINATING  ENGINEERING   PRACTICE 

tion  the  ordinary  cased  glass  may  not  be  subjected  to  changes  in 
temperature  as  great  as  in  a  glass  of  a  homogeneous  structure.  How- 
ever, if  it  is  possible  to  secure  the  cases  of  glass  having  the  same 
expansion  characteristics  the  finished  product  compares  favorably 
with  that  from  a  homogeneous  mix,  even  though  subjected  to  exces- 
sive temperature  changes. 

Many  accessories  are  made  of  flat  glass  bent  to  the  desired  shape. 
The  bending  process  is  accomplished  by  making  metal  moulds  lined 
with  paste  or  chalk,  laying  the  glass  over  the  mould,  and  placing 
it  in  an  oven  which  brings  the  glass  slowly  to  the  proper  temperature 
so  that  it  falls  of  its  own  weight,  taking  the  shape  of  the  mould. 
This  does  not  change  the  texture  or  structure  of  the  glass.  The 
bent  glass  form  may  be  cut  and  leaded  to  make  a  unit,  or  it  may  be 
left  as  taken  from  the  mould. 

Another  operation  which  has  proven  very  satisfactory  and  a  great 
time-saver  in  the  manufacture  of  globes  is  the  pressed-blown  process. 
A  mould  is  made  cone-shaped  with  a  rounded  tip,  the  top  having  the 
proper  dimension  for  the  opening  in  the  globe.  A  blank  is  formed 
in  this  mould  and  while  soft  and  plastic,  placed  in  a  second  mould 
and  the  cone-shaped  form  is  blown  by  compressed  air  to  the  desired 
shape.  This  process  insures  a  greater  uniformity  in  the  accessory 
and  retains  the  characteristics  of  blown  glass. 

"In  the  off-hand,  or  hand-made  process,  the  glass  is  gathered  in  much 
the  same  way  as  in  the  two  previous  methods.1  So  called  moulds  are  some- 
times used  to  produce  characteristic  designs  or  marks  on  the  glass  itself. 
However,  in  this  case,  the  piece  of  glass  is  dipped  into  these  moulds  before 
blowing,  so  that  the  raised  portions  chill  more  rapidly  and  retain  this 
design  during  the  process  of  making.  In  the  case  of  opalescent  glass,  this 
treatment  is  of  manifest  advantage,  as  it  results  in  the  chilling  of  the 
raised  portions.  When  the  mass  is  re-heated,  the  chilled  portions  become 
more  opaque  than  the  core,  and  when  completely  blown,  the  design  in  the 
mould  is  shown  on  the  piece  by  reason  of  this  added  opacity.  This 
method  of  dipping  is  also  used  in  the  case  of  mould  blown  glassware,  and 
has  a  tendency  when  blown  into  a  paste-mould,  of  throwing  the  design  or 
corrugations  to  the  inside,  giving  a  very  effective  lens-like  appearance  to 
the  design.  Hand-made  glass  lends  itself  to  much  more  effective  manipu- 
lation than  any  other  process.  Venetian  glass  has  always  been  made  in 
this  way.  Opportunities  are  offered  for  applying  either  to  the  core  or 
semi-finished  product  designs  in  various  colored  glasses.  When  applied 
to  the  core,  they  produce  designs  which  enlarge  with  the  blowing  of  the 
piece,  and  the  ultimate  effect  is  a  flat  design  in  color.  When  applied  to 

1  Contributed  by  Mr.  A.  Douglas  Nash. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  191 

the  semi-finished  product  the  design  is  in  relief,  this  latter  method  is 
elaborated  upon  by  certain  manufacturers  by  the  use  of  pincers  or  some 
other  suitable  tool  to  form  the  applied  glass  into  various  shapes,  producing 
very  elaborate  results.  The  well-known  Salviati  glass  is  the  best  example 
of  a  production  of  this  character.  In  the  production  of  Favrile  glass,  both 
methods  are  used.  Owing  to  the  fact  that  the  coloring  of  glass  has  a 
tendency  to  somewhat  change  its  chemical  characteristics,  these  processes 
require  unusual  care  in  annealing,  and  in  the  production  of  some  effects, 
the  loss  on  this  account  is  very  great. 

The  annealing  of  glass  is  very  important,  each  piece  requiring  a 
sufficient  period  of  time  to  cool.  The  larger  and  thicker  the  piece 
the  slower  the  cooling  process.  The  annealing  of  large  pieces  is  most 
important  as  they  are  usually  thick  and  heavy  and  breakage  after 
installation  may  be  serious,  not  only  as  to  cost  but  also  from  a  stand- 
point of  safety. 

"This  argument  leads  to  the  matter  of  annealing  as  applied  to  all 
classes  of  glassware  used  for  lighting  purposes.1  The  modern  use  of  large 
units  has  led  many  manufacturers  to  adopt  special  means  of  annealing. 
In  normal  glassware,  the  annealing  process  should  take  not  less  than 
twenty-four  hours,  during  which  time  the  article  should  be  very  gradually 
reduced  from  its  working  temperature  to  atmospheric  temperature,  but 
additional  time  should  be  given  to  this  when  the  weight  or  size  of  the 
article  varies  as  in  the  case  of  pressed  glass.  The  imperfectly  annealed 
article  may  break  from  no  apparent  cause. 

OPTICAL  PROPERTIES 

Reflection. — The  coefficient  of  reflection  as  defined  by  the  Committee  on 
Nomenclature  and  Standards  of  the  Illuminating  Engineering  Society  is: 
"the  ratio  of  total  luminous  flux  reflected  by  a  surface  to  the  total  lumin- 
ous flux  incident  upon  it.  ...  The  reflection  from  a  surface  may  be 
regular,  diffuse  or  mixed.  In  perfect  regular  reflection  all  of  the  flux  is 
reflected  from  the  surface  at  an  angle  of  reflection  equal  to  the  angle  of 
incidence.  In  perfect  diffuse  reflection  the  flux  is  reflected  from  the  sur- 
face in  all  directions  in  accordance  with  Lambert's  cosine  law.  In  most 
practical  cases  there  is  a  superposition  of  regular  and  diffuse  reflection. 

"Coefficient  of  regular  reflection  is  the  ratio  of  the  luminous  flux 
reflected  regularly  to  the  total  incident  flux. 

"  Coefficient  of  diffuse  reflection  is  the  ratio  of  the  luminous  flux  re- 
flected diffusely  to  the  total  incident  flux." 

Polished  metal,  mirror,  clear  and  prismatic  glass — in  fact  any  highly 
polished  surface — follow  the  law  of  regular  reflection  and  the  coefficient 
varies  with  the  perfection  of  the  surface  and  angle  of  incidence. 

1  Contributed  by  Mr.  A.  Douglas  Nash. 


I Q2  ILLUMINATING   ENGINEERING   PRACTICE 

Enamel  and  white  glass  with  polished  surface  follow  both  laws  of 
reflection,  while  matte  surfaces  such  as  aluminum  bronze,  depolished 
or  rough  glass,  normally  tend  to  follow  the  law  of  diffuse  reflection. 

No  surface  will  produce  perfect  diffusion  for  the  reason  that  all 
surfaces  reflect  regularly  to  some  extent.  The  quantity  of  diffuse 
reflection  will  vary  to  some  extent  according  to  the  perfection  of  the 
surface. 

Transmission. — The  light  transmission  through  glass  will  depend 
upon  its  density,  its  surface  and  index  of  refraction.  Referring  to  the 
Fresnal  formula1  for  light  transmission  through  glass,  it  is  seen  that 
through  the  ordinary  sheet  of  glass  whose  index  of  refraction  is  1.5, 
there  can  be  only  92  per  cent,  of  light  transmission,  neglecting 
absorption.  This  is  for  the  reason  that  approximately  4  per  cent, 
of  the  incident  flux  is  reflected  from  each  surface.  Some  recent 
experiments  in  the  oxidation  of  glass  surfaces  show  an  apparent  re- 
duction in  the  index  of  refraction  of  the  outer  surface  which  has 
reduced  this  reflection  from  8  per  cent,  to  approximately  3  per  cent. 
This  apparent  change  in  refractive  index  when  applied  to  a  lens  does 
not  in  any  way  change  its  focal  length. 

Light  may  be  transmitted  through  glass  in  several  different  ways,  as 

1.  Transmission  without  redirection. 

2.  Transmission  with  redirection  without  diffusion. 

3.  Transmission  with  redirection  and  diffusion. 
Transmission  without  redirection  is   the   transmission  of  light 

through  clear  glass  having  both  sides  parallel. 

Transmission  with  redirection  without  diffusion  is  the  phenomenon 
secured  with  the  use  of  totally  reflecting  prisms  and  mirror. 

Transmission  with  redirection  and  diffusion  is  that  which  is  secured 
when  light  is  passed  through  roughed  or  ground  crystal  glass  or 
through  white  diffusing  glass.  The  degree  of  redirection  and  dif- 
fusion in  white  glass  is  dependent  upon  the  quality  of  the  glass  and 
character  of  the  surface. 

The  absorption  of  light  in  glass  is  a  difficult  property  to  measure. 
It  has  been  stated  that  the  absorption  of  light  in  clear  optical  glass  is 
approximately  3  per  cent,  per  inch.  Light  absorption  in  glass  is  the 
difference  between  total  flux  of  light  on  the  glass  and  reflected  light 
plus  transmitted  light. 

Table  I-IV  shows  the  per  cent,  of  light  reflected,  transmitted 
and  absorbed  for  various  flat  and  nearly  flat  samples  of  clear 

1  Fresnal  formula,  "Light  Transmission  through  Telescopes" — F.  Kollmorgan,  paper 
read  before  the  New  York  Section  of  the  Illuminating  Engineering  Society,  Jan.  13,  1916. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES 


193 


and  diffusing  glass  and  the  per  cent,  reflected  for  opaque  surfaces. 
These  values  are  indicative  only  of  the  range  in  reflection  and  trans- 
mission for  the  several  classes  of  surfaces  and  materials,  for  the 
reason  that  the  perfection  of  the  surfaces  and  the  thickness  of  the 
glass  may  not,  and  in  some  cases  do  not,  represent  average  condi- 
tions. Further,  the  values  of  absorption  represent  the  per  cent, 
absorbed  of  the  total  flux  falling  upon  the  glass,  and  not  the  per  cent, 
absorption  of  the  light  which  enters  the  glass;  for  instance,  the  Cal- 
cite  sample  reflects  80  per  cent.,  therefore  20  per  cent,  enters  the 
glass  and  7  per  cent,  is  transmitted,  or  65  per  cent,  of  the  light 
entering  the  glass  is  absorbed. 

TABLE  I. — PER  CENT.  REFLECTED,  OPAQUE  MATERIAL;  LIGHT  INCIDENT  AT  20° 


Per  cent, 
reflection 

New  Aluminum  Bronze  (unprotected)  

CA 

Corrugated  Mirror                      .            

So 

Polished   Brass 

60 

Polished  nickel  plate  

64 

Polished  silver  plate 

QO 

Polished  Aluminum  

67 

Baked  White  Enamel  (Paint)       

72 

High  Gloss  Porcelain  Enamel 

78 

Mat  Surface  Porcelain  Enamel,  Sample  No.  i  

70 

Mat  Surface  Porcelain  Enamel,  Sample  No.  2  

76 

Regular  Surface  Porcelain  Enamel,  Sample  No   i 

77 

Regular  Surface  Porcelain  Enamel,  Sample  No.  2  

75 

*  Silvered  Mirror                                                                      

83 

*  Uranium  Glass  Silvered  Mirror 

70 

Mirrors  supplied  by  C.  A.  Matisse. 

TABLE  II. — PER  CENT.  REFLECTED,  TRANSMITTED,  AND  ABSORBED, 
CRYSTAL  GLASS;  LIGHT  INCIDENT  AT  20° 


Thickness 
(mm.) 

Per  cent, 
reflected 

Per  cent, 
trans- 
mitted 

Per  cent, 
absorbed 

4 
M 

"Pebbled"  —  smooth  side  
rough  side  
"  Roughed"  —  smooth  side  
rough  side 

18 

13 
25 

17 

81 
69 

I 
6 

3* 

7 

"Cathedral"  Glass  —  smooth  side., 
rough  side  
"Clear" 

25 
20 
II 

74 
88 

i 

i 

13 


194 


ILLUMINATING   ENGINEERING   PRACTICE 


TABLE  III. — PER  CENT.  REFLECTED,  TRANSMITTED  AND  ABSORBED;  LIGHT 
INCIDENT  AT  20° 


Thick- 
ness 
(mm.) 

Sample 

Dense 

Medium 

Light 

Per  cent, 
ref. 

Per  cent. 

trans. 

1j) 

;r  cent, 
ref. 

,r  cent, 
rans. 

tH     03 

;r  cent, 
ref. 

;r  cent, 
rans. 

c£ 

PH 

PH 

* 

PH 

PH 

PH 

2 
2 
2 

2 
2 

4 

4' 
3 

3 

4 

4 
4 
3 

2 

4 

3 
9 
4 
2 
2 
7 
6 
6 

6 
3 

Opal  glass 
Blanco  R.O.*. 

25 

39 
39 

49 
48 

49 

48 

50 
43 

54 

50 
66 
69 

56 

5 
8 

5 

9 

4 

2 

7 

Blanco*  

Acmelite*  

Monex  No.  i  

.        j      .. 

46 

44 
46 

Monex  No.  2  
Sudan:  * 
Polished  side  

59 
57 

59 
56 

29 

29 

12 
12 

Depolished  side.  . 

Magnolia  R.  O.:* 
Polished  side  

Depolished  side  

Radiant*  

Calcite  No.  i: 
Polished  side  
Iridescent  side  
Calcite  No.  2: 
Polished  side  
Iridescent  side  

80 
78 

79 
76 

7 

12 

13 
19 

74 
70 

12 

14 

Milk  glass: 
Polished  side.  .  .  . 

Roughed  side  .  . 

Veluria*  

Equalite: 
Polished  side  

66 
64 

26 
26 

36 
36 

8 
10 

48 

Semi-polished  side  

Cased  glass 
Polycase  * 

Camia*. 

Acme  Cased*  

Sheet  Glass  
Celestialite  (Three  Layers)  

Suspension  glass 
Parian  Treated  (R.I.)  *  

54 

26 

20 

Parian  Pressed*  
Carrara*  

33 

4 
6 
9 

30 
29 

37 

Blown  Alba  No   i 

2 

48 

48 
45 

43 
43 

48 
46 
48 

Blown  Alba  No.  2  
Pressed  Alba  No.  i 

Pressed  Alba  No.  2  
AlbaR.  O.  No.  i: 
Smooth  side, 

56 

42 

Rough  side.  . 

Alba  R.  0.  No.  2: 
Smooth  side  

Rough  side  

Druid* 

*  Samples  slightly  curved.     Values  therefore  questionable. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  ic 

TABLE  IV. — PER  CENT.  REFLECTED,  TRANSMITTED  AND  ABSORBED  IRIDES- 
CENT ART  GLASS;  LIGHT  INCIDENT  AT  20° 


^»7 
|i| 

Sample 

Per  cent, 
reflected 

Percent, 
transmitted 

Per  cent, 
absorbed 

Gold..    . 

13 
43 
50 
29 
4 

21 

10 
2 

66 

6l 
94 

Silver  on  Opal  

Gold  on  Opal  

Pink  on  Opal 

Deep  Blue  

PER  CENT.  REFLECTED,  TRANSMITTED  AND  ABSORBED,  ROUGH  ART 
GLASS;  LIGHT  INCIDENT  AT  20° 

M 

Art  Fire  Opal:  polished  side 
smooth  side 

17 
29 

15 
14 

68 

57 

ANALYSIS  OF  LIGHT  LOSSES  IN  ENCLOSING  FIXTURES 

The  light  lost  in  the  several  parts  of  a  fixture  is  shown  in  the  following  tabula- 
tion.   The  tests  have  been  made  on  two  street  lighting  fixtures. 

(a)  Loss  of  light  in  housing 16  per  cent. 

Loss  of  light  in  glassware 16  per  cent. 

Loss  of  light  in  complete  fixture 36  per  cent. 

(b)  Loss  of  light  in  housing 18  per  cent. 

Loss  of  light  in  glassware 15  per  cent. 

Loss  of  light  in  complete  fixture 37  per  cent. 

It  will  be  noted  that  the  sum  of  the  losses  in  the  glassware  and 
housing  is  less  than  the  loss  in  the  complete  unit.  This  is  occasioned 
by  the  fact  that  the  globe  reflects  additional  light  into  the  housing, 
thus  increasing  the  loss  in  the  housing. 


SKYLIGHT  GLASS1 

The  glass  in  plate  form  used  in  ceiling  windows  or  the  so-called 
"skylights"  backed  by  lamps  is  receiving  more  attention  than  here- 
tofore. Crystal  glass  with  various  diffusing  surfaces  is  available  in 
sufficient  characteristic  forms  to  enable  the  engineer  to  secure  al- 
most any  light  distribution  required,  from  the  slightly  diffusing  to 
the  widely  distributing.  Also  the  surface  may  be  covered  with 
prisms  to  bend  and  redirect  the  rays  of  light. 

1 1.  E.  S.  Transactions,  Vol.  9,  page  ion,  "Lighting  of  Rooms  through  Translucent  Glass 
Ceilings."  by  Evan  J.  Edwards. 


196  ILLUMINATING   ENGINEERING   PRACTICE 

REFLECTOR  DESIGN 

With  the  more  concentrated  light  sources  as  found  in  the  gas- 
filled  tungsten  or  "Mazda  C"  lamps  conies  more  accurate  light 
control  from  reflectors.  Also  the  problem  of  reflector  design  is 
simplified.  Remembering  that  the  angle  of  reflection  is  equal  to 
the  angle  of  incidence  wherever  the  surface  follows  the  law  of  regu- 
lar reflection,  it  is  obvious  that  the  widely  distributing  light  dis- 
tribution may  be  secured  in  either  of  two  ways,  viz: 

The  rays  may  cross,  or  diverge.  If  they  are  to  cross,  a  deep 
reflector  must  be  used  and  a  large  percentage  of  the  light  impinges 
upon  the  reflecting  surfaces.  Conversely  if  the  rays  diverge  little 
light  falls  upon  the  reflecting  surface.  Thus  less  control  is  secured. 
Obviously,  therefore,  the  crossed  rays  allow  a  more  accurate  light 
control  and  at  the  same  time  tend  toward  a  better  concealment  of 
the  bright  source. 

The  concentrating  reflector  must  produce  more  or  less  parallel 
rays,  and  therefore,  must  approach  the  parabolic  in  shape. 

The  majority  of  types  of  light  distribution  range  between  these 
two.  Therefore,  the  surfaces  need  but  slight  modification  to 
secure  the  required  results.  As  a  rule  when  properly  designed 
the  deeper  the  reflector  the  better  the  control  and  greater  the  light 
loss. 

To  produce  a  predetermined  light  distribution  with  a  diffusely 
reflecting  surface  is  more  difficult  and  sometimes  impossible.  How- 
ever, the  same  principle  is  followed. 

No  symmetrical  reflector,  or  one  whose  surface  is  a  surface  of 
revolution,  will  increase  to  any  marked  degree  the  light  in  a  hori- 
zontal direction  about  a  lamp.  The  so-called  deflector  was  designed 
with  this  idea  in  view,  with  a  surface  parabolic  in  shape  and  the 
source  in  the  focus.  But  in  order  that  a  fair  percentage  of  the  light 
should  fall  upon  it  its  diameter  would  have  to  be  so  great  as  to  make 
it  impracticable. 

An  unnecessary  loss  is  experienced  in  many  accessories  by 
trapping  the  light.  This  is  very  likely  to  be  serious  with  the 
ventilated  units  for  Mazda  C  lamps.  The  top  of  the  accessory  is 
usually  closer  to  the  filament  than  any  other  portion  and  subtends 
a  larger  solid  angle  of  light,  and  therefore  should  be  most  active 
and  valuable  in  light  reflection.  If  this  surface  is  not  of  the  proper 
contour  to  throw  the  light  out,  the  light  loss  in  the  unit  may  be 
excessive. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  197 

PHOTOMETRIC  PROPERTIES 

Metal  Accessories. — The  metal  accessories  have  kept  pace  with 
the  change  in  lamp  design  and  construction.  With  practically 
each  change  in  filament  dimension,  shape  or  location  it  has  been 
necessary  to  re-design  the  reflector.  With  the  advent  of  the  Mazda 
C  lamp  many  changes  were  necessary. 

Aluminum  Finished  Reflector. — The  aluminum  finished  reflectors 
are  essentially  indoor  accessories  of  the  utilitarian  type.  They  are 
made  in  deep  and  shallow  cones  and  bowls,  angle,  trough  or  show- 


Fig,  i. — Aluminum  finished 
reflectors. 


case  reflectors,  and  produce  a  complete  range  in  distribution  char- 
acteristics from  the  widely  distributing  to  the  moderately  concentrat- 
ing. They  are  designed  for  practically  all  types  of  electric  lamps 
from  the  lo-watt  Mazda  B  to  the  large  sizes  of  Mazda  C. 

In  Fig.  i  are  shown  characteristic  candle-power  distribution 
curves  for  bowl  type  accessories;  the  light  loss  to  be  expected  in  this 
type  of  reflector  varies  from  20  to  40  per  cent. 

Porcelain  Enameled  Steel. — The  enameled  steel  accessory  is  some- 
what similar  to  the  aluminum  finished  with  a  slightly  increased  re- 
flector coefficient.  It  has  a  wider  application,  as  it  may  be  used  in 
or  out  of  doors.  It  is  made  in  all  of  the  conventional  reflector 


I  g8  ILLUMINATING   ENGINEERING   PRACTICE 

shapes,  and  in  addition,  is  designed  for  numerous  asymmetric 
distributions,  where  large  flat  vertical  surfaces  are  to  be  evenly 
illuminated.  The  light  control  is  not  as  accurate  as  with  the  alumi- 
num surface  due  to  the  diffusing  qualities  of  the  enamel. 

Many  of  the  reflectors,  particularly  of  the  deep  bowl  type,  which 
are  used  with  the  large  sizes  of  Mazda  C  lamps  are  constructed  with 
ventilating  hoods,  and  as  they  are  frequently  used  with  enclosing 
glassware  the  ventilation  feature  is  doubly  important.  However, 
this  ventilating  feature  is  regarded  by  the  manufacturer  as  becom- 
ing less  important  as  the  lamps  are  now  constructed.  Where  re- 


Fig.  2. — Porcelain  enamel  reflectors. 

quired,  enameled  accessories  without  ventilators  may  be  used  with 
enclosing  gas  or  vapor-proof  glass  envelopes. 

Some  of  the  types  of  deep  bowls  have  been  constructed  with  fluted 
surfaces  for  the  purpose  of  eliminating  bright  streaks.  These  flut- 
ings  or  corrugations  also  add  to  the  rigidity  of  the  reflector.  Charac- 
teristic candle-power  distribution  curves  for  these  reflectors  are 
shown  in  Fig.  2.  The  loss  of  light  for  enameled  accessory  will  vary 
from  15  to  35  per  cent. 

Painted  Enameled  Reflectors. — The  painted  enameled  reflectors  are 
made  in  shapes  similar  to  the  more  common  types  of  porcelain 
enameled  reflectors.  However,  their  chief  quality  is  cheapness. 

The  connecting  link  between  the  metal  and  glass  accessories  is  the 


Figs.  3  and  4. — Connecting  link  between  metal  and  glass  accessories. 


Fig.  5. — Prismatic  semi-indirect  unit. 


(Facing  page  198.) 


Fig.  6. — Typical  modern  fixtures 


Fig.  7. — Fixture  to  which  may  be  attached  a  choice  from  a  number  of  interchangeable  bowls. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  199 

metal  hood  and  enclosing  or  semi-enclosing  glassware  (Figs.  3 
and  4).  Many  decorative  and  semi-decorative  units  have  been 
designed  embodying  a  hood  or  holder  to  which  is  attached  the  socket 
and  glassware. 

GLASS  ACCESSORIES 

The  glass  accessories  lend  themselves  to  practically  all  lighting 
purposes  and  are  made  up  in  innumerable  designs.  Unfortunately, 
however,  with  few  exceptions  the  accessory  is  made  to  meet  the  ideas 
or  tastes  of  the  designer  with  little  or  no  consideration  for  the  light 
distribution.  Among  the  exceptions,  may  be  cited  most  prismatic 
and  mirrored  reflectors. 

Clear  Glass. — Clear  glass  is  used  in  the  manufacture  of  a  number  of 
types  of  accessories,  namely:  Clear;  ground  or  etched;  cut;  pris- 
matic, and  mirrored. 

Clear  accessories  are  usually  globes,  the  principal  function  of 
which  is  the  protection  of  the  light  source. 

Ground  or  etched  accessories  in  some  cases  lend  themselves  to 
decoration,  but  it  is  regrettable  that  they  must  be  classed  as  lighting 
accessories,  as  their  diffusion  is  poor  and  light-redirecting  qualities 
practically  nil. 

The  only  excuse  for  the  existence  of  the  cut  accessory  is  to  serve 
as  a  medium  of  decoration,  though  in  a  few  units  it  produces 
some  sparkle  and  life.  Its  redirecting  qualities  are,  usually  of  little 
importance. 

The  so-called  "daylight"  unit  is  properly  an  accessory,  which, 
when  used  with  an  artificial  illuminant,  will  produce  a  light  equiva- 
lent in  color  to  daylight  (north  sky  or  sunlight).  This  corrective 
process  usually  consists  in  the  use  of  the  subtractive  method  of  color 
correction  or  the  reduction  of  all  of  the  light  in  proportion  to  the  ratio 
of  the  blue  in  the  artificial  light  to  the  blue  in  daylight.  The  blue  in 
most  artificial  illuminants  is  approximately  10  per  cent,  of  north  sky 
or  20  per  cent,  of  sunlight.  Therefore,  the  maximum  theoretical 
efficiency  obtainable  is  10  per  cent,  for  north  sky,  and  20  per  cent, 
for  sunlight. 

However,  where  a  whiter  light  is  required  than  that  produced  by 
the  bare  lamp,  it  has  been  found  satisfactory  to  employ  an  accessory 
which  absorbs  not  over  50  per  cent.  This  unit,  of  course,  must  not 
be  regarded  as  a  color-matching  unit.  A  slight  reduction  in  the  red 
component  frequently  produces  a  very  noticeable  change  in  the 
apparent  color  of  fabrics,  particularly  where  the  blues  predominate. 


200 


ILLUMINATING   ENGINEERING  PRACTICE 


Glass  manufacturers  have  taken  advantage  of  this  fact  and  have 
made  accessories  with  a  thin  casing  of  blue  glass,  usually  on  the 
inside,  thus  not  changing  the  appearance  of  the  unit  during  the  day- 
light hours,  -but  at  night  producing  a  somewhat  whiter  light  than 
would  otherwise  be  secured. 

One  claim  for  these  modified  units  is  that  the  light  apparently 
mixes  to  better  advantage  with  daylight  or  twilight  than  the  light 
from  the  unmodified  unit. 

When  a  unit  producing  true  daylight  is  to  be  installed  where  it 
can  be  contrasted  with  the  unmodified  artificial  light,  it  appears  very 
blue  and  observers  do  not  believe  it  to  produce  light  of  real  daylight 
quality. 

From  tests  made  at  the  Electrical  Testing  Laboratories  there  is 
an  indication  that  transparent  colored  glass  and  gelatine  increases 
in  absorption  toward  the  blue  end  of  the  spectrum.  The  following 
table  shows  the  transmission  values  for  red,  green  and  blue  light 
through  corresponding  colors  in  glass  and  gelatine. 


Color  of 
substance 

Color  of 
light 

Per  cent,  transmitted 

Jena  glass 

Wratten  filter 

Red  . 

Red. 

92 

55 
30 

90 
36 
23 

Green  

Green  
Blue 

Blue  

Both  the  glass  and  the  gelatine  filters  were  supposedly  designed 
for  monochromatic  light  transmission,  and  the  difference  in  trans- 
mission values  between  the  two  substances  is  probably  accounted  for 
in  that  the  color  in  one  case  is  purer  than  in  the  other ;  therefore,  more 
nearly  monochromatic.  This  absorption  of  light  militates  further 
against  the  efficient  production  of  an  accurate  daylight  unit  by  the 
subtractive  method. 

Prismatic  Accessories. — Unlike  the  majority  of  accessories,  the 
prismatic  units  are  usually  designed  according  to  carefully  worked 
out  prototype  curves.  Light  control  is  accomplished  by  the  use  of 
totally  reflecting  prisms  which  follow  the  general  contour  of  the  glass. 
Furthermore,  prisms  may  be  refracting  as  well  as  reflecting.  In 
some  units  results  have  been  secured  by  the  use  of  both  kinds  of 
prisms.  If  the  contour  of  the  glass  and  shape  of  the  prisms  are 
properly  formed,  almost  any  light  distribution  may  be  secured. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  201 

Corrugations  have  been  placed  in  glass  for  the  purpose  of  diffusing 
light.  These  corrugations  have  been  frequently  called  diffusing 
prisms. 

Prismatic  accessories  are  made  in  numerous  designs,  each  having 
its  characteristic  distribution  and  function  to  fulfill.  The  conven- 
tional forms  of  prismatic  reflectors  are  well  known;  therefore,  only 
the  newer  types  are  here  discussed.  Totally  enclosing  prismatic 
units  are  made  to  control  the  light  quite  as  accurately  as  the  pris- 
matic reflector  with  but  slightly  increased  loss.  In  this  way  the 
light  source  can  be  entirely  enclosed  and  still  secure  the  desired  light 
distribution.  Combinations  of  prismatic  glass  with  white  diffusing 
glass  make  possible  light  control  and  elimination  of  glare  such  as 
would  be  impossible  with  either  one  alone  (Fig.  5) .  A  so-called  semi- 
direct  unit  has  recently  been  developed  consisting  of  a  prismatic 
reflector  designed  after  a  prototype  curve  using  a  clear  glass  or 
"velvet"  finish  glass  envelope  conforming  closely  to  the  contour  of 
the  reflector.  Between  these  two  is  placed  any  fabric  or  paper  to 
correspond  with  the  surrounding  decorations.  With  this  unit  it  is 
possible  to  secure  almost  any  desired  ratio  between  the  direct  and 
indirect  components  of  light  unit  brightness  and  at  the  same  time 
secure  decoration  and  color  effects  from  the  transmitted  light. 

Asymmetric  prismatic  reflectors  have  a  large  field  of  usefulness. 

The  refractor  unit  is  notable  in  that  it  will  to  a  marked  degree  re- 
direct a  large  portion  of  the  light  at  or  near  the  horizontal.  This 
accessory  has  also  been  made  in  the  form  of  a  band  refractor  which 
intercepts  only  the  light  above  the  horizontal,  and  this  light  may  be 
redirected  wherever  required,  adding  considerably  to  the  light  in  the 
lower  hemisphere.  The  band  carrying  the  refracting  prisms  is  sur- 
rounded by  a  second  band  carrying  corrugations  or  ribbings  which 
diffuse  the  light  in  a  plane  normal  to  the  surface,  this  producing  a 
nearly  uniform  brightness  over  the  entire  band  rather  than  a  bright 
spot  at  its  center. 

An  enclosing  prismatic  accessory  is  made  with  a  standard  reflector 
for  the  upper  portion  and  refracting  prisms  for  the  lower  portion. 
The  refracting  prisms  break  up  and  redirect  the  light  falling  upon 
them,  thus  helping  to  eliminate  excessive  glare.  The  same  general 
function  is  performed  by  the  reflector-refractor,  which  with 
its  combination  of  reflecting  and  refracting  prisms  breaks  up  and 
redirects  the  light  as  desired. 

A  range  in  candle-power  distribution  curves  which  may  be  secured 
from  prismatic  accessories  is  shown  in  Fig.  8.  To  the  left  is  an 


202 


ILLUMINATING   ENGINEERING   PRACTICE 


Fig.  8. — Prismatic  reflectors. 


Fig.  9. — Prismatic  accessories. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES 


203 


asymmetric  reflector,  the  center  a  concentrating  type,  the  right  a 
distributing  type.  The  losses  to  be  expected  in  these  reflectors 
range -from  12  to  14  per  cent. 

In  Fig.  9  to  the  right  will  be  found  a  reflector-refractor  showing 
a  loss  of  light  of  about  20  per  cent,  thus  showing  large  redirection 
from  an  enclosing  medium  with  a  relatively  small  loss.  To  the  left 
will  be  found  the  candle-power  distribution  characteristic  of  a  semi- 
indirect  prismatic  unit  (see  Fig.  5). 

Mirrored  Accessories. — The  mirrored  reflector,  as  in  the  case  of 
the  prismatic,  is  usually  designed  to  produce  a  predetermined  light 


Fig.   10. — Mirrored  reflectors. 

distribution.  Its  efficiency  is  high  and  light  control  excellent.  The 
problem,  however,  is  to  retain  a  permanent  reflecting  surface.  This 
is  a  comparatively  simple  matter  where  not  subjected  to  excessive 
heat  or  moisture.  Deterioration  from  the  former  cause  has  proven 
a  very  formidable  obstacle  since  the  widespread  use  of  the  Mazda 
C  lamp.  The  mirror  reflector  must  consistently  follow  the  changes 
in  lamp  construction,  filament  location  and  design,  for  the  reason 
that  it  functions  by  the  principle  of  regular  reflection.  Therefore 
its  efficiency  is  closely  related  to  its  contour  and  location  of  light 
center.  With  the  introduction  of  the  concentrated  filament  it  was 


204  ILLUMINATING   ENGINEERING  PRACTICE 

found  that  the  corrugations  in  the  reflectors  made  for  the  Mazda  B 
lamps  were  not  sufficiently  fine  or  numerous  to  prevent  bright 
streaks.  This  led  to  the  development  of  a  new  series  of  reflectors 
with  very  fine  waves  or  corrugations. 

The  flat  corrugated  mirror  strips  in  trough  reflectors  are  still 
much  used.  With  this  type  of  reflector  extremely  accurate  light 
control  in  a  plane  normal  to  the  axis  of  the  reflector  can  be  secured, 
as  the  strips  can  be  made  as  wide  or  as  narrow  as  desired  and  each 
installation  may  have  a  particular  reflector  designed  for  it. 

In  Fig.  10  is  shown  characteristic  distributions  of  mirrored  acces- 
sories. The  light  loss  in  these  units  is  from  15  to  20  per  cent. 

Diffusing  Glassware. — Diffusing  glassware  as  used  in  lighting  ac- 
cessories furnishes  a  wide  range  in  reflection,  transmission  and  diffu- 
sion, and  this  range  may  be  varied  to  a  considerable  extent  in  any 
one  type  of  glassware  by  changing  its  density,  its  thickness,  its  sur- 
face and  its  contour.  Added  flexibility  is  frequently  secured  by 
coating  the  glass  with  a  white  enamel.  The  enameled  surface  has 
a  high  reflecting  power  and  low  transmission. 

Opal  glass  is  used  in  the  manufacture  of  reflectors,  bowls  and 
globes.  By  the  proper  selection  of  thickness  and  densities  varied 
effects  may  be  secured.  The  dense  opal  accessory  when  properly 
shaped  may  produce  an  excellent  reflector  so  far  as  light  control  is 
concerned. 

When  used  in  bowls  it  can  be  thin  with  high  transmission  or  dense 
with  little  transmission.  The  diffusing  qualities  are  very  good, 
particularly  in  all  cases  when  the  surfaces  are  roughed. 


Per  cent, 
reflected 

Per  cent. 

transmitted 

Dense  (4  samples) 

80  to  76 

7  to  12 

Medium  (2  samples)  

74  to  70 

12 

Light  (9  samples) 

CO  to  4^ 

2O  to  40 

The  characteristic  candle-power  distributions  to  be  expected  from 
bowl  reflectors  of  the  opal  type  are  shown  in  Fig.  n.  To  the  left  is 
the  pressed  reflector;  to  the  right  is  the  blown.  The  light  loss  in 
these  reflectors  ranges  from  12  to  20  per  cent. 

Accessories  made  of  cased  glass  are  usually  of  the  totally  enclosing 
type  as  its  principal  purpose  is  high  transmission  coupled  with  good 
diffusion.  Some  so-called  reflectors  are  made  of  cased  glass  but  they 


LITTLE:  MODERN  LIGHTING  ACCESSORIES 


205 


should  rightly  be  classed  among  the  shades.  Occasionally  the  cas- 
ings are  made  sufficiently  thick  to  reduce  the  transmission  to  a  com- 
paratively low  figure. 

Bowls  have  been  made  of  cased  glass  but  the  units  are  usually  un- 
satisfactory, resulting  in  a  very  high  brightness  and  small  reflection. 


Fig.  ii. — Opal  reflectors. 


Per  cent, 
reflected 

Per  cent, 
transmitted 

Dense  (i  sample,  3-layer  glass) 

T4 

26 

Medium  (2  samples)  

36 

Light  (2  samples)  

48 

43  to  50 

Fig.  12  shows  a  characteristic  candle-power  distribution  for  a 
cased  glass  bowl  reflector.  It  will  be  noticed  that  the  transmission 
is  high  and  reflection  low,  the  loss  in  this  type  being  approximately 
8  per  cent. 

The  active  interest  in  diffusing  glassware  found  its  beginning  in 
the  suspension  type.  Other  diffusing  accessories  were  made  in  opal, 
cased  and  roughed  crystal  glass,  but  not  until  the  development  of 


2O6 


ILLUMINATING   ENGINEERING   PRACTICE 


the  alabaster  type  did  the  use  of  diffusing  glassware  receive  its  full 
impetus.  The  flexibility  of  this  glassware  makes  it  particularly 
valuable. 


Fig.  12. — Cased  glass  reflector. 


Fig.  13. — Suspension  glass  accessories. 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  207 


Per  cent, 
reflected 

Per  cent, 
transmitted 

Dense  (2  samples)  .. 

<;6 

•32   to  4.2 

Medium  (5  samples). 

4.8  to  4.1 

46  to  48 

Light  (5  samples)  

37  to  29 

69  to  50 

In  Fig.  12  will  be  seen  candle-power  distributions  for  suspension 
glass  accessories.  To  the  left  are  two  types  of  distribution  curves 
both  showing  considerable  transmission  and  comparatively  little  re- 
flection. The  loss  in  these  accessories  varies  from  8  to  12  per  cent. 
To  the  right  is  a  suspension  glass  dish.  The  loss  in  this  accessory 
is  ii  per  cent. 

Suspension  glass  lends  itself  readily  to  either  blown  or  pressed 
accessories  made  in  either  iron  or  paste  molds.  As  its  -density 
varies  from  the  almost  transparent  to  the  almost  opaque,  so  also  do 
its  diffusive  qualities.  In  the  dense  glassware  fairly  accurate  light 
control  may  be  secured.  Therefore,  it  is  used  to  good  advantage 
in  reflectors  and  bowls  and  the  less  dense  glass  is  used  in  globes. 

FIXTURES 

The  problem  of  good  fixture  design  is  complex  and  few  designers 
approach  it  from  the  same  standpoint.  The  introduction  of  bowls  of 
diffusing  glassware  has  to  a  very  considerable  extent  curtailed  the 
demand  for  the  conventional  (old-fashioned)  fixture.  This  curtail- 
ment has  been  obviously  caused  by  the  entrance  into  the  field  of  the 
glass  manufacturer.  In  many  cases  the  glass  superseded  the  metal 
work  in  fixture.  Had  the  fixture  houses  been  as  active  in  pushing 
the  glass  bowl  type  of  unit  as  they  were  in  pushing  older,  more  con- 
ventional types,  it  probably  would  not  have  been  necessary  for  the 
glass  manufacturer  to  enter  the  field,  and  the  types  of  fixtures  might 
have  followed  a  different  style  of  design. 

The  fixture  business  may  be  divided  into  two  principal  classifica- 
tions, the  stock  fixture  and  the  special  fixture.  The  stock  fixture 
follows  to  some  extent  a  definite  period  design.  The  special  fixture 
is  supposedly  made  to  harmonize  with  a  particular  environment.  As 
examples  of  the  type  of  recent  stock  fixtures  might  be  cited  the  can- 
delabra wall  bracket,  usually  using  frosted  round  bulb  electric  lamps; 
the  center  candelabra  chandelier,  similarly  equipped;  the  chandelier 
in  the  ring  or  bracket  form  using  the  same  round  bulb  frosted  lamps; 


208  ILLUMINATING   ENGINEERING   PRACTICE 

the  dome  in  art  glass  or  silk;  the  table  lamp  with  glass  or  silk  shades, 
Fig.  6.  Possibly  the  only  characteristic  shape  on  the  table  lamp 
shade  of  silk  is  that  of  the  frustum  of  a  cone  with  the  top  diameter 
only  slightly  less  than  the  bottom. 

The  officers  of  a  much  imitated  fixture  house  which  boasts  of  the 
pick  of  the  trade  and  carries  no  stock  fixtures  assert  that  there  has 
been  no  advance  or  characteristic  change  in  fixture  design  for  the  past 
twenty  years  other  than  a  tendency  toward  a  larger  number  of  lamps 
of  lower  intensity.  They  state  that  the  crystal  fixture  is  more 
popular  than  ever.  The  side-wall  bracket  is  coming  into  great  favor, 
in  the  majority  of  cases  using  the  bare  frosted  lamp,  and  in  some  cases 
a  silk  shade  or  eye  shield.  In  almost  no  case  is  glassware  of  any 
description  used.  They  do  state,  however,  that  the  table  lamp  is  used 
to  supplement  the  wall  brackets  and  chandeliers.  Further,  appar- 
ently no  effort  is  made  to  redirect  or  control  in  any  way  the  light 
from  the  small  round  bulb  frosted  lamps.  The  light  distribution 
characteristics  of  these  fixtures  is  of  no  consequence  whatever  to 
these  fixture  designers.  Even  in  the  case  of  enclosing  glassware  the 
tendency  is  toward  cluster  rather  than  single  unit  lamps.  When  the 
diffusing  bowl  type  of  unit  is  employed  it  is  frequently  supplemented 
by  candle  brackets. 

One  of  the  representative  manufacturers  stated  that  the  resultant 
illumination  produced  was  almost  never  considered  as  part  of  his 
work,  or  part  of  the  artistic  and  aesthetic  feature  of  the  installation 
as  a  whole.  This  state  of  affairs  should  be  looked  upon  with  con- 
siderable alarm  by  the  illuminating  engineers,  particularly  at  this 
time  when  light  source  brightnesses  are  so  decidedly  on  the  increase. 

An  exception  to  this  practice  was  found  in  one  of  the  largest  and 
best  fixture  houses  in  New  York  where  the  quality  of  light,  light  dis- 
tribution and  light  control  are  the  first  conditions,  and  the  design 
is  worked  around  these.  This  house  would  not  consent  to  showing 
designs  or  photographs  of  any  fixtures,  as  such,  without  knowing 
where  and  under  what  conditions  the  fixtures  were  to  be  used. 
Here  at  least  good  taste  and  quality  of  light  are  paramount,  and  the 
purpose  of  the  fixture  in  many  cases  is  disguised  in  the  design.  This 
does  not  mean,  however,  that  unnatural  and  disconcerting  conditions 
are  tolerated. 

A  very  popular  and  satisfactory  diffusing  bowl  unit  is  found  in  the 
alabaster  accessory.  The  stone  as  it  is  taken  from  the  Italian  quarry 
is  quite  translucent  and  in  some  places  almost  transparent,  thus 
producing  a  beautiful  effect  of  fire  and  life  without  excessive  bright- 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  209 

ness.  Unfortunately  it  must  be  used  with  discretion  as  it  cracks 
readily  and  will  blacken  if  not  properly  protected  from  the  lamp. 
Here  again  little  attention  is  given  to  shapes  or  designs  which  might 
produce  an  advantageous  light  distribution  but  as  they  are  usually 
employed  to  produce  a  generally  diffused  illumination  and  supple- 
mented with  localized  lighting,  their  distribution  characteristics  are 
of  little  importance.  Beautiful  designs  have  been  worked  out  on 
these  bowls,  in  some  cases  the  depressions  are  colored  with  a  sepia 
stain  giving  them  a  rich  day  as  well  as  night  value.  The  density  of 
the  stone  is  quite  sufficient  to  keep  the  surface  brightness  down  to  a 
satisfactory  value. 

The  total  disregard  of  quality,  fitness  and  distribution  of  light  is 
not  so  prevalent  among  the  glass-  and  reflector-manufacturers  who 
also  make  fixtures.  In  many  cases  they  are  attempting  to  secure 
the  desired  weighting  of  transmission  and  reflection,  and  the  tend- 
ency is  toward  a  consideration  for  quality  by  tinting  the  glass. 
On  the  other  hand,  glass  manufacturers  are  prone  to  consider  their 
one  or  two  types  of  glassware  the  panacea  for  all  lighting  ills, 
whereas  a  slight  modification  in  the  mix  or  density  of  the  ware 
would  make  success  of  failure.  Such  a  step  in  this  direction  has 
been  taken  by  a  fixture  producer,  in  the  design  of  a  single  stem 
from  which  are  mounted  either  gas  or  electric  lamps  and  to  which 
may  be  readily  attached  any  one  of  a  number  of  bowls  having 
different  shapes,  densities  and  colors,  Fig.  7. 

An  improvement  in  the  resultant  illumination  from  semi-direct 
fixture  is  being  accomplished  by  placing  a  thin  diffusing  glass  plate 
over  the  bowl.  This  eliminates  chain  shadows  and  simplifies  the 
cleaning  problems. 

Indirect  fixtures  are  now  made  utilizing  the  accurately  designed 
mirror  reflector  inside  a  diffusing  glass  bowl,  and  by  means  of  an 
auxiliary  lamp  or  a  diffusing  cup  in  the  bottom  of  a  mirror  reflector 
the  bowl  is  illuminated  to  the  desired  brightness.  This  arrangement 
to  a  very  marked  degree,  eliminates  the  argument  against  indirect 
fixtures,  namely,  that  the  fixture  appears  dark  against  the  illuminated 
ceiling. 

Table  lamps  are  also  designed  with  some  conception  of  the  result- 
ing light  distribution.  The  lamps,  using  silk  shades  as  above  de- 
scribed, are  frequently  equipped  with  real  reflectors  which  control  the 
light  produced.  This  shape  will  allow  of  a  considerable  upward 
component  for  a  semi-indirect  unit  or  may  be  equipped  with  a 
reflector  throwing  a  large  portion  of  the  light  downward.  Fre- 
14 


2IO  ILLUMINATING   ENGINEERING    PRACTICE 

quently  the  silk  alone  is  used  as  a  reflecting  surface.  Much  flexi- 
bility can  be  secured  as  the  silk  may  be  left  highly  transluent  or 
diffusing  and  additional  layers  may  be  added  to  secure  the  desired 
transmission  and  different  colors  for  different  effects. 

In  an  effort  to  make  semi-indirect  units  more  universal  and  allow 
them  to  be  used  even  where  highly  reflecting  ceilings  are  not  avail- 
able or  in  those  locations  where  the  ceilings  are  too  high  above  the 
logical  locations  of  the  unit  the  fixture  manufacturer  has  designed 
a  small  portion  of  ceiling  to  go  with  the  bowl.  The  fixture  therefore 
consists  of  a  diffusing  bowl  and  a  reflecting  surface  a  short  distance 
above.  This  method  serves  to  enlarge  slightly  the  light  giving  area, 
and  thus  to  decrease  correspondingly  the  fixture  brightness.  The 
upper  reflecting  surface  has  been  changed  in  size,  location  and  shape 
by  the  several  manufacturers,  but  always  serves  the  same  purpose. 

A  survey  of  the  field  indicates  that  excellent  accessories  of  a  wide 
variety  are  available.  As  a  rule,  however,  these  follow  conventional 
lines  according  to  well  recognized  concepts  of  design  and  use.  Only 
to  a  slight  extent  are  designers  undertaking  to  provide  accessories 
which  represent  adaptation  of  simple  means  of  directing,  diffusing 
and  tinting  the  light  along  unconventional  lines. 

References 

E.  B.  ROWE. — "Some  tendencies  in  the  design  of  illuminating  glassware." 
Electrical  Engineering,  Sepember,  1914. 

JAMES  R.  CRAVATH. — "Glass  globes  for  street  lamps."  Municipal  Journal, 
August  27,  1914. 

RENE  CHASSERIAUD. — "The  art  of  logical  lighting  (French)."  Societe 
Beiges  des  Electriciens  (Brussells),  May,  1914. 

GUIDO  PERI. — "Present  status  and  tendencies  in  electric  illumination 
(Italian)."  L'Industria  (Milan),  November  i,  1914. 

H.  B.  WHEELER. — "Lighting  of  show  windows."  Illuminating  Engineering 
Society  Transactions,  September,  1913. 

W.  W.  COBLENTZ. — "The  diffuse  reflecting  power  of  various  substances." 
Bulletin  of  Bureau  of  Standards,  April  i,  1913. 

H.  J.  TAITE  and  T.  W.  ROLPH—  "Notes  on  metal  reflector  design." 
General  Electric  Review,  May,  1914. 

A.  L.  POWELL.— "An  investigation  of  reflectors  for  tungsten  lamps." 
General  Electric  Review,  November,  1912. 

M.  LUCKIESH.—" Investigation  of  diffusing  glassware."  Electrical  World, 
November  16,  1912. 

W.  W.  COBLENTZ. — "Diffuse  reflecting  power  of  various  substances." 
Journal  Franklin  Institute,  November,  1912. 

L.  BLOCK.— "Reflectors  and  accessories  for  lighting  inner  rooms  with  metal 


LITTLE:  MODERN  LIGHTING  ACCESSORIES  211 

filament     lamps    (German)."     Elektrotechnik   Und   Maschinenbau,   October 
13,  1912. 

DR.  L.  BLOCK. — "Reflectors  for  metal-filament  lamps."  London  Elec- 
trician, March  21,  1913. 

A.  L.  POWELL  and  G.  H.  STICKNEY.— "  Data  concerning  incandescent 
reflectors."  Electrical  World,  September  6,  1913. 

VAN  RENSSELAER  LANSINGH. — "Characteristics  of  enclosing  glassware." 
Illuminating  Engineering  Society  Transactions,  September,  1913. 

W.  T.  MACCALL. — "Half  frosted  lamps  in  reflectors."  London  Electrician, 
October  3,  1913. 

A.  L.  POWELL. — "Reflectors  for  tungsten  lamps  in  industrial  and  office 
lighting."  Electrical  Engineering,  October,  1913. 

DR.  L.  BLOCH. — "Choice  of  reflectors  for  street  lighting."  London  Elec- 
trician, May  31,  1912. 

L.  BLOCH. — "Choice  of  reflectors  and  proper  heights  for  metal  filament 
street  lamps."  Elektrotechnik  und  Maschinenbau,  December  3,  1911. 

R.  HORATIO  WRIGHT. — "The  mazda  lamps  with  a  few  common  types  of 
reflectors."  Sibley  Journal  of  Engineering,  November,  1910. 

C.  TOONE. — "Globes,  shades  and  reflectors."  London  Electrical  Review, 
June  16,  1911. 

P.  G.  NUTTING,  L.  A.  JONES  and  F.  A.  ELLIOTT. — "Tests  of  some  possible 
reflecting  power  standards."  Illuminating  Engineering  Society  Transactions, 
Volume  9,  No.  7,  1914. 

LEONARD  MURPHY  and  H.  L.  MORGAN. — "Distribution  and  efficiency  tests 
on  lamp  shades  and  reflectors."  London  Electrical  Review,  July  7,  1911. 

GEO.  H.  MCCORMACK,  ALBERT  JACKSON  MARSHALL,  L.  W.  YOUNG;  Intro- 
ductory remarks  by  BASSETT  JONES,  JR. — "Symposium  on  illuminating  glass- 
ware." Illuminating  Engineering  Society  Transactions,  September,  1911. 

THOMAS  W.  ROLPH. — "Reflectors  for  incandescent  lamps."  Electric 
Journal,  May,  1910. 

J.  R.  CRAVATH. — "Show  window  illumination."  Central  Stations,  May 
1910. 

C.  E.  FERREE  and  G.  RAND. — "Some  experiments  on  the  eye  with  inverted 
ieflectors  of  different  densities."  Illuminating  Engineering  Society  Transac- 
trons,  December  20,  1915. 

FRANK  A.  BENFORD.— "The  parabolic  mirror."  Illuminating  Engineering 
Society  Transactions,  December  20,  1915. 

HAYDEN  T.  HARRISON. — "Efficiency  of  projectors  and  reflectors."  Ab- 
stract of  a  paper  read  before  the  Liverpool  Engineering  Society. 


LIGHT  PROJECTION:  ITS  APPLICATIONS 

BY    E.    J.    EDWARDS    AND   H.    H.    MAGDSICK 

Light  projection,  as  the  term  is  commonly  employed,  covers  the 
redirection  of  light  flux  from  artificial  sources  by  means  of  suitable 
optical  systems  so  that  it  may  be  utilized  within  solid  angles  which 
are  small  as  compared  with  those  encountered  in  equipment  for  gen- 
eral illumination  purposes.  It  was  in  connection  with  such  applica- 
tions in  a  few  restricted  fields  that  some  of  the  more  important  prin- 
ciples of  optics  and  illuminating  engineering  were  long  since  devel- 
oped and  applied.  During  the  past  few  years  these  applications 
have  multiplied  rapidly,  occupying  the  attention  of  many  illuminat- 
ing engineers  and  giving  rise  to  numerous  papers  in  the  Transactions 
of  the  Illuminating  Engineering  Society  and  articles  in  the  technical 
press  dealing  with  the  principles  of  optics,  searchlighting  for  military 
and  navigation  purposes,  flood-light  projectors  for  displaying  sur- 
faces at  a  distance,  headlighting  for  vehicles,  orientation  lighting 
for  the  navigator,  light  signals,  and  apparatus  for  the  projection 
of  enlargements  of  transparencies. 

Two  general  classes  of  apparatus  are  used  to  direct  the  flux  from 
a  source  into  the  desired  small  angle:  Opaque  reflector  systems  con- 
trolling the  light  by  the  principle  of  specular  reflection,  and  lens 
systems  depending  upon  the  refractive  properties  of  glass.  Fre- 
quently the  two  forms  of  control  are  combined  in  the  same  device. 

In  Fig.  i,  A,  is  illustrated  the  action  of  a  simple  convex  lens.  A 
light  ray  emerging  from  the  focus,  F,  is  refracted  in  passing  through 
the  lens  so  as  to  be  projected  parallel  with  the  axis,  while  from  a 
larger  source  as  shown  at  the  focus,  a  cone  of  light  is  projected  with 
an  angle  of  divergence,  26,  depending  upon  the  size  of  the  source, 
the  focal  length  of  the  lens  and  the  angle,  a,  at  which  it  is  emitted. 
The  greatest  angle  of  divergence  is  that  of  the  cone  issuing  at  the 
axis  of  the  lens.  These  statements  apply  to  lenses  intercepting  the 
flux  in  a  relatively  small  solid  angle.  As  the  diameter  of  a  lens  in- 
creases relative  to  the  focal  length,  the  thickness,  and  hence  the 
absorption,  increase  rapidly  and  the  control  of  the  emerging  rays  is 
limited  by  the  increasing  spherical  and  chromatic  aberration.  To 

213 


214 


ILLUMINATING   ENGINEERING   PRACTICE 


reduce  these  elements  of  inefficiency,  Fresnel  nearly  one  hundred 
years  ago  built  a  lens  of  concentric  rings,  Fig.  i,  B;  in  effect  a  large 
convex  lens  with  sections  of  the  glass  removed.  He  also  added  con- 
centric prism  rings  to  direct  additional  light  into  the  beam  by  total 
reflection.  Later  these  prisms  were  given  a  curved  surface  and  re- 


Axis 


B 


Fig.  i. — Light  projection  with  lenses. 


fraction  was  combined  with  reflection  to  produce  the  desired  results. 
It  will  be  noted  that  the  sections  give  rise  to  a  series  of  dark  rings 
when  viewed  within  the  beam,  since  the  light  striking  the  risers  is 
deflected  at  a  large  angle  from  the  axis.  In  Fresnel  lenses  of  reason- 


Axis 


Axis 


Fig.  2. — Light  projection  with  opaque  reflectors. 

ably  effective  angle,  the  solid  angle  subtended  by  the  lens  at  the  focus, 
the  contour  of  the  surface  may  be  so  corrected  as  to  secure  very  ac- 
curate control  of  light.  They  are  frequently  referred  to  as  stepped 
or  as  corrugated  lenses. 

Rays  emerging  from  a  source  at  the  center  of  a  sphere  are  reflected 
from  the  polished  surface  as  shown  in  Fig.  2,  A.     Used  in  this  manner 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION  215 

as  an  accessory  with  a  lens  on  the  other  side  of  the  source,  the  mirror 
increases  the  amount  of  light  intercepted  by  the  lens,  providing  the 
source  is  at  least  partially  transparent.  With  the  source  placed  on 
the  axis  of  a  spherical  mirror  at  half  the  radius,  rays  are  returned 
with  only  a  small  divergence  from  the  parallel  when  the  effective 
angle  is  not  large.  Mangin  devised  a  spherical  mirror  of  silvered 
glass  with  the  radius  of  the  inner  surface  less  than  that  of  the  outer, 
Fig.. 2,  B.  The  varying  degree  of  refraction  introduced  by  this  con- 
cavo-convex lens  is  utilized  to  keep  the  divergence  of  the  beam 
within  narrow  limits  for  effective  angles  up  to  as  much  as  120°. 

The  greatest  efficiency  and  accuracy  in  concentrating  light  with 
an  opaque  reflector  is  secured  with  a  parabolic  contour,  since  all 
rays  from  the  focus  are  reflected  parallel  with  the  axis  no  matter 


o — 


Axis 


B 
Fig.  3. — Light  projection  with  opaque  reflectors. 

how  large  the  effective  angle  is  made.  The  divergence  from  a  source 
as  in  Fig.  3,  A,  is  greatest  at  the  axis  and  decreases  with  increasing 
angles.  Only  within  the  angle  of  the  cone  showing  the  smallest 
divergence,  that  is  the  cone  emanating  from  the  edge  of  the  mirror, 
does  the  beam  contain  light  from  all  parts  of  the  surface,  and  hence 
only  in  this  region  does  the  measured  candle-power  obey  the  inverse- 
square  law.  Beyond  this  limiting  cone,  light  is  received  from  a  de- 
creasing zone  of  the  reflector  until  at  the  edge  of  the  cone  only  the 
point  at  the  axis  is  effective.  Fig.  3,  J5,  shows  one  combination  of 
reflecting  surfaces  and  lens  among  several  that  may  be  employed  to 
meet  various  requirements. 

In  all  of  the  projection  devices  described  above  a  part  of  the  beam 
receives  light  from  the  entire  surface.  In  some  cases  this  is  at  the 
axis  only;  in  others,  over  a  wider  angle.  The  brightness  of  the  sur- 


2l6  ILLUMINATING   ENGINEERING   PRACTICE 

face  is  in  every  case  the  brightness  of  the  source  at  the  respective 
angle  multiplied  by  the  coefficient  of  reflection  or  transmission  of 
the  system.  The  intensity  of  the  beam  within  this  range  is,  there- 
fore, the  product  of  the  brightness  and  the  projected  area  of  the  sur- 
face; variations  in  the  focal  length  and  the  effective  angle  do  not 
change  the  result.  The  multiplying  factor  of  the  system  is  then 
approximately  the  ratio  of  the  squares  of  the  diameter  of  the  mirror 
and  the  diameter  of  the  source.  Table  I,  giving  the  brightness  of 
the  various  sources  used  in  projection  apparatus,  indicates  their 
relative  value  so  far  as  the  production  of  the  maximum  beam  in- 
tensities is  concerned. 

In  most  applications  a  beam  can  advantageously  be  utilized  with 
a  divergence  so  great  that  the  total  amount  of  flux  in  the  beam  is  of 
equal  or  greater  importance  than  the  central  density.  The  effective 
angle  of  the  system,  the  size  of  the  source  and  the  focal  length  are 
important  factors  in  determining  the  width  of  the  beam,  the  total  flux 
and  its  distribution.  Table  II  gives  the  solid  angles  subtended  at 
the  focus  by  parabolic  reflectors  and  lenses  of  various  proportions. 
The  latter  are  most  often  applied  where  accuracy  of  control  is  re- 
quired; the  former  where  it  is  desired  to  intercept  the  flux  in  a  rela- 
tively large  solid  angle.  The  average  opaque  projector  system 
directs  from  30  to  60  per  cent,  of  the  available  light  into  the  beam ; 
with  lens  systems,  typical  effective  angles  are  so  small  that  only 
5  to  10  per  cent,  is  transmitted.  The  cost  of  the  respective  types 
of  apparatus  for  different  sizes  is,  of  course,  often  the  determining 
factor  in  their  adoption;  in  general  the  cost  of  lenses  increases  the 
more  rapidly  with  larger  size. 

TABLE  I. — INTRINSIC  BRILLIANCY  OF  COMMON  PROJECTION  SOURCES 


Source 

Candle-power  per  sq.  inch 

Flame  Arc  for  search  lighting  

250  000—350  ooo 

Carbon  Arc  "        "            "      

80,000—  90,000 

Magnetite  Arc  

4  ooo~-     6  ooo 

Mazda  C  Projection  Type  

o  ooo~  1  8  ooo 

Mazda  C  Regular. 

3  coo 

Mazda  B  Concentrated  
Mazda  B  Regular  .    . 

1,200 

7  ^O 

Kerosene  Mantle.. 

2OO~'5OO 

Acetylene. 

60 

Gas  Mantle  

•7O—  CO 

Kerosene  Flame 

S—  TO 

EDWARDS   AND   MAGDSICK:   LIGHT  PROJECTION 


217 


TABLE  II.— PERCENTAGE  OF  TOTAL  SOLID  ANGLE  SUBTENDED  BY  PARABOLIC 
REFLECTORS  AND  CONDENSING  LENSES 


Parabolic  reflectors 

Condensing  lenses 

Ratio  of  diameter 
of  opening  to  focal 
length  (R) 

Percentage  of  total 
solid  angle 

Ratio  of  diameter  to 
focal  length 
(R) 

Percentage  of  total 
solid  angle 

2 

2O.  O 

0-3 

0.6 

3 

36.0 

0.4 

I.O 

4 

50.0 

0-5 

1.5 

5 

61  .0 

0.6 

2  .  I 

6 

69.2 

0.7 

2.8 

7 

75-4 

0.8 

3-6 

8 

80.0 

0.9 

4-4 

9 

83-5 

.0 

S-3 

10 

86.2 

.1 

6.2 

.2 

7-i 

Percentage  of  Total  Solid  Angle 

•3 

8.1 

cos  -  +  i 

•4 

9.1 

2              V  inn 

•5 

IO.O 

—  Ps  loo 

2 

2.0 

14.6 

J?2 

./Y 

2-5 

10.  I 

~  #2+i6    X  I0° 

Percentage  of  Total  Solid  Angle 
/i  -  cos  6\ 

=  (°-S  ~ 

/     />S    1<J<J 

/  —  —  I  X  100 

There  are  four  principal  surfaces  employed  in  opaque  projectors. 
Those  of  mirrored  glass  and  silvered  metal  have  a  coefficient  of 
reflection  of  the  order  of  85  per  cent.  Polished  aluminum  reflects 
slightly  more  than  60  per  cent,  of  the  incident  light,  and  a  nickel- 
plated  brass  surface  has  an  efficiency  of  less  than  55  per  cent.  All  of 
the  metal  surfaces  tarnish  and  require  repolishing  or  replating  from 
time  to  time.  Silvered  metal  deteriorates  rapidly  where  air  circu- 
lates over  it,  particularly  in  a  salt  atmosphere  and  where  fumes  from 
stacks  are  present.  The  nickeled  and  aluminum  surfaces  depreciate 
less  rapidly.  The  aluminum  has  the  further  advantage  that  re- 
polishing  does  not  also  in  time  involve  replating  as  with  the  other 
metal  units.  Silvered  glass  is  usually  found  the  most  desirable  and 
economical  in  the  long  run,  although  where  there  is  no  intense  heating 
and  the  reflectors  may  be  tightly  enclosed,  silvered  metal  is  found 
very  satisfactory.  The  light  absorption  by  lenses  varies  with  the 
thickness  of  the  glass  and  the  nature  of  the  construction;  10  to  15  per 


2l8 


ILLUMINATING   ENGINEERING   PRACTICE 


cent,  may  be  taken  as  typical  values.     With  Fresnel  lenses  there  is  a 
further  loss  due  to  the  f  "»gs  produced  by  the  risers. 

The  large  proportion  oi  t.  projection  field  served  by  the  parabolic 
reflector  makes  a  further  analysis  of  its  properties  with  different 
sources  desirable.  The  following  curves,  Figs.  4,  5,  and  6,  and 
accompanying  formulae  are  taken  from  Benford's  paper1  on  this 
subject.  In  Fig.  4  are  shown  the  beam  characteristics  that  are 
approached  as  the  source  approaches  a  point  radiating  equally  in  all 
directions.  The  rays  are  parallel  and  the  apparent  candle-power  is, 


Fig.  4.  —  Parabolic  mirror  and  point  source  beam  characteristics. 

of  course,  different  at  each  distance  measured.     The  density  of  the 
flux  at  any  radius  is  given2  by  the  formula,  D  =         —  ^  • 


results  are  shown  for  three  reflectors  of  equal  diameter  but  of  differ- 
ent focal  length  and  effective  angle. 

In  Fig.  5  a  similar  analysis  is  made  for  a  spherical  source  of  0.5  in. 
diameter  and  a  brilliancy  of  1000  c.p.  per  sq.  in.  In  this  case  the 
equation  for  the  axial  density  of  the  beam  becomes 


sL* 


tan2 


Hence 


/„  =  -jrR2  Bm. 


1  Frank  A.  Benford,  Jr.,  "The  Parabolic  Mirror,"  Trans.  I.  E.  S.,  Vol.  X,  page  905. 

2  The  following  symbols  are  employed:  E  =  illumination  on  a  plane  normal  to  beam,  in 
foot-candles;  Ia  =  intensity  of  source  at  angle  a  from  axis  of  mirror,  in  candles;  IB  =  in- 
tensity of  beam,  in  candles;  B  =  brilliancy  of  light  source,  in  candles  per  sq.  in.;  m  = 
coefficient  of  reflection  of  mirror;  F  =  focal  length  of  mirror,  in  inches;  R  =  radius  of  mirror, 
in  inches;  L  =  distance  from  focal  point  to  point  in  beam,  in  feet;  r  —  radius  of  source,  in 
inches;  5  *=  area  of  light  source,  in  sq.  in.;  a  —  angle  measured  about  focus,  in  degrees. 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION 


219 


The  intensity  varies,  for  fixed  focal  length,  with  the  square  of  the 
tangent  of  one-fourth  the  effective  angle;  for  fixed  angle,  as  the 
square  of  the  focal  length.  Also,  the  axial  intensity  is  seen  to  depend 
upon  the  brightness  of  the  source  but  is  not  affected  by  its  size;  it  is 


Fig.  5. — Parabolic  mirror  and  spherical  source  beam  characteristics. 

S 

equal  for  all  parabolic  mirrors  having  the  same  diameter.  The  same 
intensity  will  be  directed  at  all  angles  within  which  light  is  received 
from  the  entire  surface  of  the  reflector.  This  angular  spread  is 
determined  by  the  size  of  the  source  and  its  angular  radius  viewed 


Fig.  6. — Parabolic  mirror  and  disk  source  beam  characteristics. 

from  the  edge  of  the  reflector.     The  intensity  at  other  angles  is  pro- 
portional to  the  area  of  the  mirror  contributing  light. 

These  characteristics  of  the  beam  apply  at  distances  beyond  the 
point  at  which  the  rays  from  the  extreme  edge  of  the  reflector  cross 


22O  ILLUMINATING  ENGINEERING   PRACTICE 

the  axis.     This  point  of  maximum  density  from  which  the  inverse 
square  law  takes  effect  is  found  from  the  equation 


R( 


I2T 

For  a  disk  source  the  characteristics  are  given  in  Fig.  6.  Here 
again  IB  =  irR^Bm. 

With  a  disk  source  a  wider  angular  opening  than  180°  is  not  effect- 
ive, since  the  projected  area  becomes  zero  at  90°  from  the  axis. 
The  effective  diameter  of  reflector  C  is  therefore  reduced  to  2 A. 
The  distance  from  the  focus  at  which  the  inverse  square  region  begins 
is  in  this  case 


L 

Lo  = 


i2r  cos  a 


EQUIPMENTS    FOR    VEHICLE    HEADLIGHTING 

The  opaque  projectors  find  by  far  their  greatest  application  on 
vehicles;  the  number  of  automobiles,  street  and  interurban  railway 
cars,  electric  and  steam  locomotives  equipped  with  projectors  is,  no 
doubt,  in  excess  of  3,000,000  in  the  United  States  alone. 

The  first  object  in  equipping  automobiles  with  projectors  was,  of 
course,  to  light  the  road  ahead  for  the  driver.  It  is  desirable  that  the 
driver  of  an  automobile  be  able  to  see  his  way  for  several  hundred 
feet  in  advance,  and  since  he  must  provide  his  own  lamps  and  direct 
the  light  unfavorably  for  lighting  the  roadway,  it  becomes  necessary  to 
project  a  high  intensity.  It  was  the  effort  to  accomplish  this,  as  well 
as  to  give  ease  of  control,  that  brought  about  the  rapid  change  from 
oil  and  acetylene  units  to  the  electric  system  employing  closely  coiled 
low-voltage  filaments  in  deep  projectors,  giving  both  accurate  con- 
trol and  high  efficiency.  The  intensities  now  vary  throughout  a 
wide  range  up  to  hundreds  of  thousands  of  candle-power  with  an 
average  of  about  25,000.  If  there  were  but  one  automobile  and  a 
lonely  road,  the  headlighting  problem  might  be  considered  solved. 
But  the  higher  intensity  lighting  equipments  have,  in  solving  the 
problem  of  lighting  the  road,  introduced  a  new  and  serious  problem 
in  that  they  temporarily  blind  the  driver  or  pedestrian  who  happens 
to  come  within  their  angle  of  action. 

"Glare"  is  the  one  word  most  used  in  referring  to  the  blinding 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION  221 

effect  of  high  candle-power  units.  This  question  probably  commands 
more  widespread  interest  at  the  present  time  than  any  other  prob- 
lem within  the  scope  of  the  illuminating  engineer.  A  few  states  and 
many  cities  have  enacted  legislation  designed  to  regulate  the  use  of 
projector  lamps  to  eliminate  dangerous  glare.  Other  states  and 
cities  have  such  laws  in  contemplation.  In  Table  III  is  found  a  sum- 
mary of  the  automobile  laws  of  the  various  states  as  obtained  in  re- 
sponse to  a  general  letter  sent  to  the  Secretaries  of  State  under  date 
of  June,  1916.  It  is  seen  that  a  small  percentage  have  laws  per- 
taining to  glare,  and  that  there  seems  to  be  lack  of  definiteness  in  the 
laws  which  do  exist.  It  should  be  possible  with  more  general  knowl- 
edge as  to  the  causes  of  glare  and  with  a  more  widespread  under- 
standing of  the  methods  of  measuring  light,  to  create  laws  which  will 
be  definite,  consistent  with  a  consideration  of  the  factors  involved, 
and  stated  in  terms  which  permit  of  verification  by  measurement. 

It  is  generally  agreed  that  the  main  factors  involved  in  producing 
glare  are  included  in  the  following: 

1.  Luminosity  of  background. 

2.  Solid  angle  subtended  by  source  projected  area  at  eye  of  observer;  in  other 
words,  source  size  and  distance. 

3.  Luminous  intensity  of  source  in  direction  in  question. 

Automobile  headlighting  units  are  limited  by  cost  and  appearance 
considerations  to  sizes  under  i  ft.  in  diameter,  and  the  size  can 
be  considered  as  a  constant  in  a  consideration  of  the  glare  problem. 
The  luminosity  of  the  background  under  worst  conditions  is  zero,  the 
complete  darkness  of  the  country  road,  and  it  is  likely  to  be  for  some 
time,  until  all  roads  are  artificially  illuminated  at  night.  Therefore, 
the  third  factor,  the  luminosity  of  the  background,  is  also  a  constant 
so  far  as  the  present  problem  is  concerned.  There  remains  then  only 
one  controllable  factor,  the  luminous  intensity. 

There  is  likely  to  be  a  great  difference  of  opinion  as  to  the  limit  of 
intensity  which  would  be  fairly  safe  and  yet  endurable.  An  in- 
tensity of  100,000  candle-power  is  unquestionably  bad,  and  there  is 
hardly  an  appreciable  reduction  in  the  glare  in  cutting  down  to  10,000 
candle-power,  assuming,  of  course,  the  worst  background  conditions. 
It  is  unfortunate  that  no  consistent  method  has  been  devised  for  the 
measurement  of  interference  with  vision,  making  it  possible  to  de- 
termine the  relation  between  glare  effect  and  candle-power  for  some 
fixed  road  condition.  Observations  have  indicated  that  such  a  curve 
taken  on  a  dark  country  road  would  be  of  the  general  form  of  Fig.  7. 


222 


ILLUMINATING   ENGINEERING  PRACTICE 


TABLE  IIL4 


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EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION 

TABLE  IllB 


223 


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the  glare  vanishes  completely  only  when  the  candle-power  reaches 
zero.  When  the  background  is  not  dark,  there  can,  of  course,  be 
considerable  intensity  without  marked  interference. 


224 


ILLUMINATING   ENGINEERING   PRACTICE 


There  are  many  devices  on  the  market  which  reduce  the  glare 
by  cutting  down  the  intensity  of  the  beam.  They  are  diffusing  doors 
of  various  forms  and  degree.  As  a  rule,  they  slightly  reduce  inter- 
ference at  the  maximum  glare  angle  by  diminishing  beam  candle- 
power  to  a  small  fraction  of  the  previous  value,  and  increase  hundreds 
of  times  the  solid  angle  in  which  glare  is  experienced.  A  well-focused, 
accurately  made  parabolic  headlighting  unit  may  produce  blinding 
glare  in  the  angle  of  the  beam,  but  it  has  the  one  inherent  virtue  that 
except  for  the  filament  itself,  its  field  of  action  is  limited  within  a 
small  angle.  The  approaching  driver  may  face  the  beam  at  a 
considerable  distance,  but  it  likely  to  escape  it  when  within,  say, 


Conditic 


100* 


on  of  To 


ally 


Blinding 


Gla 


10,000         20,000       30,000 


40,000         80,000        60,000 
Beam  Candle-Power 


70,000   80.000    90,000  100,000 


Fig.  7. — Nature  of  relation  between  beam  candle-power  and  visibility  of  objects  viewed 
against  beam  where  the  background  is  totally  dark. 

ioo  ft.  of  the  approaching  car.  There  is  no  escape  from  the 
diffusing  equipment.  One  is  like  the  small-pox,  serious  when  en- 
countered but  not  difficult  to  avoid;  the  other  like  the  measles,  not 
so  serious,  but  unavoidable,  it  seems.  If  one  of  these  diseases  could 
be  eliminated,  many  would  vote  that  the  measles  should  go.  Fig.  8 
illustrates  light  distribution  from  three  typical  classes  of  equipment; 
an  unmodified  parabola  with  covers  of  clear  glass,  partially  frosted 
"lens"  and  all-frosted  diffusing  glass. 

If  the  one  object  in  regulation  were  to  eliminate  glare,  the  answer 
would  be  simple :  Eliminate  the  concentrating  headlights.  Limits  to 
the  glare  effect  mainly  protect  the  approaching  driver;  the  problem 


EDWARDS   AND   MAGDSICK:   LIGHT  PROJECTION 


22; 


must  be  considered  from  the  point  of  view  of  the  driver  behind  the 
headlights  as  well.  There  are  also  pedestrians  and  the  occupants  of 
unlighted  vehicles,  whose  safety  depends  upon  the  ability  of  the  auto- 
mobile driver  to  see  them  in  sufficient  time  to  avoid  running  them 
down.  A  just  regulation  should  do  more  than  place  upper  limits  of 
permissible  intensity;  there  should  be  lower  limits  in  so  far  as  road 
illumination  is  concerned.  If  the  beams  from  automobile  lamps  are 
to  be  at  all  times  capable  of  good  road  illumination  and  at  the  same 


56.000 


20"    16 


8°      4°      0°     4°      8C 
Angle  from  Axis 


12°    16°    20° 


Fig.  8. — Beam  candle-power  of  parabolic  automobile  projector  with  6-8  volt,  3.0  ampere 

Mazda  C  lamp. 

time  incapable  of  causing  glare  under  average  conditions,  there  seems 
to  be  but  one  solution,  and  that  is  to  greatly  reduce  or  entirely 
eliminate  the  light  from  the  angles  above,  say,  4  ft.  from  the  ground, 
and  retain  the  light  at  the  lower  angles. 

Many  devices  have  been  designed  for  reducing  or  eliminating  the 
upward  light,  redirecting  the  intercepted  light  in  downward  di- 
rections. The  simplest  method  of  eliminating  strong  upward  light 
with  accurately  made  headlamps  is  to  tilt  them  downward  by  an 
angle  equal  to  half  the  angle  of  spread  of  beam;  many  headlamps, 

is 


226  ILLUMINATING   ENGINEERING   PRACTICE 

however,  are  not  made  sufficiently  accurate  to  have  any  well-defined 
beam.  Another  method  commonly  used  is  to  set  the  light  source 
back  of  the  focal  point  of  the  reflector  and  to  cover  the  upper  half  of 
the  door  with  an  obscuring  material.  Obviously,  this  method  is 
inefficient.  The  Patent  Office  records  show  a  wide  variety  of 
devices  for  diverting  the  light  from  directions  above  the  horizontal. 
One  is  a  cup-shaped  spherical  reflector  placed  over  the  lamp  bulb  to 
return  the  upward  light  back  along  its  initial  path.  When  placed 
over  the  bulb,  it  is  assumed  that  the  filament  is  placed  back  of  the 
focus.  These  devices  are  frequently  seen  placed  on  the  lower  side  of 
the  bulb,  thus  utilizing  the  upper  instead  of  the  lower  half  of  the 
parabolic  reflector,  and  when  so  used  the  filament  must  be  forward 
of  the  focus  in  order  to  be  effective.  It  not  infrequently  happens 
that  the  filament  is  in  focus  as  adjusted  by  the  owner,  in  which  case 
the  device  has  no  effect  except  to  reduce  the  efficiency  of  the  lamp. 

In  another  class  of  devices  use  is  made  of  compound  curvatures  in 
the  reflector.  There  is  the  offset  parabola  where  the  upper  half  has  a 
focal  point  back  of  that  of  the  lower  half,  so  that  the  filament  may  be 
placed  back  of  the  focus  of  the  lower  half  at  the  same  time  that  it  is 
placed  forward  of  the  focus  of  the  upper  half.  A  tilted  upper  half, 
where  the  upper  surface  has  been  revolved  downward  about  the 
focus  as  a  center,  is  another  device  described.  Still  another  is  a 
combination  consisting  of  a  parabolic  lower  part  and  an  ellipsoidal 
upper  part.  This  device  if  perfectly  made  would  give  no  light  above 
the  horizontal,  not  even  the  direct  light  from  the  filament.  Proper 
adjustment  requires  that  the  filament  be  placed  a  little  more  than 
half  its  axial  length  back  of  the  focus  of  the  parabolic  part.  The 
ellipsoid  is  arranged  to  have  one  of  its  foci  at  the  proper  position  of 
the  filament  and  the  other,  through  which  the  intercepted  rays  are 
directed,  at  a  point  on  the  axis  of  the  lamp  within  the  plane  of  the 
front  glass.  There  are  also  a  number  of  prismatic  glass  covers  that 
reduce  the  upward  light,  bending  the  reflected  rays  downward  and 
to  the  side  of  the  road.  These  devices  seem  to  be  limited  in  the  de- 
gree to  which  they  can  cut  down  upward  intensities,  because  in 
being  designed  to  take  care  of  the  light  coming  from  the  reflector, 
they  are  sure  to  throw  some  of  the  direct  light  from  the  filament 
upward  in  narrow  high-intensity  beams,  although  this  may  be 
obviated  by  screening  the  tip  of  the  bulb.  Figs.  9,  10  and  n  are 
photographs  by  C.  A.  B.  Halvorson,  Jr.,  of  a  screen  illuminated  at  a 
distance  of  10  ft.  by  three  types  of  equipment,  star  frosted,  pris- 
matic and  paraboloid-ellipsoid. 


Fig.  9. — Screen  il- 
luminated at  10  ft.  by 
parabolic  reflector  with 
star  frosted  "lens." 


Fig.  10. — Screen  il- 
luminated at  10  ft.  by 
parabolic  reflector  with 
prismatic  cover. 


Fig.  ii. — Screen  il- 
luminated at  10  ft.  by 
paraboloid  -  ellipsoidal 
reflector  with  clear 
glass  cover. 


(Facing  page  226.) 


EDWARDS    AND   MAGDSICK:    LIGHT   PROJECTION  227 

No  one  of  the  so-called  non-glare  devices  that  are  now  in  general 
use  can  be  said  to  be  a  complete  solution  of  the  problem.  Each 
may  have  its  favorable  point  or  points,  just  as  the  unmodified 
parabolic  lamp  has  its  advantage.  An  equipment  which  gives  no 
light  above  the  horizontal,  may  give  good  road  surface  illumination 
at  the  same  time  that  it  is  incapable  of  glare  on  a  dark  road,  but  it 
can  blind  the  approaching  driver  coming  up  into  view  on  a  convexity 
in  the  road,  and  has  the  further  disadvantage  that  it  ordinarily  must 
show  up  vehicles  and  other  objects  by  their  lower  extremities  only 
and  may  miss  entirely  the  near  objects  when  approaching  the  foot  of 
a  hill.  A  lamp  with  no  light  above  the  horizontal  is  sure,  on  account 
of  the  varying  curvatures  in  road  profile,  to  have  a  widely  varying 
range  of  throw.  From  the  driver's  standpoint  it  has  great  advan- 
tage in  a  fog  since  there  is  none  of  the  usual  luminous  haze  between 
the  driver's  eyes  and  the  road. 

The  details  of  the  various  devices  which  have  appeared  are 
interesting  but  they  are  not  as  important  at  the  present  time  as 
the  study  of  the  underlying  principles.  If  the  best  solution  to  the 
problem  were  a  matter  of  general  agreement,  a  device  which  would 
accomplish  the  result  would  probably  soon  appear.  As  a  matter  of 
fact,  some  of  those  already  on  the  market  give  results  which  the 
inventors  believe  to  be  the  best  solution.  Some  of  the  states  have 
evidently  assumed  that  it  is  necessary  only  to  place  upper  limits  of 
intensity  above  3  or  4  ft.  from  a  level  road;  presumably  it  is 
considered  unnecessary  to  place  lower  limits  of  intensity  for  lower 
angles.  Assuming  that  the  best  answer  to  the  glare  problem  is  the 
elimination  of  light  above  the  horizontal,  it  should  be  possible  to 
draught  a  regulation  which  would  be  definite  and  in  terms  capable  of 
measurement.  It  would  be  necessary  to  use  only  one  technical 
term.  Such  a  law  might  specify  that  the  head  lighting  beam  shall 
not  have  an  intensity  at  any  angle  above  the  horizontal  exceeding  a 
certain  amount,  say,  20  candle-power,  and  that  it  shall  have  not 
less  than  an  average  of,  say,  10,000  candle-power  measured  at  equal 
vertical  angular  increments  from  the  axis  down  to  the  road,  at  a 
distance  of  100  ft.  If  desirable,  lower  limits  of  intensity  at  the 
lower  angles  to  the  side  might  be  specified  in  order  to  insure  that  the 
driver  can  at  all  times  see  the  curb  or  other  sidewise  limits  to  the 
road.  The  point  to  be  emphasized  is  that  once  general  agreement 
is  obtained  as  to  the  best  solution  of  the  problem,  the  necessary  regu- 
lations can  be  stated  in  simple  terms  involving  only  luminous  in- 
tensity measurements  in  addition  to  simple  measurements  of  length. 


228  ILLUMINATING   ENGINEERING   PRACTICE 

It  may  safely  be  said  that  the  present  tendency  is  toward  cutting 
down  or  entirely  eliminating  the  upward  light,  but  it  appears  that 
this  method  in  itself  can  never  be  entirely  satisfactory  to  the  motor- 
ist. Much  of  the  pleasure  and  sense  of  security  in  night  driving  come 
from  bringing  into  view  the  overhanging  foliage  and  other  high 
objects  along  the  road,  as  is  done  with  the  usual  parabolic  units  of 
high  power.  Evidently  there  is  no  harm  done  as  long  as  there 
are  no  eyes  ahead  to  be  blinded.  This  feature  may  have 
prompted  the  recommendation  of  the  glare  committee  of  the 
Illuminating  Engineering  Society  to  the  effect  that  unmodified 
parabolic  equipments  could  be  used  if  they  were  always  ex- 
tinguished when  meeting  another  driver.  This  plan  seems  to 
have  disadvantages,  for  on  a  dark  road  the  sudden  change  is 
likely  to  interfere  with  the  vision  of  both  drivers  due  to  the  length 
of  time  required  for  adaptation,  and,  to  an  extent,  greater  than 
would  be  obtained  with  the  glare  of  the  undimmed  lamps. 

Perhaps  the  best  solution  is  a  regulation  such  as  outlined  above, 
but  taken  to  apply  only  when  meeting  other  vehicles  on  unlighted 
roads.  Such  a  regulation  would  require  on  every  automobile 
used  at  night  an  equipment  giving  no  upward  light,  but  would 
allow  of  any  kind  of  additional  equipment  that  might  be 
desired.  It  would  seem  that  an  equipment  consisting  of  one  or 
two  high-powered  parabolic  units  with  two  lower-powered  non-glare 
(no  upward  light)  lamps  would  be  satisfactory.  The  glareless  lamps 
would  be  used  for  all  night  driving,  both  city  and  country,  and  the 
additional  equipment  of  parabolic  lamps  could  be  employed  at  times 
when  no  harm  would  result  to  others. 

Modification  of  the  color  quality  of  the  light  emitted  from  a  head- 
lamp is  sometimes  secured  by  means  of  yellow  glass  in  the  reflector 
or  cover.  Two  advantages  are  sought  by  the  suppression  of  the 
shorter  wave  lengths;  increased  acuity  and  decreased  scattering  of 
light.  The  former  is  seldom  realized  since  the  better  acuity  usually 
fails  to  compensate  for  the  lower  intensity.  The  latter  effect  is 
more  often  of  importance  since  the  rays  scattered  by  a  haze  or  fog 
produce  a  luminous  veil  that  may  seriously  interfere  with  vision; 
an  appreciable  reduction  of  this  veiling  is  obtained  with  the  yellow 
glasses.  The  same  purpose  is  served  by  the  use  of  ordinary  head- 
lamps and  a  yellow  disc  on  the  wind-shield  or  yellow  glasses  worn 
by  the  driver.  A  further  step  in  this  direction  has  been  attempted 
by  making  the  reflector  of  glass  with  fluorescent  properties  and  thus 
converting  the  shorter  wave  lengths  instead  of  absorbing  them. 


EDWARDS   AND   MAGDSICK:    LIGHT   PROJECTION 


229 


The  plan  possesses  little  utility,  however,  since  the  transformed 
light  is  not  projected  into  the  beam  by  the  reflector  but  issues  as 
from  a  diffusely  emitting  medium. 


EQUIPMENTS  FOR  RAILWAY  HEADLIGHTING 

On  street  cars  for  ordinary  city  service,  the  head  lamps  need 
serve  only  as  markers.  For  suburban  and  interurban  runs  with  the 
higher  speeds  and  dark  roads,  a  higher  intensity  is  required  both 


16,000 
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Angle  from  Axia 

Fig.   12. — Beam  candle-power  of  typical  electric  street  railway  head-lamp.     Parabolic  re- 
flector of  1 54  in.  focus  and  %>%  in.  diameter. 

as  a  warning  at  greater  distances  of  the  approach  of  a  car,  and  to 
illuminate  objects  on  the  track  at  a  sufficient  distance  to  allow  the 
car  to  be  stopped  before  reaching  them.  Fig.  12  gives  photometric 
data  for  several  lamps  for  headlighting  typical  of  those  used  in  this 
service.  The  advantage  of  the  more  concentrated  low- voltage  source 
in  increasing  the  beam  candle-power  is  apparent,  but  this  greater 


230 


ILLUMINATING   ENGINEERING    PRACTICE 


concentration  is  not  always  required.  Since  high-voltage  direct 
current  is  available,  the  magnetite  arc  has  been  found  to  be  particu- 
larly useful  in  this  field  when  a  high  intensity  beam  is  wanted.  The 
large  amount  of  steadying  resistance  stabilizes  the  arc,  and  when  the 
equipment  includes  a  lens  cover,  good  control  is  secured,  with  the 
results  shown  in  Fig.  13. 


400,000 


4-Ampere 

High -Efficiency 

Electrode 


4°    3°    2° 

Angle  from  Axis 

Fig-   13- — Beam  candle-power  of  luminous  arc  interurban  head-lamp  with  12  in. 
semaphore  lens. 

The  proper  headlighting  equipment  for  steam  and  electric  loco- 
motives has  been  exhaustively  studied  by  railway  associations, 
individual  roads  and  utility  commissions.  The  headlamp  in  this 
case  may  be  made  to  serve  as  a  marker  for  the  head  end  of  a  train  and 
as  a  warning  of  the  approach  of  a  train,  for  the  illumination  of  way- 
side objects,  for  displaying  numbers  in  the  case  and  to  enable  the 
engineman  to  see  objects  on  the  track  at  a  distance  so  great  that  he 
may  stop  the  train  before  reaching  them.  All  of  these  requirements 


EDWARDS   AND.  MAGDSICK :   LIGHT   PROJECTION  231 

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DISTANCE  IN  FEET  DISTANCE  IN  FEET 

Fig.   14. — Locomotive  beam  intensities  required  to  render  dummies  visible. 

SECONDS 
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DISTANCE  IN  FEET 
Fig.  15. — Deceleration  curves  for  heavy  express  train  with  older  and  modern  braking  systems. 


232 


ILLUMINATING   ENGINEERING   PRACTICE 


can  probably  be  met  with  the  best  apparatus  available  at  present, 
providing  the  brakes  are  applied  immediately  whenever  any  indica- 
tion of  an  object  on  the  track  is  seen.  However,  its  use  may  lead  to 
some  difficulty  in  temporarily  blinding  a  person  passing  through  the 
beam  and  thus  introducing  an  element  of  confusion  at  multiple  track 
crossings,  and  in  interfering  with  the  visibility  and  correct  reading 


15000 


200  400  600  600        1000 

DISTANCE   IN   FEET 
Fig.  16.— Visibility  of  signals  and  objects  with  various  beam  intensities. 

of  color  and  position  of  semaphore  and  hand  signals,  classification 
lights,  etc. 

From  Minick's  admirable  summary3  and  presentation  of  the 
findings  of  the  Headlight  Committee  of  the  Railway  Master  Mechan- 
ics Association  and  other  investigators,  covering  the  several  classes 
of  oil,  acetylene,  incandescent  electric  and  arc  lamps,  are  taken 
Figs.  14  to  17.  Fig.  14  shows  the  beam  intensity  required  to  see 

»  J.  L.  Minick,  "The  Locomotive  Headlight;"  Trans.  I.  E.  S.,  Vol.  9,  page  909. 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION 


233 


at  different  distances  dummies  of  the  size  of  a  man  dressed  in  light, 
medium  and  dark  clothing.  The  curves  at  the  left  refer  to  the 
tests  on  the  oil,  acetylene  and  incandescent  electric  lamps;  those  on 
the  right  to  the  arc  tests.  There  is  a  marked  advantage  in  favor  of 
the  more  yellow  light  sources  due,  no  doubt,  in  part  to  the  lack  of 
steadiness  in  an  arc  and  to  the  fact  that  there  is  a  considerable  pro- 
portion of  blue  rays  for  which  the  eye  does  not  focus  accurately; 
thus  the  visibility  of  a  distant  object  is  reduced  with  a  given  inten- 
sity of  illumination.  Fig.  15  shows  deceleration  curves  for  a  heavy 
express  train  running  at  60  miles  per  hour  with  both  the  older  and 
more  modern  braking  systems.  It  is  evident  that  to  stop  the  train 


3000 


PWREES 

Fig.   17. — Candle-power  specifications  for  locomotive  head  lamps  recommended  by  Com- 
mittee of  Railway  Master  Mechanics  Assn. 

before  reaching  a  detected  object  requires  the  use  of  exceedingly 
high  beam  candle-powers.  From  Fig.  16,  recording  the  test  data 
indicating  the  range  within  which  the  various  signals  may  be 
identified  without  danger  of  error  for  different  beam  intensities, 
it  would  appear  that  only  relatively  low  values  of  beam  candle- 
power  would  meet  the  requirements  from  this  standpoint  with  the 
prevailing  signal  sources.  It  is  possible  that  the  substitution  of 
sources  of  higher  intensity  or  greater  concentration  in  the  signals 
would  make  the  use  of  high  candle-power  lamps  satisfactory  in 
every  respect.  The  conclusion  of  the  Master  Mechanics  Com- 
mittee was  that  the  intensity  of  locomotive  headlights  should  fall 


234 


ILLUMINATING    ENGINEERING   PRACTICE 


within  the  limits  given  in  Fig.  17.  These  limits  cover  the  range 
in  which  fall  the  oil  and  acetylene  lamps  with  which  most  loco- 
motives are  still  operated. 

It  would  appear  that  the  estimation  of  the  relative  importance  of 
the  several  factors  involved  must  determine  the  choice  of  headlighting 
characteristics.  For  multiple  tracks  and  winding  roads  this  will 
lead  to  different  conclusions  than  for  single-track  roads  without 
block  signals.  The  Interstate  Commerce  Commission,  which  re- 
cently undertook  the  supervision  of  these  devices  used  in  interstate 


1,200.000 


10°    8°     6°     4°      2°      0°  '  2°     4 
Angle  from  Axis 

Fig.  1 8. — Distribution  of  light  from  incandescent  headlight.     Parabolic  reflector  of  2% -in. 
focal  length  and  20  in.  diameter. 

traffic,  ruled  that  after  October  i,  1916,  all  new  locomotives  for  road 
service  and  those  given  a  general  overhauling  must  be  so  equipped 
that  a  person  of  normal  vision  at  the  engine  may  be  able  to  see  a 
dark  object  of  the  size  of  a  man  at  a  distance  of  1000  feet  or  more, 
under  normal  weather  conditions.  Furthermore,  all  locomotives 
must  be  so  equipped  before  January  i,  1920.  This  is,  of  course, 
very  different  from  the  recommendations  of  the  report  above  cited. 
As  a  result  of  the  new  ruling  it  would  seem  that  electric  units  will 
be  utilized  in  order  to  obtain  the  necessary  intensity,  and  that  since 


Fig.   19. — Locomotive  type  incandescent  head  lamp. 


Fig.  20. — Hand-controlled  commercial  searchlighting  equipments. 

(Facing  page  234.) 


Fig.   2 i . 


Fig.  22. 


Fig.   23. 


EDWARDS    AND   MAGDSICK:    LIGHT   PROJECTION  235 

arc  lamps  have  been  found  to  possess  less  suitable  characteristics 
and  to  be  not  so  well  adapted  to  the  desirable  electric  systems  in  this 
service,  incandescent  lamps  will  be  favored.  The  demand  for  mir- 
rored glass  reflectors  may  be  expected  to  increase  since  the  silvered 
metal  parabolas  which  have  been  employed  most  in  the  past  can- 
not so  easily  be  maintained  in  the  condition  required. 

It  appears  that  the  rulings  of  the  commission  can  be  met  by  the 
36,  72  and  io8-watt  6- volt  gas-filled  tungsten-filament  lamps 
and  the  150  and  25o-watt  32-volt  lamps.  The  io8-watt,  6-volt 
and  2 50- watt  32-volt  provide  a  good  factor  of  safety  and  will  prob- 
ably be  most  often  employed.  Fig.  19  illustrates  one  of  the  larger 
incandescent  headlighting  reflectors.  In  Fig.  18  are  given  the  pho- 
tometric results  with  three  different  lamps  in  this  reflector.  The 
folly  of  the  headlighting  legislation  of  a  number  of  states  (see  Table 
III)  requiring  the  use  of  a  source  of  1500  unreflected  candle-power  is 
apparent,  since  equal  beam  intensities  may  be  secured  with  con- 
siderably reduced  wattages.  Requirements  as  to  diameter,  visual 
tests,  etc.,  all  are  unnecessarily  indefinite  and  lead  to  needless  con- 
fusion. The  entrance  of  the  Interstate  Commerce  Commission 
into  the  field  promises  to  relieve  the  chaotic  condition  resulting  from 
the  legislation  of  the  individual  states,  but  it  would  seem  entirely 
feasible  that  its  requirements  be  stated  in  the  form  of  a  specifica- 
tion of  the  beam  characteristics  and  the  method  of  measurement. 


SEARCHLIGHTING  EQUIPMENTS 

Searchlighting  equipments  were  developed  principally  for  the  mili- 
tary service.  They  have  been  employed  by  the  army  and  navy  for 
more  than  50  years  as  one  of  the  most  effective  means  of  defense 
against  night  attack,  for  locating  enemy  vessels  and  fortifications 
as  well  as  for  signaling  purposes.  About  30  years  ago  the  first 
accurately  ground  parabolic  mirrors  became  available  and  these  with 
the  direct-current  carbon  arc  have  been  the  standard  equipment. 
No  radical  improvements  in  either  the  light  source  or  optical 
system  were  made  until  recently,  when  the  increasing  range  of 
torpedoes  made  these  developments  particularly  desirable. 

Fig.  20  shows  a  number  of  small  hand-controlled  searchlighting 
equipments  such  as  are  employed  also  in  commercial  work  and  in 
navigation.  It  will  be  seen  that  the  electrodes  are  in  a  horizontal 
position,  with  the  positive  tip  at  the  focus  of  the  mirror  inasmuch  as 
most  of  the  light  is  radiated  from  this  surface.  Fig.  21  is  a  mili- 


236 


ILLUMINATING   ENGINEERING   PRACTICE 


tary  equipment  provided  with  automatic  control  and  feeding 
mechanism.  In  addition  to  the  clear  glass  front  cover,  there  is  an 
iris  shutter  for  the  purpose  of  quickly  shutting  off  the  beam  or 
making  it  available  at  full  candle-power;  a  considerable  delay  in 
securing  full  intensity  would  be  involved  if  the  arc  were  extinguished. 
In  field  operation  the  equipments  are  mounted  on  trucks  with  ele- 
vating platforms  as  shown  in  Fig.  22  and  provided  with  reels  of  cable 
for  connection  with  the  energy  supply.  For  rapid  signaling  Vene- 
tian blinds  or  louvers  are  used  in  front  of  the  cover  glass,  Fig.  23. 
Some  data  for  typical  equipments  are  given  in  Table  IV: 

TABLE  IV. — TYPICAL  CARBON-ARC  SEARCHLIGHTING  PROJECTORS 


Nominal 
diameter 
of  mirror 
in  inches 

Amperes 

Actual  diame- 
ter of  mirror 
in  inches 

Focal 
length 
in  inches 

Reflector 

9 

IO 

Mangin  Mirror 

13 

20 



Mangin  Mirror 

18 

35 

19^6 

7% 

Mangin  Mirror 

24 

50 

25  H 

IO 

Parabolic  Mirror 

30 

80 

31  Me 

»H 

Parabolic  Mirror 

36 

no 

37 

14% 

Parabolic  Mirror 

60 

175 

Parabolic  Mirror 

The  36-in.  size  has  been  standard  in  the  navy,  while  the  6o-in. 
size  is  used  very  generally  in  land  fortifications.  The  beam  intensity 
of  such  units  is  of  the  order  of  60,000,000  and  200,000,000  candle- 
power,  respectively.  It  will  be  noted  that  the  focal  length  is  in 
each  case  about  40  per  cent,  of  the  diameter,  corresponding  to  an 
effective  angle  of  120°  to  130°.  Within  this  angle  is  included  a  large 
percentage  of  the  light  emitted  by  the  arc;  to  increase  the  angle  for 
a  given  diameter  would  be  to  decrease  the  beam  intensity  on  account 
of  the  greater  divergence  resulting  from  the  increased  angle  sub- 
tended by  the  source. 

Since  it  is  especially  necessary  to  maintain  the  arc  steady  and  at  the 
focus  of  the  reflector,  very  careful  attention  must  be  given  to  the 
electrical  characteristics,  the  feeding  mechanism,  the  uniformity 
of  the  electrodes  f  to  secure  constant  rate  of  consumption,  and  a 
proper  selection  of  sizes  of  electrodes.  High  current  density  and 
small  crater  result  in  high  intrinsic  brilliancy  and  beam  intensity. 
The  efficiency  is  increased  as  the  diameter  of  the  negative  electrode 
is  decreased  and  the  arc  lengthened,  with  the  accompanying  reduc- 


EDWARDS   AND   MAGDSICK:    LIGHT   PROJECTION  237 

tion  in  the  angle  of  shadow.  The  small  negative  is  also  advantage- 
ous in  steadying  the  arc;  but  if  the  current  density  is  carried  too  high, 
the  electrode  spindles,  that  is,  oxidizes  near  the  tip  and  thus  further 
reduces  the  diameter. 

Chillas  has  reported  the  results  of  an  investigation4  which  showed 
that  by  reducing  the  positive  electrode  size  to  the  point  where  the 
arc  crater  covers  practically  the  entire  tip  and  using  a  small  negative 
provided  with  a  copper  coating  to  increase  the  conductivity  and  pre- 
vent spindling,  equilibrium  conditions  are  attained  more  rapidly 
after  starting,  the  arc  is  more  steady,  there  is  a  higher  average  bright- 
ness of  the  positive,  a  smaller  dispersion  of  the  beam  and  hence  a 
considerable  increase  in  its  intensity.  These  advantages  are  secured 
at  a  sacrifice  of  electrode  life  and  with  the  necessity  for  some  adjust- 
ment of  the  arc  from  time  to  time.  The  size  of  electrodes  recom- 
mended is  about  ij^  in.  positive  and  %  in.  negative  in  the  36-in. 
reflector,  and  for  the  6o-in.,  i%-in.  positive  and  ^-in.  negative. 
Heretofore  a  2-in.  positive  has  ordinarily  been  employed  in  the  6o-in. 
lamp. 

The  recent  developments  in  the  application  of  flame  electrodes 
at  high  current  densities  have  produced  a  notable  advance  in  the 
performance  of  searchlighting  equipments.  The  electrode  diame- 
ters are  only  about  %  in.  for  the  positive  and  %g  m-  f°r  the  nega- 
tive. The  arc  is  somewhat  longer  than  with  pure  carbons  and  the 
negative  electrode  is  inclined  at  an  angle  of  about  20°  below  the  axis. 
At  the  high  currents  employed,  the  luminous  flame  is  confined  in  the 
deep  crater  of  the  positive,  where  the  gases  are  superheated  to  an 
exceedingly  high  temperature,  producing  a  brightness  of  about 
350,000  candles  per  sq.  in.  The  positive  electrode  is  continually 
rotated  and  thus  the  crater  kept  symmetrical;  the  negative  may 
also,  with  advantage,  be  rotated.  At  the  high  temperature  involved, 
some  provision  must  be  made  against  spindling  and  for  cooling  the 
electrodes.  In  one  form  of  lamp  this  is  done  by  bathing  the  tips 
with  burning  alcohol  vapor,  which,  with  the  radiating  discs  on  the 
holder,  acts  as  a  cooling  agent  and  prevents  oxidation  of  the  carbon 
shell.  In  another  form,  the  holders  are  also  provided  with  radiating 
discs  which  are  cooled  by  a  blast  of  air;  the  positive  electrode  is  fed 
through  a  quartz  tube  to  prevent  spindling. 

The  Navy  Department  tests5  have  shown  that  in  addition  to  its 

*  R.   B.   Chillas,  Jr.,   "Operating  Characteristics  of  Searchlight  Carbons;"  Journal  of 
the  United  States  Artillery,  page  191,  March-April,  1916. 

*  Lieut.  C.  A.  McDowell,  "Searchlights;"  Proc.  A.  I.  E.  E.,  Vol.  24.  page  207. 


238  ILLUMINATING    ENGINEERING    PRACTICE 

higher  efficiency  of  light  production,  the  flame  arc  directs  a  greater 
percentage  of  the  flux  into  the  effective  angle  of  the  mirror.  The 
small  source  results  in  a  narrow  angle  of  divergence — only  i°  to  2°, 
as  compared  with  2^°  to  3°  for  the  beam  from  standard  carbon  arcs. 
In  general,  it  is  reported  that  these  factors  combine  to  produce  with 
the  flame  arc  units  beam  intensities  about  five  times  as  great  as  those 
from  standard  carbon  lamps. 

The  formula  Ji  =  YI(I  —  P)ZLK,  giving  the  intensity  reaching 

the  observer  from  the  object  illuminated  is  frequently  referred  to 
as  indicating  that  the  range6  of  a  beam  is  proportional  to  the  fourth 
root  of  the  intensity.  On  the  other  hand,  it  is  contended  by  some 
that  since  the  brightness  of  an  object  remains  the  same  at  all  dis- 
tances, that  is,  the  luminous  density  on  the  retina  is  constant,  visi- 
bility is  dependent  only  upon  the  illumination  of  the  object  and  that 
the  range,  therefore;  varies  with  the  square  root  of  the  beam  intensity 
rather  than  the  fourth  root.  For  an  object  subtending  a  large  angle 
this  would  doubtless  be  true,  but  it  is  still  a  moot  question  whether 
for  small  angles  visibility  is  determined  by  the  total  flux  or  by  "the 
flux  density.  The  factor  of  acuity  doubtless  is  of  the  greatest  im- 
portance. The  dimensions  of  the  object;  the  color,  form  and  nature 
of  its  surface;  the  degree  of  contrast  with  surroundings;  the  influence 
of  telescope,  glasses  or  spectacles  and  the  physiological  peculiarities  of 
the  observer's  eye  all  enter  into  the  range  at  which  a  beam  is  effective. 
These  factors  have  been  analyzed  by  Blondel7  who  states  that  the 
range  increases  even  less  rapidly  than  the  fourth  root  of  the  intensity. 
To  multiply  the  range  five  fold  under  atmospheric  conditions  giving 
70  per  cent,  transmission  per  kilometer,  he  estimates  that  the  intens- 
ity would  have  to  be  increased  42,000  fold  for  typical  military  work. 

The  impression  prevails  that  blue  light  is  particularly  desirable 
in  the  rays  of  a  searchlighting  beam  since  the  surfaces  observed  are 
often  bluish  gray  and  because  of  the  Purkinje  effect.  Whenever  a 
preponderance  of  blue  rays  is  reflected  an  advantage  probably  ex- 
ists, but  in  the  usual  case  it  would  seem  to  be  detrimental  since  the 
eye  will  not  focus  for  the  blue  rays  when  the  longer  wave  lengths 
predominate,  and  vision  is,  therefore,  impaired. 

The  present  European  war  has  brought  about  a  number  of  inno- 

6  In  this  formula  Ji  =  intensity  directed  toward  eye  of  observer;  J  =  intensity  of  search- 
light beam;  L  =  distances  of  illuminated  object,  observer  assumed  near  searchlight;  P  = 
absorption  of  atmosphere;  K  =  coefficient  of  reflection  of  object. 

7  Prof.   A.    Blondel,    "A    Method   for   Determining   the   Range   of   Searchlights;"   The 
Illuminating  Engineer  (London),  Vol.  8,  page  85,  153. 


Fig.  24. 


Fig.  25. 


Fig.  26.  Fig.  27. 

Figs.  24,  25,  26,  27. — Commercial  flood-lighting  projectors. 

(Facing  page  238.) 


Fig.  28. — Representative  flood-lighting  installation. 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION  239 

vations  in  searchlighting  equipments,  such  as  the  use  of  a  Fresnel 
lens  above  the  arc  with  units  directed  upward  in  anti-aircraft 
work,  thus  replacing  a  mirror  below  the  arc,  which  would  be  subject 
to  cracking  by  the  molten  carbon.  For  short  ranges  incandescent 
electric  lamps  with  their  steadier  light,  greater  portability  and  ease 
of  control,  have  been  employed  to  advantage.  Oxy-acetylene  equip- 
ment has  found  similar  application. 

FLOOD  LIGHTING 

Flood  lighting  of  the  exteriors  of  structures  with  sources  concealed 
at  a  distance  is  more  a  problem  of  aesthetics  than  of  optics. 

Although  arc  projectors  had  been  employed  for  temporary 
lighting  spectacles  of  this  nature,  the  general  application  was  not 
found  feasible  until  the  concentrated  tungsten-filament  lamps  of 
high  efficiency  were  developed.  With  these  units  of  relatively 
small  size,  the  necessary  flexibility  in  installation  and  control  of 
intensity  and  direction  were  attained,  so  that  artistic  results  might 
be  secured.  Flood  lighting  supplements  the  older  forms  of  display 
illumination;  it  lends  itself  particularly  to  the  fields  of  sculpture, 
monumental  public  buildings  and  commercial  structures.  It  finds 
application  also  in  the  illumination  of  large  outdoor  spaces  devoted 
to  pageants  or  to  sports,  in  the  yards  of  industrial  plants  and 
railroads. 

Desirable  distributions  of  light  for  the  majority  of  installations 
range  from  an  angle  of  divergence  of  6°  to  one  of  30°.  This  is 
determined  by  the  area  of  the  surface,  its  distance  from  the  units  and 
the  angle  at  which  the  beam  is  incident.  A  small  amount  of  scat- 
tered light  is  usually  not  detrimental.  The  problem  of  reflector 
design  is,  therefore,  one  of  securing  high  over-all  efficiency  and 
adjustment  of  beam  spread  rather  than  of  narrow  divergence  and 
accurate  control.  Short  focal  lengths  are,  then,  desirable  so  as  to 
secure  a  large  effective  angle  with  a  reasonable  diameter  and  cost  of 
reflector.  It  would  seem  that  the  tendency  has  been  too  general 
to  secure  spreading  of  the  beam  by  placing  the  source  out  of  focus 
in  a  parabolic  reflector,  which  results  in  marked  lack  of  uniformity 
in  the  spot.  Except  for  the  narrow  beam  units,  the  rational  method 
is  to  proceed  with  the  design  of  the  reflector  from  the  desired 
distribution  curve  and  the  limiting  dimensions,  just  as  with  any  other 
specular  surface  equipment.  In  this  manner  units  are  secured  which 
not  only  produce  a  given  spread  with  reasonable  uniformity  but, 


240  ILLUMINATING   ENGINEERING    PRACTICE 

by  careful  design,  also  permit  a  considerable  adjustment  of  beam 
divergence. 

Typical  commercial  flood-lighting  projectors  are  shown  in  Figs. 
24-27.  All  of  these  have  reflectors  of  mirrored  glass  protected  in 
various  ways  to  withstand  high  temperatures  and  atmospheric 
conditions  to  which  they  may  be  subjected.  A  reflecting  surface 
of  this  class  is  the  only  one  to  be  recommended  in  the  great  majority 
of  installations. 

The  projectors  of  Figs.  24  and  25  are  designed  for  use  with  250- 
watt  flood-lighting  lamps.  In  both  cases  the  contour  of  the  reflector 
departs  somewhat  from  a  parabola  to  give  greater  uniformity  of  beam 
with  varying  divergence  as  the  position  of  the  lamp  is  adjusted. 
The  back  part  of  the  reflectors  is  spherical  to  accommodate  the  lamp 
bulb  and  direct  the  light  back  through  the  source,  making  possible 
a  unit  of  short  focal  length  and  considerable  depth,  hence  of  high 
efficiency.  The  one  unit  is  enclosed  in  a  ventilated  weatherproof 
housing  with  heat-resisting  glass  cover.  The  other  has  a  similar 
cover  but  is  tightly  enclosed  without  a  housing  about  the  reflector; 
the  copper  backing  provides  the  necessary  strength  and  the  dull 
black  enameling  facilitates  radiation  sufficiently  to  render  ventila- 
tion unnecessary.  Both  units  combine  compactness  and  low  cost. 

Fig.  26  shows  a  unit  usually  employed  with  the  5oo-watt  lamp. 
It  is  of  parabolic  contour,  relatively  more  shallow  but  giving 
a  more  concentrated  beam.  For  extreme  concentration  a  reflector 
of  this  type  is  to  be  recommended  with  a  2 50- watt  lamp.  The  pro- 
jector of  Fig.  27,  designed  for  iise  with  looo-watt  lamps,  is  a  para- 
bolic reflector  that  is  shallow  and  hence  inefficient  in  utilizing  the 
flux;  it  is  a  desirable  unit  for  few  applications. 

In  Fig.  28  is  given  the  distribution  of  candle-power  from  a  typical 
2  50- watt  projector  with  the  lamp  in  two  positions.  With  the  proper 
equipment  it  is  usually  possible  to  deliver  from  30  to  50  per  cent,  of 
the  total  flux  from  the  lamp  on  the  surface  of  the  structure  to 
be  illuminated.  An  experience  of  several  years  in  flood  lighting 
has  demonstrated  that  a  unit  of  the  general  type  of  Fig.  24  or  25 
can  most  often  be  used  advantageously  and  efficiently.  The  higher 
efficiency  of  the  larger  sizes  of  lamps  favor  their  use,  but  the  better 
control  of  direction  and  intensity  with  the  smaller  units  frequently 
out-weighs  this.  It  is  desirable  always  to  have  every  part  of  the 
surface  receive  light  from  several  projectors  in  order  to  eliminate 
the  striations  (images  of  the  filament)  and  to  provide  against  ap- 
parent lack  of  intensity  at  any  point  when  individual  lamps  burn  out. 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION 


241 


The  intensity  need  by  no  means  be  made  equal  for  all  parts  of  a 
structure;  rather,  the  brightness  should  be  so  distributed  as  to 
display  the  structure  as  nearly  as  possible  as  the  architect  or  sculptor 
intended.  Frequently  certain  features  can  be  emphasized  with 
advantage  over  the  results  secured  with  daylight.  In  general, 
desirable  average  intensities  are  dictated  by  the  reflecting  character- 
istics of  the  building  in  both  amount  and  direction,  the  bright- 
ness of  surroundings,  the  average  distance  from  which  it  is  to 
be  viewed  and  maximum  radius  of  visibility  desired,  as  well  as 
nature  of  the  structure  itself.  There  is  seldom  danger  of  overlight- 
ing  if  the  installation  is  properly  made. 


50,000 


25°   20 


20°    25° 


15°    10 s    5"     0°      5U     10° 

Angle  from  Axis 

Fig.  29. — Beam  candle-power  of  typical  complete  flood-lighting  unit  with  250- watt  Mazda 
C  flood-lighting  lamp  in  two  positions. 

The  latitude  in  direction  of  light  and  intensities  employed  may 
be  indicated  by  reference  to  a  few  representative  installations. 
Fig.  28  is  a  structure  of  simple  Doric  form  in  light  Bedford  stone  and 
granite.  Considerable  choice  is  here  offered  both  in  the  size  of 
units  and  their  location.  The  projectors  are  placed  on  the  roof  of 
a  four-story  building  diagonally  across  the  street  and  the  electrical 
power  provided  is  slightly  more  than  %  watt  per  square  foot  of 
building  surface. 

The  granite  building  of  Fig.  30  with  its  massive  Corinthian 
columns  and  decorations  in  relief,  required  particular  attention 
from  the  standpoint  of  direction.  The  light  sources  are  placed 

16 


242  ILLUMINATING   ENGINEERING   PRACTICE 

across  the  street  and  slightly  higher  than  the  bank  building.  About 
i  watt  per  square  foot  is  provided,  and  2 50- watt  units  are  employed 
in  order  to  secure  the  necessary  control  of  distribution  to  em- 
phasize the  architectural  features. 

The  monument  shown  in  Fig.  31  is  284  ft.  high  and  stands  in  an 
open  circle.  To  light  the  narrow  shaft  most  efficiently  requires  the 
use  of  parabolic  reflectors  giving  a  concentrated  distribution  of  light. 
The  projectors  are  placed  in  four  groups  on  the  surrounding  build- 
ings at  a  distance  of  230  ft.,  and  a  total  of  25  kw.  is  employed. 

The  Wool  worth  Tower,  Fig.  32,  receives  its  illumination  from  550 
projectors  of  the  2 50- watt  size.  The  average  power  consumption 
increases  from  0.75  watt  per  sq.  ft.  at  the  lower  section  to  four  times 
this  value  at  the  top.  The  use  of  small  units  with  considerable  lati- 
tude of  adjustment  was  here  required  because  of  the  necessity  for 
mounting  the  equipment  on  the  Tower  itself  and  the  desirability  of 
preserving  the  vertical  lines  which  form  the  main  architectural 
feature.  The  glazed  terra  cotta  surface  of  this  Tower8  complicated 
the  design  of  the  system. 

LIGHTHOUSES 

Lighthouses  differ  from  the  projector  applications  discussed  above 
in  that  they  exist  for  orientation  purposes  rather  than  for  the  illumi- 
nation of  other  objects.  Questions  of  visibility  here  pertain  to  a 
point  source,  that  is,  one  subtending  an  angle  of  less  than  30  seconds, 
the  limit  for  the  resolving  power  of  the  eye. 

Metallic  reflectors  were  at  one  time  employed  in  this  service  but 
are  now  found  in  only  a  few  installations  on  lightships.  Lens  sys- 
tems form  the  standard  equipment,  and  their  application  in  this  field 
is  notable  for  the  large  effective  angles  and  hence  the  high  efficiencies 
obtained.  The  careful  correction  of  these  lenses  has  led  to  a  degree 
of  control  surprising  in  view  of  the  extended  sources  of  relatively  low 
intrinsic  brilliancy  employed.  Reliability,  simplicity  and  low  cost  of 
operation,  rather  than  extreme  intensities,  are  the  primary  requisites 
in  the  majority  of  lighthouses.  From  the  optical  standpoint,  electric 
arc  or  concentrated  incandescent  lamps  are  most  nearly  ideal,  but 
since  central  electric  service  is  seldom  available,  their  application 
requires  an  installation  of  high  initial  and  operating  cost  with  skilled 
attendance.  For  these  reasons,  oil  lamps,  of  both  the  wick  and  in- 
candescent mantle  type,  are  still  generally  employed.  The  former 
is  the  most  reliable  of  all  sources;  the  latter  excels  it  in  brightness  and 

•  Electrical  World,  Vol.  68,  page  412. 


Fig.   30. — Representative  flood-lighting  installation. 


(Facing  Pag*  242.) 


Pig.   31. — Representative  flood-lighting  installation. 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION  243 

has  the  lowest  operating  cost  of  any  lamp  used  in  the  service.  Elec- 
tric lamps  are  installed  in  some  of  the  more  important  lighthouses 
where  high  intensity  is  necessary.  They  are  also  found  on  all  the 
larger  light  vessels. 

The  lens  systems  are  divided  into  orders  according  to  their  focal 
lengths,  ranging  from  150  mm.  for  the  6th  order  to  920  mm.  for  the 
ist  and  1330  mm.  for  the  hyper-radial.  For  fixed  beams,  giving  a 
band  of  light  continuous  in  a  horizontal  plane,  the  lenses  are  cylin- 
drical in  form  about  a  vertical  axis,  Fig.  33.  The  light  issues  as  a 
belt  of  narrow  vertical  divergence;  this  angle  and  the  intensity  of  the 
beam  vary  directly  with  the  focal  length  for  a  given  light  source. 
The  central  part  of  a  typical  lens  covers  an  angle  at  the  source  of 
nearly  60°  and  contributes  about  60  per  cent,  of  the  light.  This 
portion  of  the  lens  is  dioptric,  redirecting  the  light  by  refraction 
only.  The  upper  and  lower  parts  of  the  lens  system  are  catadioptric, 
acting  by  both  refraction  and  total  reflection.  The  lower  prisms 
cover  about  20°  and  furnish  10  per  cent,  of  the  beam;  the  upper,  nearly 
50°  and  30  per  cent,  of  the  light.  Frequently  a  dioptric  belt  of 
about  80°  effective  angle  is  employed  alone. 

If  lenses  developed  about  a  horizontal  axis  are  used,  both  vertical 
and  horizontal  concentration  is  secured  and  a  very  intense  narrow 
cone  of  light  results,  varying  for  a  given  source  roughly  as  the  square 
of  the  focal  length  of  the  lens.  Such  a  hemispherical  lens,  Fig.  34, 
with  a  spherical  mirror  on  the  opposite  side  of  the  source  gives  a 
powerful  beam  in  one  fixed  direction,  as  for  range  lighting  along 
a  channel.  Two  such  hemispheres,  known  as  the  bi- valve  lens,  give 
high  intensity  beams  at  180°  and  are  utilized  rotating  about  the 
source  to  produce  the  highest  powered  flashing  effects.  Another 
lens  giving  four  flashes  per  revolution  is  shown  in  Fig.  35.  By  vary- 
ing the  design,  any  desired  sequence  of  flashing  with  controlled 
period  of  flash  and  interval  may  be  secured. 

Variations  from  a  fixed  beam  are  introduced  in  part  to  differen- 
tiate lighthouses  from  each  other  and  from  shore  stations.  Where 
low  intensity  suffices,  this  is  often  accomplished  by  an  occulting 
device  which  covers  the  source  at  characteristic  intervals,  or  by 
rotating  the  lens  after  screening  sections  of  it.  If  spherical  mirrors 
are  used  as  screens,  the  beam  intensity  is  thereby  also  increased. 
The  other  important  reason  for  the  use  of  the  flashing  lens  is  the 
enormous  increase  in  beam  intensity  realized;  this  is  practically  in- 
versely proportional  to  the  ratio  of  period  of  flash  to  interval  between 
flashes. 


244  ILLUMINATING   ENGINEERING    PRACTICE 

The  lenses  shown  in  the  illustration  represent  a  recent  develop- 
ment in  that  they  are  ground  by  machinery;  hence  all  sections  are 
interchangeable  among  different  units  of  the  same  type.  This 
is  not  the  case  with  the  hand-made  imported  lenses  previously  used; 
yet  the  new  lenses,  designed  by  Hower,  are  exceedingly  accurate, 
with  a  divergence,  it  is  reported,  of  less  than  one  degree  in  some  sizes, 
and  with  little  scattered  light. 

Patterson  and  Budding9  found  that  the  visibility  of  a  point  source 
is  proportional  to  the  candle-power  and  inversely  to  the  square  of  the 
distance;  that  visibility  is  independent  of  brightness  for  sources  sub- 
tending an  arc  of  less  than  two  minutes.  Their  investigation  showed 
values  for  the  range  of  lights  of  different  colors  only  slightly  less 
than  the  following  reported  by  the  German  lighthouse  Board  of  Ham- 
burg as  the  results  of  their  tests  in  1894: 
R  =  i.53\/I  For  white  light  in  clear  weather,  where  R  represents 

the  range  in  miles  and  7  the  candle-power. 
R  =  i.09\/i    For  white  light  in  rainy  weather. 
R  =  1.63^/1     For  green  light  in  clear  weather. 

It  will  be  seen  that  for  ordinary  atmospheric  conditions  relatively 
low  intensities  would  suffice  to  be  visible  at  the  geographic  limit. 
Many  of  the  larger  incandescent  mantle  oil  lanterns  give  intensities 
of  the  order  of  several  hundred  thousand  candle-power.  Electric 
units  give  beams  that  are  measured  in  millions;  the  largest  is  the 
Navesink  equipment  at  the  entrance  to  New  York  Harbor  reported 
variously  as  from  25,000,000  to  60,000,000  candle-power.  In 
many  installations  the  duration  of  the  flash  is  o.i  second  or  even  less. 
This  is  probably  shorter  than  the  time  required  at  low  illuminations 
to  produce  the  same  sensation  as  a  steady  beam  of  the  same  inten- 
sity. The  results  produced  by  different  durations  of  flash  and  inter- 
vening periods  are  only  partially  known;  nevertheless  the  work  of 
Blondel  and  Rey  leads  them  to  conclude  that  for  maximum  utiliza- 
tion of  a  source  at  range  limits  short  flashes  are  required. 

There  is  a  marked  tendency  toward  using  numbers  of  buoys  in- 
stead of  erecting  a  few  lighthouses  of  high  intensity.  With  Pintsch 
gas  or  acetylene  these  buoys  frequently  operate  for  periods  as  high  as 
nine  months  or  a  year  without  attention.  They  can  be  operated 
with  interrupted  beams  by  means  of  mechanism  actuated  by  the  gas 
pressure,  which  turns  the  main  burner  off  and  on.  With  the  large 
buoys  it  is  also  found  economical  to  utilize  valves  which  are  kept 
closed  during  the  day  by  the  daylight  radiation. 

»  Proc.  Phys.  Soc.  London,  24,  page  379,  IQI3- 


Fig.  32.— Representative  flood-lighting  installation. 


(Facing  page  244.) 


Fig.  33. — Fourth  order  six-panel  fixed  lens. 


Fig.  34. — Fourth  order  range  lens. 


Fig.  35. — Fourth  order  four-panel  flashing  lens. 


Fig.  36. — Signalling  projector  for  aircraft 


EDWARDS   AND   MAGDSICK:   LIGHT   PROJECTION 


245 


LIGHT  SIGNALS    ' 

Other  applications  of  light  signals  are  principally  in  the  railway 
and  military  fields.  Table  V,  taken  from  a  paper  by  Gage,10  shows 
the  usual  sizes  and  types  of  semaphore  lenses  with  the  axial  candle- 
power  values  and  beam  spread  for  both  the  long-time  and  one-day 
kerosene  burners,  which  flames  give  about  one  and  two  candle-power, 
respectively.  The  optical  lens  is  of  the  usual  Fresnel  type  with  the 
edge  of  the  prismatic  rings  toward  the  flame;  the  inverted  has  these 
pointing  outward  and  requires  a  cover  glass.  The  inverted  lens  has 
the  advantage  that  none  of  the  light  is  deflected  by  the  risers  of  the 
prisms.  The  values  in  the  table  are  for  clear  lenses.  In  most 
signals  colored  glasses11  are  employed.  With  the  same  sources,  the 

TABLE  V. — DATA  FOR  OPTICAL  LENSES 


With  long-time  Burner 

With    one-day    burner 

Diam., 

Focus, 

Spread,  ft.  per  100 

Spread,  ft.  per  100 

inches 

inches 

r^     /^1 

C*       A\ 

A 

S 

power, 

Of  50  per  cent, 
intensity, 
D 

Extreme, 
E 

power, 

Of  50  per  cent, 
intensity, 
G 

Extreme 
H 

4 

»H 

37-5 

14.0 

16.6 

30.6 

24-3 

26.7 

4 

3/-6 

40.5 

12.2 

14.4 

32.8 

21  .0 

23-3 

4H 

2^ 

39-6 

15.2 

17-4 

32.3 

25   5 

28.1 

4* 

sM 

42.0 

12-5 

14-7 

33   5 

21.5 

23-7 

4% 

3 

44-5 

12.9 

15   3 

36.3 

22.3 

24-7 

4H 

3H 

48.0 

II.  9 

14.1 

38.5 

20.  6 

22.7 

5 

3>£ 

57-0 

II.  7 

13-8 

46-5 

2O.  2 

22.3 

sK 

3H 

69.0 

11.75 

14.0 

56.2 

20.4 

22.6 

6 

3*i 

82.0 

10.6 

12.6 

67.1 

18.4 

20.3 

6}i 

3?i 

90.5 

10.5 

12.4 

74-2 

18.1 

20.0 

8^ 

4 

130.0 

8.4 

ii  .7 

106.5 

14-6 

20.2 

SK 

5 

142  .0 

7-4 

8.7 

116.0 

12.7 

14-0 

I 

DATA  FOR  INVERTED  LENSES 

4 

3H 

35-4 

14-5 

17-5 

29.0 

24.0 

31   o 

4K 

2>i 

42.0 

17.0 

21  .1 

34.2                28.0 

38.3 

4H 

3 

51.8 

16.1 

19.8            42.3              26.4 

35-7 

S 

3^ 

62.5 

14.2 

17.75 

51  o               23.4 

32.1 

5^ 

2K 

59  0 

17-0 

19  3 

48.0                 28.0                 53-0 

5^ 

3^ 

66.8 

13-8 

16.75 

55   3                  22.7                  30.3 

6  £6 

3?4 

89.8 

12.7 

16.5 

73-2                  20.8 

29.6 

7^ 

3 

94  5 

13-5 

23-7 

77-1                   22.3 

42.8 

8^ 

3H 

I2O.O 

ii.  8 

19-8                97-8                   19-5                  35-7 

10  H.  P.  Gage,  "Types  of  Signal  Lenses,"  TRANS,  I.  E.  S..  Vol.  9.  page  486. 

11  For  a  resum6  of  the  subject  of  color  and  vision,  see  "Color  and  It  Applications"  by 
M.  Luckiesh. 


246  ILLUMINATING   ENGINEERING   PRACTICE 

effective  range  in  miles  for  commercial  colored  lenses  is  reported 
by  the  Railway  Signal  Association  (1908)  as: 

Red 3     to  3. 5 

Yellow i      to  i .  5 

Green 2 . 5  to  3 .  o 

The  range  for  a  clear  lens  is  estimated  at  from  8  to  1 2  miles.  The 
visibility  is  decreased  when  the  field  surrounding  the  lens  is  slightly 
illuminated,  as  in  a  slight  haze  or  when  other  sources  are  near  by. 

Small  electric  lamps  are  rapidly  coming  into  use  in  semaphore 
signals  and,  with  the  stronger  intensities  produced  by  these  more 
concentrated  sources,  they  are  found  to  be  more  satisfactory  by 
day  than  are  the  arms.  In  one  type  of  equipment  use  is  made  of 
rows  of  lenses  in  the  three  arm  positions.  On  the  C.  M.  &  St. 
P.  R.  R.,  three  signals,  red,  green  and  white,  are  aligned  vertically. 
Behind  each  lens  are  two  lamps,  one  operating  at  low  efficiency,  to 
prevent  failure  of  the  signal.  The  normal  daylight  range  is  3000 
feet,  and  under  the  worst  conditions  when  opposed  to  direct  sun- 
light the  range  is  not  less  than  2000  feet.  It  is  reported  that  they 
are  seen  more  easily  than  semaphore  arms  under  all  circumstances 
and  that  they  show  two  or  three  times  as  far  as  the  latter  in  a 
snowstorm. 

Military  searchlighting  projectors  have  been  used  to  transmit  sig- 
nals at  night  more  than  50  miles  by  training  the  beam  on  a  cloud. 
They  are  also  used  in  the  navy  directly  for  day  signaling  over  con- 
siderable distances,  and  have  the  advantage  that  the  narrow  beam 
precludes  observation  by  other  vessels  even  though  only  a  few  degrees 
removed.  The  type  of  shutter  equipment  used  is  illustrated  in  Fig. 
23.  Several  small  incandescent  lamps  mounted  in  the  ring  focus  of 
a  cylindrical  Fresnel  lens  are  used  with  a  Morse  key  for  night  signal- 
ling in  the  navy  at  moderate  distances,  superseding  the  Ardois  and 
other  devices.  In  the  European  war,  extensive  use  is  made  among 
the  land  forces  of  i.2-c.p.  metal-filament  lamps  equipped  with  para- 
bolic reflectors.12  Morse  signals  are  reported  to  have  been  read  at 
ii  miles  at  the  rate  of  17  words  per  minute  with  this  apparatus. 
Fig.  36  illustrates  a  150- watt  signalling  projector  employed  on 
British  aircraft.  The  properties  of  the  spherical  and  parabolic 
mirrors  as  well  as  the  dioptric  lens  are  utilized. 

PROJECTION  OF  TRANSPARENCIES 

One  of  the  most  familiar  applications  of  lens  systems  in  lighting 
equipment  is  for  the  projection  of  lantern  slides  and  motion  picture 

15  Illuminating  Engineer,  London,  Vol.  8,  page  62. 


EDWARDS   AND    MAGDSICK :   LIGHT    PROJECTION 


247 


films.  The  scope  of  this  lecture  permits  reference  only  to  the 
fundamental  optical  systems  and  the  light  sources  for  the  most  com- 
mon classes  of  equipment.  Numerous  treatises  of  a  more  detailed 
nature  are  available;  some  of  the  more  recent  ones  are  mentioned  in 
the  appended  bibliography. 

The  elements  of  the  optical  system  for  lantern-slide  projection  are 
shown  in  Fig.  3  jA .  The  condenser  intercepting  the  flux  from  the  lamp 
becomes  a  secondary  source  having  a  brightness  differing  from  the 
intrinsic  brilliancy  of  the  light  source  by  only  the  percentage  of 
losses  in  the  glass,  and  directs  a  converging  beam  through  the  slide 
into  the  objective  lens.  The  focal  length  of  the  latter  is  determined 
by  the  distance  to  the  screen  and  the  size  of  the  picture  desired 


Light  Source 


Condensing  Lens 

Slide  Holder 


Screen 


Mirror 


Screen 


Fig.  37. — A,  Simple  optical  system  for  the  projection  of  lantern  slides.     B,  Simple  optical 
system  for  the  projection  of  motion  pictures. 

Focusing  for  the  different  distances  is  accomplished  by  adjusting  the 
position  of  the  objective  with  reference  to  the  slide.  If  the  objective 
were  limited  to  a  very  small  aperture,  the  source  of  light  would  have 
to  be  highly  concentrated  in  order  that  the  rays  might  be  accurately 
controlled  and  concentrated  at  this  point.  In  practice,  these  may 
be  made  of  considerable  size;  hence  it  is  possible  to  secure  the  re- 
quired illumination  from  a  somewhat  extended  source.  Cost  con- 
siderations determine  the  best  combination  of  source  brightness  and 
objective  diameter.  To  secure  uniform  results  over  the  entire  pic- 
ture, it  is  necessary  that  from  any  point  in  it  a  view  through  the 
objective  and  slide  holder  disclose  condenser  surface  covering  the 
entire  area.  In  order  to  keep  the  condenser  diameter  within  reason- 


248  ILLUMINATING    ENGINEERING   PRACTICE 

able  limits,  it  is  important  to  place  the  slide  holder  close  to  it. 
Mounting  the  light  source  near  the  condenser  results  in  the  utiliza- 
tion of  the  flux  in  a  relatively  large  solid  angle,  and,  therefore,  makes 
for  efficiency.  The  usual  opening  in  the  slide  holder  is  3  X  3/4  in. 
To  illuminate  all  parts  and  avoid  spherical  and  chromatic  aberration 
requires  a  beam  of  a  diameter  even  greater  than  the  diagonal  of 
the  opening;  thus  a  considerable  percentage  of  the  light  is  lost. 

In  motion  picture  work,  Fig.  37  B,  the  intensity  requirements 
are  far  more  severe  and  the  brightness  of  the  light  source  is  corre- 
spondingly important.  The  aperture  of  the  plate  through  which  the 
film  is  fed  has  an  area  of  0.680  X  0.906  in.  It  is,  therefore,  placed 
well  forward  of  the  condenser  in  the  narrower  part  of  the  beam. 
Additional  losses  are  encountered  through  the  necessity  for  a  shutter, 
usually  a  sectored  disc,  to  cut  off  the  light  during  the  period  of 
film  shifting,  which  occurs,  with  the  usual  pictures,  16  times  every 
second.  Since  this  frequency  would  be  apparent  as  a  distinct 
flicker,  a  two-wing  or  three- wing  shutter  is  provided  so  that  the  light 
may  be  shut  off  32  or  48  times  per  second. 

Kerosene  and  acetylene  flames,  incandescent  mantles  and  Nernst 
glowers  and  oxy-hydrogen  lime  light  sources,  have  all  been  em- 
ployed in  the  projection  of  lantern  slides.  To-day  electric  arc  and 
incandescent  lamps  are  used  almost  exclusively. 

The  positive  crater  of  the  direct-current  arc  is  particularly  de- 
sirable as  a  source  of  light  because  of  its  high  intrinsic  brilliancy. 
It  is  not  practicable  to  utilize  the  maximum  brightness  since  the 
electrodes  must  be  so  arranged  that  the  positive  tip  is  at  an  angle 
with  the  condenser  or  that  the  negative  shades  a  part  of  it.  In 
order  to  keep  the  arc  steady,  it  is  desirable  to  have  a  small  negative 
electrode,  and  this  is  secured  with  the  necessary  current-carrying 
capacity  by  coating  the  carbon  with  metal.  For  lantern  slide  pro- 
jection,13 currents  of  from  4  to  25  amperes  are  found  ample,  with 
electrodes  ranging  from  6  to  13  mm.  in  diameter.  For  the  ordi- 
nary motion  picture  films,  currents  of  from  40  to  no  amperes  are 
employed  with  positive  electrodes  ranging  from  13  to  25  mm.  in 
diameter  and  negative  electrodes  of  from  8  to  22  mm.,  depending 
upon  the  current  and  the  composition. 

Alternating-current  lamps  of  low  amperage  are  operated  with  a 
long  arc.  Since  the  arc  is  continuously  reversing,  there  is  no  sharply 
defined  crater  of  high  brilliancy  on  either  electrode.  Such  lamps  are 
distinctly  inferior  in  efficiency  to  the  direct-current  arcs,  although 

l*  R.  B.  Chillas,  Jr.,  "Projection  Engineering;"  Trans.  I.  E.  S.,  Vol.  u,  page  1097. 


EDWARDS   AND    MAGDSICK :   LIGHT   PROJECTION 


249 


ample  for  most  lantern  slide  work.  For  motion  picture  projection, 
the  alternating-current  electrodes  are  operated  close  together  to 
secure  better  craters.  The  electrodes  are  inclined  to  each  other  so 
as  to  expose  as  much  as  possible  of  one  of  the  tips  to  the  condenser. 
However,  the  brightness  of  the  source  is  still  lower  than  with  direct- 
current,  and  considerable  shading  results  due  to  the  interference  of 
the  other  electrode.  Shutters  employed  with  alternating-current 
equipment  are  of  the  two- wing  type;  the  three-wing  shutter  with  a 
frequency  of  48  per  second  gives  rise  to  stroboscopic  effects  with 
6o-cycle  current. 

Incandescent  lamps  of  special  concentrated-filament  construc- 
tion are  used  for  the  projection  of  lantern  slides  under  all  condi- 
tions, and  take  care  of  the  requirements  amply.  Recently  the  gas- 
filled  tungsten-filament  lamps  have  also  been  successfully  applied 


Condenser 


Objective 


Light  Source 


Per  Cent -100  20.2   14.0 
Lumens-23,600  4770  3300 
Fig.  38. — Typical  efficiency  chart  for  motion  picture  projection  with  mazda  lamp;  machine 
operating  without  film. 

to  motion  picture  projection.  It  will  be  seen  from  Table  I  that  the 
brilliancy  of  such  sources  is  still  below  that  of  the  carbon  arc; 
nevertheless,  their  application  is  feasible  because  of  other  advan- 
tages gained.  Among  these  is  a  somewhat  more  efficient  utiliza- 
tion of  the  flux  due  to  the  fact  that  the  source  can  be  placed  closer 
to  the  condensing  lens.  When  used  with  objectives  of  the  larger 
apertures  the  incandescent  filament  is  found  to  be  sufficiently  con- 
centrated, and  the  intensification  of  flicker  and  irregularities  pro- 
duced by  such  lenses  with  arc  sources  is  obviated.  The  steadiness 
of  the  light  and  the  elimination  of  operating  difficulties  are  quite  as 
important  as  the  reduction  in  operating  cost  realized.  In  Fig.  38 
are  shown  the  utilization  and  losses  of  the  flux  in  such  apparatus. 
Although  the  losses  in  the  system  may  appear  high,  it  should  be 
noted  that  the  results  for  each  part  of  the  apparatus  are  superior 
to  those  secured  with  most  equipment  in  use  to-day.  The  illumina- 


250  ILLUMINATING    ENGINEERING    PRACTICE 

tion  intensity  on  a  picture  area  of  150  square  feet  is  seen  to  be  in 
excess  of  5  foot-candles. 

The  question  of  the  most  desirable  intensity  for  motion  picture 
projection  is  one  on  which  a  difference  of  opinion  still  exists.  The 
Committee  on  Glare  of  the  Illuminating  Engineering  Society14  has 
recommended  a  brightness  for  the  picture  corresponding  to  a  screen 
illumination  of  2.5  foot-candles  with  no  film  in  the  machine,  with  a 
factor  of  5  either  way.  A  brightness  which  is  too  high  causes  not 
only  fatigue  to  the  eye,  but  also  makes  the  flicker,  wandering  of  the 
arc,  etc.,  more  pronounced.  It  appears  that  the  present  high- 
current  arc  installations  are  operating  in  the  upper  range  of  desirable 
intensities. 

BIBLIOGRAPHY 

GENERAL  PRINCIPLES  or  LIGHT  PROJECTION 

BENFORD,  F.  A.,  JR.— "The  Parabolic  Mirror;"  Trans.  I.  E.  S.,  Vol.  10,  p.  905. 

GAGE,  S.  H.  and  H.  P.— "Optic  Projection,"  Comstock. 

NATIONAL  LAMP  WORKS  or  G.  E.  Co. — "  Mazda  Lamps  for  Projection 
Purposes;"  Eng.  Dept.  Bulletin  No.  23. 

ORANGE,  J.  A. — "Photometric  Methods  in  Connection  with  Magic  Lantern 
and  Moving  Picture  Outfits,  and  a  Simple  Method  of  Studying  the  Intrinsic 
Brilliancy  of  Projection  Sources;"  G.  E.  Review,  Vol.  19,  p.  404. 

PORTER,  L.  C. — "Photometric  Measurements  of  Projectors;"  Lighting  Jour- 
nal, Vol.  4,  p.  7. 

"New  Developments  in  the  Projection  of  Light;"  Trans.  I.  E.  S.,  Vol.  10, 
p.  38. 

AUTOMOBILE  HEADLIGHTING 

CLARK,  EMERSON  L. — "Automobile  Lighting  from  the  Lighting  Viewpoint;" 
Bull.  Soc.  Auto.  Engs.,  April,  1916,  p.  45. 

Discussion. — "Headlight  Glare;"  Bull.  Soc.  Auto.  Engs.,  Feb.,  1916, 
p.  296. 

Symposium. — "Glare-Preventing  Devices  for  Headlights;"  Trans.  Soc. 
Auto.  Engs.,  Vol.  9,  Part  II,  p.  284. 


RAILWAY  HEADLIGHTING 

American  Ry.  Master  Mechanics  Ass'n. — "Report  of  Headlight  Committee," 
1914. 

Ass'n.  of  Ry.  Elec.  Engineers. — "Report  of  Committee  on  Locomotive 
Headlights;"  Ry.  Elec.  Eng.,  Vol.  5,  p.  199.  . 

BABCOCK,  A.  H.— "Southern  Pacific  Six- Volt  Electric  Headlight  Equip- 
ment;" Ry.  Elec.  Eng.,  Vol.  7,  p.  233. 

14  Committee  on  Glare,  "Diffusing  Media;  Projection  and  Focusing  Screens,  "Trans. 
I.  E.  S.,  Vol.  ii,  page  92. 


EDWARDS   AND   MACDSICK:    LIGHT   PROJECTION  251 

BAILEY,  P.  S. — "Incandescent  Headlights  for  Street  Railway  and  Locomo- 
tive Service;"  G.  E.  Review,  Vol.  19,  p.  638. 

HARDING,  C.  F.,  AND  TOPPING,  A.  N. — "Headlight  Tests;"  Trans.  A.  I.  E.  E. 
Vol.  29,  p.  1053. 

MINICK,  J.  L.— "The  Locomotive  Headlight;"  Trans.  I.  E.  S.,  Vol.  9,  p.  909. 

PORTER,  L.  C. — "Meeting  the  Federal  Headlight  Requirements;"  Ry. 
Elec.  Eng.,  Vol.  7,  p.  468. 

Ry.  Elec.  Engineer,  Vol.  3. — "Electric  Headlights — Wisconsin  Railroad 
Commission  Tests." 

SCRUGHAN,    J.    G.— "Electric    Headlight  Tests;"   Ry.   Elec.   Eng.,   Vol.   5, 

P-  349- 

Symposium  (Succ,  CHAS.  R.,  DENNINGTON,  A.  R.,  PORTER,  L.  C.). — "Theory, 
Design  and  Operation  of  Head-Lamps;"  Elec.  World,  Vol.  62,  p.  741. 

SEARCHLIGHTING 

BLONDEL,  A. — "A  Method  for  Determining  the  Range  of  Searchlights;" 
Illuminating  Eng.  (London),  Vol.  8,  pp.  85,  153. 

CHILLAS,  R.  B.,  JR.— " Searchlight  Carbons;"  Journal  of  U.  S.  Artillery, 
March-April,  1916,  p.  191. 

Electrical  World. — Vol.  64,  p.  181;  "Search  Lamp  with  Vapor-cooled  Elec- 
trodes" (Beck).  Vol.  68,  p.  611;  "High-Intensity  Searchlight  for  Governmental 
Purposes"  (Sperry). 

MCDOWELL,  LIEUT,  C.  S.— "Searchlights;"  Proc.  A.  I.  E.  E.,  Vol.  34,  p.  195. 
"Illumination  in  the  Navy;"  Trans.  I.  E.  S.,  Vol.  n,  p.  573. 

NERZ,  F. — "Searchlights;  Their  Theory,  Construction  and  Applications;" 
Van  Nostrand. 

Symposium  (LEDGER,  P.  G.,  AYRTON,  MRS.  HERTHA,  TROTTER,  A.  P.,  etc.) 
— "Searchlights;  Their  Scientific  Development  and  Practical  Applications;" 
Illuminating  Eng.  (London),  Vol.  8,  pp.  53-84. 

WEDDING,  W. — "A  New  Searchlight"  (Beck);  Electrotechnische  Zeit- 
schrift,  1914,  p.  901. 

FLOOD  LIGHTING 

BAYLEY,  G.  L. — "Illumination  of  Panama-Pacific  Exposition;"  Elec. 
World,  Vol.  65,  p.  391. 

Elec.  Review  and  Western  Electrician,  Vol.  67,  p.  1104;  "Indianapolis 
Bank  Adopts  Flood  Lighting."  Vol.  67,  p.  724,  "Flood  Lighting  of  Building 
Fronts  from  Ornamental  Cluster  Posts." 

Electrical  World,  Vol.  67,  p.  1173;  "Flood  Lighting  a  Flag;"  Vol.  67, 
p.  1462;  "Adding  Hours  to  Summer  Days  for  Outdoor  Recreations."  Vol. 
68,  p.  453;  "Niagara  Falls  Flood-Lighted." 

HARRISON,  WARD  and  EDWARDS,  EVAN  J.— "Recent  Improvements  in  In- 
candescent Lamp  Manufacture;"  Trans.  I.  E.  S.,  Vol.  8,  p.  533. 

Lighting  Journal,  Volume  4,  p.  18;  "Projectors  for  Flood  Lighting." 

MACGREGOR,  R.  A.— "Lighting  the  Soldiers'  and  Sailors'  Monument;" 
Ltg.  Journal,  Vol.  4,  p.  175. 

MAGDSICK,  H.  H.—" Flood  Lighting  the  World's  Tallest  Building;"  Elec. 
World,  Vol.  68,  p.  412. 


252  ILLUMINATING   ENGINEERING   PRACTICE 

PORTER,  L.  C. — "Pageant  Lighting;"  Ltg.  Journal,  Vol.  3,  p.  169. 

RYAN,  W.  D'A. — " Spectacular  Illumination;"  G.  E.  Review,  Vol.  17,  p.  329. 

"Illumination  of  the  Panama-Pacific  International  Exposition;"  G.  E. 
Review,  Vol.  18,  p.  579. 

SUMMERS,  J.  A. — "Flood  Lighting  the  State  House  at  Boston;"  Ltg.  Journal, 
Vol.  4,  p.  2. 

UHL,  A.  W. — "Flood  Lighting  of  a  Great  Outdoor  Pageant;"  Ltg.  Journal, 
Vol.  4,  p.  172. 

LIGHTHOUSES 

Encyclopaedia  Brittanica,  nth  Edition. 

HASKELL,  RAYMOND. — "Lighthouse  Illumination;"  Trans.  I.  E.  S.,  Vol. 
10,  p.  209. 

MACBETH,  GEO.  A. — "Lighthouse  Lenses;"  Proc.  Engs.  Soc.  Western 
Penn.,  Vol.  30,  p.  231. 

LIGHT  SIGNALS 

CHURCHILL,  WM. — "Red  as  a  Danger  Indication;"  Trans.  I.  E.  S.,  Vol.  9, 

P-  37i. 

GAGE,  H.  P.— "Types  of  Signal  Lenses;"  Trans.  I.  E.  S.,  Vol.  9,  p.  486. 

LUCKIESH,  M. — "Color  and  Its  Applications,"  Van  Nostrand. 

MCDOWELL,  LIEUT.  C.  S. — "Illumination  in  the  Navy;"  Trans.  I.  E.  S., 
Vol.  ii,  p.  573- 

SAUNDERS,  J.  E. — "Recent  Developments  in  Light  Signals  for  Control  of 
High-Speed  Traffic;"  Elec.  Journal,  Vol.  13,  p.  443- 

STEVENS,  THOS.  S. — "Illumination  of  Signals;"  Trans.  I.  E.  S.,  Vol.  9,  p.  387. 

PROJECTION  OF  TRANSPARENCIES 

CHILLAS,  R.  B.,  JR. — "Projection  Engineering;"  Trans.  I.  E.  S.,  Vol.  n, 
p.  1097. 

GAGE,  S.  H.  and  H.  P.— "Optic  Projection." 

ORANGE,  J.  A. — "Optic  Projection  as  a  Problem  in  Illumination;"  Trans. 
I.  E.  S.,  Vol.  11,  p.  768. 

TAYLOR,  J.  B. — "The  Projection  Lantern;"  Trans.  I.  E.  S.,  Vol.  n,  p.  414. 


THE  ARCHITECTURAL  AND  DECORATIVE  ASPECTS  OF 

LIGHTING 

BY    GUY   LOWELL 

There  is  surely  no  scientific  profession,  there  is  no  branch  of  the 
engineering  fraternity  for  which  a  thorough  artistic  training  is  more 
desirable  than  the  profession  of  illuminating  engineering.  We  can 
see,  however,  by  looking  over  the  list  of  lectures  in  the  usual  engi- 
neering courses  that  the  technical  knowledge  which  one  should  have 
is  so  great — there  are  so  many  scientific  subjects  to  be  discussed — 
that  there  can  be  but  little  time  left  in  the  curriculum  for  the  study 
of  the  fine  arts.  Yet  after  all  the  aims  of  the  illuminating  engineer 
and  of  the  artist  are  similar — it  is  to  reach  the  mind  through  the 
eyes.  The  point  of  view  of  the  engineer  is,  however,  largely  objec- 
tive. He  often  seems  to  think  that  his  mission  is  ended  when  he 
has  made  it  possible  to  convey  to  the  mind  the  facts — as  they  are. 
The  artist  idealizes  and  wishes  to  state  the  facts  as  they  might  or 
should  be,  or  as  we  say,  colloquially,  he  wants  to  show  them  in  the 
best  possible  light.  These  two  methods  of  seeing — the  subjective 
method  and  the  objective  method,  are  often  not  very  different,  and 
I  want  to  spend  my  time  this  morning  considering  the  common  aims 
of  the  illuminating  engineer  and  the  artist,  and  show  how  close  to- 
gether the  paths  of  the  two  really  lie. 

We  have  been  taught  that  were  it  not  for  the  reflected  light  that 
comes  from  all  the  different  objects  on  this  earth  of  ours,  our  world 
would  appear  to  be  in  darkness,  because  we  could  not  see  the  objects 
around  us.  There  might  be  sources  of  light,  like  the  fire,  the  incan- 
descent filament,  or  the  electric  arc  which  would  be  visible  in  them- 
selves, but  the  light  that  comes  from  the  heavens  or  from  some  man- 
made  source  must  be  reflected  from  an  object  in  greater  or  less  in- 
tensity for  us  to  be  able  to  see  it.  Furthermore,  we  all  know  that 
the  effect  that  an  object  makes  on  the  retina  and  thereby  on  the 
mind  is  dependent  on  the  way  the  light  is  reflected  from  an  object, 
and  partly  therefore  on  the  way  the  light  falls  on  that  object. 

Since  it  is  this  pattern  made  by  rays  of  varying  intensity  and  of 
varying  color  on  the  retina,  calling  up  various  reminiscences  to  our 
mind,  that  enables  us  to  see — to  understand  what  lies  before  us,  it 

253 


254  ILLUMINATING   ENGINEERING   PRACTICE 

follows  that  the  type  of  lighting  that  sets  in  motion  the  most  power- 
ful train  of  associative  ideas  is  the  one  that  may  have  the  greatest 
emotional  effect;  but  the  intensity  of  the  emotional  effect  is  not 
measured  by  the  intensity  of  the  light  even  though  the  intensity  of 
that  light  may  affect  the  clearness  with  which  we  judge  of  the  phys- 
ical aspect  of  the  object  on  which  it  falls.  We  are  not  always  neces- 
sarily interested,  however,  in  the  physical  aspect — in  the  intricate 
details  of  the  object  at  which  we  are  looking.  We  are  often  more 
interested  in  the  memories  it  calls  up.  Let  me  illustrate  what  I 
mean  by  an  example. 

When  I  realized  some  weeks  ago  that  I  was  going  to  talk  to  the 
members  of  this  society  on  the  aesthetic  principles  instead  of  the 
scientific  principles  involved  in  some  of  the  every-day  problems  of 
lighting,  it  occurred  to  me  to  get  a  variety  of  opinions  on  the  mental 
reaction  produced  by  such  a  simple  source  of  light  as  one  bright 
star  in  the  midnight  sky.  So  I  asked  three  people  among  my  neigh- 
bors— one  a  distinguished  astronomer,  the  next  a  young  girl  just 
back  from  college,  and  the  third  an  immigrant  woman  whose  husband 
worked  as  gardener  on  the  place — what  their  thoughts  would  be 
were  they  to  wake  up  in  the  middle  of  a  wintry  night,  and  as  they 
came  back  to  consciousness  were  to  see  through  the  window  a  bright 
star.  The  astronomer  said  he  would  begin  to  wonder  which  star  it 
was  among  all  the  myriads  in  the  heavens;  the  young  girl  with  a 
mind  full  of  classical  poetry  said  she  would  think  of  the  mytholog- 
ical stories  connected  with  the  stars;  the  working  woman  said, 

"  Sure  if  it  was  a  single  star  on  a  wintry  night  I  would  think  of  the  Star  of 
Bethlehem." 

But  when  I  said  to  each  one  of  my  three  friends,  "  Supposing  you 
were  told  that  it  was  not  a  star  after  all  but  a  distant  electric  light, 
what  would  you  do?  "  They  all  three  made  a  similar  answer,  "  We'd 
turn  over  and  go  to  sleep." 

Now  the  interest  in  these  answers  lies  here.  No  one  of  the  three 
was  interested  in  the  one  little  bright  spot  in  the  sky  as  a  source  of 
light;  so  long  as  it  was  a  star,  it  called  up  a  whole  series  of  associative 
thoughts. 

Whether  it  calls  up  with  its  suggestion  of  infinite  distances  and 
infinite  time  a  whole  theory  of  cosmic  philosophy;  or  whether  it 
suggests  to  the  pagan  mind  the  mythological  intrigues  of  a  Jupiter, 
a  Mars  and  a  Venus;  or  whether  the  star  recalls  one  of  the  most 
touching  stories  of  our  Christian  faith — the  story  of  the  Star  of 
Bethlehem  seen  by  the  watching  shepherds  from  the  hillside  nearly 


LOWELL:  ASPECTS  OF  LIGHTING  255 

two  thousand  years  ago,  certain  it  is  that  most  of  us  when  we  see 
the  brilliant  star  set  in  its  wonderful  background  of  midnight  blue, 
project  into  our  thoughts  the  reminiscence  of  some  earlier  associated 
idea,  and  thereby  enjoy  intellectual  pleasures  that  we  could  not  get 
from  the  mere  contrast  alone  of  a  brilliant  light  against  a  dark  back- 
ground. That  one  little  spark  of  light  in  the  sky  is  able  to  suggest 
a  whole  train  of  speculative  thought,  and  serves  as  a  strong  stimulus 
to  the  imagination;  in  other  words,  fulfills  the  functions  of  a  work  of 
art,  for  in  stimulating  the  imagination  it  has  called  up  thoughts  of 
beauty. 

What  I  want  to  consider  more  particularly  to-day  is  the  artistic 
function  of  lighting  and  show  how  the  lighting  scheme  of  the  scene 
at  which  we  are  looking  may  quite  independently  of  its  efficiency, 
technical  excellence  or  physiological  advantages,  control  the  emo- 
tional reaction  which  it  produces — influences  the  aesthetic  result 
produced.  Now  instead  of  a  single  star,  the  scene  at  which  we  are 
perhaps  looking  may  be  the  harmonious  grouping  of  the  many  differ- 
ent objects  in  a  natural  landscape  or  inside  a  room,  all  reflecting 
different  kinds  of  light  in  different  ways  and  combining  to  make  up 
the  picture  that  is  conveyed  to  the  mind  by  the  eye.  I  have  already 
said  that  the  strength  of  the  intellectual  reaction  made  by  the  pic- 
ture we  see  is  in  no  sense  dependent  on  the  intensity  of  the  lighting, 
nor  necessarily  on  the  clearness  of  vision  with  which  we  see  the  ob- 
jects in  our  scene. 

Let  me  show  you  again  that  the  artistic  effect  is  quite  as  much 
due  to  the  train  of  associative  ideas  it  calls  up  as  to  the  clearness 
with  which  we  see  that  scene.  For  this  purpose  I  am  going  to  ust 
as  an  illustration  an  outdoor  scene,  since  we  can  select  some  beautiful 
view  and  nature  will  kindly  shift  the  light  for  us,  so  that  in  our  out- 
door laboratory  we  may  judge  of  the  changing  thoughts  produced 
by  the  same  or  similar  scenes  but  under  different  conditions  of  light- 
ing. In  order  to  show  the  difference  between  a  scene  clearly  defined 
because  of  its  uniform  lighting,  and  a  similar  scene  where  only  the 
important  elements  are  brought  out  by  the  artist,  I  would  ask  you 
to  compare  a  photograph  of  some  familiar  object  with  a  painting 
by  an  artist  of  that  same  object. 

Photography  is  of  use  because  it  provides  an  illustration  of  the  way 
we  really  see  things  in  that  it  gives  a  record  full  of  detail  of  what  we 
see.  The  image  permanently  produced  on  the  photographic  plates 
after  chemical  development  is  monochromatic  it  is  true  and  cannot 
by  black  and  white  present  all  the  different  colors  nor  are  the  light 


256  ILLUMINATING   ENGINEERING   PRACTICE 

values  in  the  photograph  always  relatively  right,  but  the  direct 
photograph  being  what  one  might  call  a  mechanical  record  of  the 
scene  before  us  provides  us  with  an  interesting  way  of  comparing 
actuality  with  the  way  an  artist  would  treat  a  similar  scene,  for  the 
artist  first  looks,  then  apprehends,  and  then  selects  from  all  that  he 
sees  only  that  which  he  desires  to  record. 

The  painter  with  his  easel  set  up  about  to  paint  a  landscape  or  a 
portrait  waits  till  the  lighting  on  his  subject  is  just  right,  of  the  proper 
concentration  or  diffusion,  from  the  right  direction,  of  the  right  color, 
and  is,  therefore,  dependent  on  the  vagaries  of  nature.  And  to  him 
the  proper  lighting  of  his  subject  is  of  tremendous  artistic  importance. 
Artificial  light  in  the  hands  of  the  illuminating  engineer  can  be  con- 
trolled and  arranged  as  the  artist  wishes,  and  the  architect  in  the 
planning  of  his  lighting  scheme  considers  the  same  rules  of  compo- 
sition, studies  the  same  effects  of  contrasts,  produces  by  the  position 
of  his  sources  of  light  the  same  harmonies  of  line  that  the  painter 
patiently  waits — often  day  after  day — for  nature  to  produce. 

Right  here  we  must  emphasize  once  more  the  fact  that  uniform 
visibility  and  great  distinctness  of  vision  are  not  necessarily  desir- 
able; it  is  wrong  to  assume  that  because,  for  instance,  much  time, 
thought  and  money  have  been  spent  on  some  decorative  detail,  or 
even  on  some  art  object  among  a  collector's  treasures,  that  it  must  be 
clearly  brought  out  in  the  picture  as  a  whole — that  what  is  costly 
and  of  value  should,  to  use  a  naval  expression,  have  high  visibility. 

The  artist  does  not  want  you  to  see  everything  with  equal  dis- 
tinctness. In  his  composition  as  in  a  symphonic  poem  some  of  the 
most  beautiful  passages,  though  full  of  suggestion,  are  low  in  tone, 
thus  bring  out  in  greater  contrast  the  general  theme — throwing  a 
high  light  on  some  other  beautiful  part.  The  musical  composer  only 
puts  into  his  composition  what  he  believes  to  be  of  importance  to  the 
creating  of  a  proper  impression  of  the  whole;  the  artist  or  the  worker 
in  black  or  white  leaves  out  what  he  does  not  want.  The  artist  who 
arranges  the  light  sources,  who  provides  the  illumination  of  a  building 
must  do  the  same,  and  the  elimination,  in  the  picture  that  presents 
itself  to  the  eye,  of  the  undesired  elements  by  one  method  or  another, 
should  be  an  important  part  of  his  artistic  result. 

The  illumination  of  work  shops,  clerical  offices,  manufacturing 
plants,  mercantile  buildings — as  well  as  schools  and  buildings  more 
directly  under  governmental  control  has  been  carefully  studied,  and 
we  have  been  told  at  this  convention  here  of  the  increased  efficiency, 
the  better  health  and  the  greater  freedom  from  accidents  that  have 


LOWELL:  ASPECTS  OF  LIGHTING  257 

been  brought  about  by  a  proper  and  efficient  system  of  lighting  and 
by  the  proper  treatment  of  the  wall  surfaces  and  ceilings,  so  that  they 
will  not  absorb  an  undue  amount  of  light.  That  is  a  practical 
problem  that  you  gentlemen  are  well  qualified  to  solve;  but  the 
architect  is  at  times,  when  he  is  not  building  loft  buildings,  offices, 
hospitals  or  industrial  plants,  but  is  designing  buildings  that  are  to 
serve  for  rest  and  for  recreation  rather  than  for  work  and  efficiency, 
called  upon  to  forget  cost  of  operation  and  to  neglect  efficiency  in 
order  to  produce  a  greater  emotional  effect.  I  am  making  a  plea  for 
the  architect.  You  as  engineers  have  not  done  your  complete  duty 
when  you  have  thrown  enough  light  by  some  economical  system  that 
does  not  require  the  paying  of  too  large  tribute  to  the  electric  light 
company,  to  enable  one  to  see  clearly  all  part  of  some  new  building. 

You  may  feel  that  I  am  talking  too  much  about  beauty,  and  too 
little  about  lumens  and  amperes,  but  after  all  I  am  only  telling  you 
how  the  trained  artist  with  his  surety  of  taste  resulting  from  his  long 
study  of  composition  must  always  study  to  make  lighting  right, 
aesthetically,  for  that  enables  him  to  show  the  form  and  the  color  of 
what  he  represents  in  the  most  artistically  effective  way 

That  from  an  architect's,  as  well  as  the  artist's  point  of  view  is  the 
artistic  function  of  artificial  lighting. 

The  architect,  however,  is  constantly  trying  to  apply  his  artistic 
ideals  to  the  practical  solution  of  his  problem.  He  recognizes  at 
times  that  the  utilitarian  must  prevail,  but  he  also  believes  that  there 
are  times  when  the  aesthetic  appearance  is  of  paramount  importance 
and  his  resulting  lighting  scheme  may  be  neither  economical  nor 
physiologically  correct. 

We  are  often  told  that  whatever  is  scientifically  right  must  be  good 
artistically,  and  that  whatever  in  our  universe  is  functionally  correct 
and  calculated  to  its  needs  with  nicety  is  beautiful  for  that  reason. 
I  do  not  entirely  believe  that  myself,  but  I  am  going  to  concede  to  a 
gathering  of  the  scientific-minded  like  this  that  the  scientific  solution 
is  undoubtedly  the  best  for  most  problems;  but  qualify  it  by  saying 
that  the  scientific  mind  often  finds  it  hard  to  grasp  what  the  artistic 
problem  really  is,  for  science  is  dealing  with  facts,  is  interpreting  them 
and  converting  them  to  use,  but  is  not  interested  in  the  emotional 
effect,  for  that  is  dependent  on  the  different  reaction  on  different 
individuals. 

For  the  understanding  of  many  of  these  problems  where  the  artistic 
and  the  scientific  seem  to  come  in  conflict  real  powers  of  imagination 
seem  necessary.  What  is  imagination?  Imagination  might  be 
17 


258  ILLUMINATING   ENGINEERING   PRACTICE 

defined  as  the  power  to  realize  that  there  are  variations  from  the  rule 
and  that  such  variations  require  a  special  treatment.  If  you  agree 
with  that  definition  of  imagination  consider  the  artistic  treatment  of 
the  lighting  problem  as  a  possible  variation  from  the  rule  and  allow 
your  imagination  full  play. 

So  the  lighting  scheme  laid  out  by  an  architect  in  connection  with 
a  building  may  be  for  one  or  two  purposes: 

(a)  Primarily  for  use  and  not  for  decoration. 

(b)  To  produce  a  decorative  effect  without  special  care  being  taken 
to  have  it  economical  and  efficient  from  the  engineering  standpoint. 

In  a  practical  system  of  architectural  lighting  the  usual  object  is 
to  reproduce  in  so  far  as  economical  utilitarian  consideration  will 
allow  a  properly  diffused  light  resembling  daylight  if  possible,  and 
in  sufficient  quantity  to  enable  one  to  do  one's  work  or  see  about  the 
lighted  rooms  or  spaces  with  absolute  ease.  The  best  way  to  ob- 
tain such  scientifically  worked  out  lighting  so  that  it  shall  be  efficient 
and  economical  is  really  a  practical  question  and  not  an  aesthetic  one. 

In  what  I  consider  an  artistic  scheme  the  sources  of  light 'screened 
or  unscreened  are  grouped  in  such  a  way  as  to  produce  not  diffusion 
but  contrasts.  The  spots  of  strong  reflected  light  and  the  spots  of 
deep  shadow  are  composed  much  as  artists  would  compose  light  and 
dark  spots  in  a  drawing  or  painting.  I  have  an  admirable  illustra- 
tion in  mind  of  two  art  museums  with  these  two  absolutely  different 
types  of  artificial  lighting.  They  are  the  Art  Museum  in  Boston 
and  the  private  collection  of  Mrs.  Gardner  near  it.  At  the  Museum 
we  tried  to  arrange  the  light  so  that  it  will  as  nearly  as  possible 
reproduce  in  direction  and  color  the  daylight — those  are  the  condi- 
tions that  exist  during  the  greater  part  of  the  time  that  the  Museum 
is  open — and  there  every  object  can  be  clearly  seen  and  studied. 
Mrs.  Gardner  lights  her  rooms  with  a  few  candles  placed  around  so 
that  some  one  particularly  interesting  object  can  be  seen,  standing 
out  as  it  were  from  the  surrounding  shadow.  No  indirect  lighting 
system  in  her  house  could  begin  to  have  the  same  charm. 

Even  in  an  art  museum  the  chosen  method  of  lighting  might 
depend  on  whether  it  is  for  use  or  for  artistic  effect. 

In  my  garage,  in  my  kitchen,  and  in  my  work  room,  I  try  to 
diffuse  the  electric  light  as  much  as  possible  by  reflecting  surfaces. 
In  my  own  dining  room  and  parlor,  however,  though  I  have  electric 
light  brackets  on  the  walls,  they  are  never  turned  on,  and  the  room 
is  either  lit  entirely  by  candles,  or  by  candles  and  portable  lamps. 

Let  me  illustrate  these  two  different  ways  of  looking  at  the  same 


LOWELL:  ASPECTS  OF  LIGHTING  259 

thing — that  is,  the  objective  and  the  subjective.  Consider,  first  of 
all,  a  photograph  of  a  bridge;  every  detail  is  clearly  brought  out  in 
the  picture,  the  arches  of  the  cement  bridge,  the  trolley  poles,  the 
roadway,  the  ugly  buildings,  all  jumbled  together.  No  artist  com- 
posed the  picture — it  is  just  a  record  of  homely  facts.  Now  let  us 
see  a  bridge  through  the  eye  of  an  artist.  Perhaps  it  is  a  little  un- 
fair to  contrast  with  the  photograph  of  a  modern  cement  bridge, 
say,  one  of  Whistler's  lithographs,  but  this  shows  in  a  simple  drawing 
the  beauty  the  artist  saw  in  what  is  really  a  very  ugly  bridge.  He 
tried  to  express  only  as  much  of  the  bridge  as  seemed  to  him  in  his 
mood  at  the  time  as  necessary  to  call  up  a  certain  impression.  In 
other  words  he  threw  the  light  on  only  the  essentials  and  left  the 
unessentials  undefined.  To  some  this  drawing  calls  up  a  long  train 
of  associative  ideas,  to  others  it  represents  little  more  than  a  beauti- 
ful pattern  in  black  and  white.  I  would  have  you  consider  the 
Presentation  in  the  Temple,  by  Rembrandt.  Here  we  have  the 
strong  lighting  of  the  important  figures,  the  background  subdued, 
and  only  half  felt  to  be  there,  like  the  subdued  accompaniment  to 
the  principal  melody  in  music.  We  might  almost  think  that 
Rembrandt  had  invented  the  modern  theatrical  spot  light  in  his 
desire  to  accent  strongly  the  personages  in  his  picture,  and  this 
characteristic  of  strongly  marked  high  light  is  produced  in  all  his 
paintings,  because  he  knew  that  skilfully  disposed  lights,  despite 
the  strong  contrasts,  produce  an  agreeable  pattern  of  lights  and 
shadows.  There  is  a  simple  way  to  study  composition,  by  taking  the 
paintings  of  the  acknowledged  masters,  and  when  we  are  sure  that 
we  like  a  certain  work  try  to  analyze  its  composition,  judge  the  com- 
posing of  light  and  try  to  express  in  ideas,  in  words,  wherein  its  ex- 
cellence lies. 

Nature,  too,  has  a  lot  to  teach  us.  We  suddenly  come  on  an 
opening  in  the  woods,  and  the  scene  before  us  seems  to  make  a 
satisfying  and  inspiring  picture.  To  what  is  the  charm  due?  Is 
it  the  color  of  the  young  green  leaves  with  the  sun  shining  through 
them;  is  it  the  sweep  of  the  tree  trunks  and  the  branches  into  a 
smooth  and  flowing  pattern;  is  it  the  distant  vista  of  lake  or  moun- 
tain? It  may  be  one  or  all  of  these,  but  there  is  always  the  light 
which  above  all  is  just  right.  Were  it  to  come  from  a  different 
direction,  the  leaves  would  be  in  shadow,  the  dark  lines  of  the 
branches  would  make  a  different  pattern,  the  high  lights  would  be 
differently  placed.  But  to  have  just  the  right  picture  you  must  see 
your  scene  as  the  artist  would  with  its  chosen  lighting. 


260  ILLUMINATING   ENGINEERING   PRACTICE 

Now  let  us  consider  scenes  by  other  painters.  Gainsborough, 
full  of  vigor  with  strongly  marked  lines  in  the  composition;  Turner 
with  a  satisfying  and  harmonious  sweep  of  line  from  one  side  of  the 
canvas  to  the  other;  Rembrandt  with  his  high  lights  like  the  strong 
blare  of  the  trumpet  in  an  orchestral  piece. 

You  see  efficiency  and  intensity  of  lighting  are  left  far  behind  in 
our  minds  when  it  comes  to  tracing  harmonious  patterns  and  pro- 
ducing wonderful  blendings  of  color  and  light  and  shade. 

Think  of  the  advantage  you  have  as  artists  if  you  will  so  consider 
yourselves.  You  hold  in  your  hand  brushes  dipped  in  light;  you 
have  a  pallette  set  with  all  the  colors  that  we  find  in  nature.  You 
can  make  your  high  lights  shimmer  at  will.  You  can  throw  the 
confused  detail  into  the  mysterious  and  shadowy  background. 
Equipped  with  a  sound  technical  knowledge,  the  effects  you  can 
produce  are  only  limited  by  your  artistic  training.  But  the  road  to 
art  is  long.  A  short  lecture  like  this  can  only  show  that  there  is 
such  a  road;  it  cannot  for  a  moment  do  more  than  that.  For  the 
power  to  understand  the  artistic  impulse,  the  power  to  create  what 
is  artistically  good,  must  come  as  the  result  of  years  of  thought  and 
study. 

We  have  given  a  hasty  glance  without  attempting  to  classify 
them  at  the  lighting  schemes  of  nature  in  outdoor  landscapes.  The 
most  direct  copy  of  those  effects  of  lighting  we  find  on  the  stage  of 
the  modern  theatre. 

There  the  aim  is  to  produce  illusions,  to  produce  the  illusion  of 
reality,  not  necessarily  as  we  have  ourselves  experienced  it,  but  as 
we  can  conceive  that  it  might  exist,  and  there  are  no  limits  to-day 
to  what  one  can  do.  For  that  reason  the  conventions  for  lighting 
of  the  stage  of  the  last  generation  are  disappearing.  The  strange 
effect  produced  by  footlighting,  with  the  resulting  prominent  chins 
and  receding  foreheads,  is  giving  way  to  a  flood  of  colored  light  pro- 
ducing the  effect  of  a  shadowless  stage  where  the  whole  company 
is  suffused  in  light.  We  are  now  looking  at  a  pattern  of  color  and 
even  the  advent  of  solid  properties  on  the  stage  has  failed  to  give 
us  quite  the  real  sense  of  solidity  that  comes  with  the  feeling  of  mod- 
elling one  gets  from  natural  unilateral  lighting.  The  thoughtful 
stage  manager  tells  us  that  we  have  not  solved  his  problem  because 
he  has  to  work  with  color  and  has  to  give  up  attempts  at  modelling. 
But  since  he  states  that  the  problem  needs  solving,  you  gentlemen 
must  help  him. 

I  have  purposely  made  this  comparison  of  unilateral  lighting  out- 


LOWELL:  ASPECTS  OF  LIGHTING  261 

doors,  and  diffused  lighting  on  the  stage  because  the  stage  manager 
with  all  possible  kinds  of  lighting  at  hand  has  to  resort  to  the  subter- 
fuge of  painted  shadows  in  order  to  make  objects  on  his  stage  appear 
real,  to  appear  solid.  He  paints  a  shadow  under  the  cornice  of  a 
building,  he  shades  one  side  of  a  real  round  column  with  paint,  he 
even  darkens  the  eye  sockets  and  wrinkles  of  the  actors.  Though 
he  is  working  in  a  space  suffused  with  light  he  must  paint  in  shadows 
to  make  his  picture  real  and  solid.  And  painting  will  not  take  the 
place  of  real  high  lights  and  real  shadows  as  nature  produces  them. 

It  is  exactly  the  same  thing  in  lighting  our  buildings,  we  must  make 
them  seem  real,  and  what  is  more,  make  them  seem  solid. 

We  thus  see  in  our  examples  chosen  from  natural  landscapes  and 
from  the  work  of  the  landscape  painters  that  the  emotional  reaction 
of  the  scene  before  us  when  we  are  in  the  open  looking  at  a  natural 
landscape — that  the  mood  produced  in  us  by  the  artist's  painting  is 
largely  affected  by  the  way  the  lighting  is  done.  We  also  see  how 
the  theatrical  manager,  with  the  wonderful  power  of  creating  im- 
pressions, of  inducing  sensations,  which  is  given  him,  is  again  abso- 
lutely dependent  on  the  varying  effects  of  lighting  for  painting  his 
stage  lights  and  his  shadows.  But  the  final  effect  produced  on  the 
individual  is  dependent  on  the  taste  of  the  individual.  And  the 
individual's  taste  is  the  result  of  experience,  of  education,  of  varying 
reminiscences,  and  it  therefore  is  impossible  to  dogmatize  and  say 
that  such  and  such  a  way  is  the  best  way  to  light  a  given  subject. 

Some  one  asked  me  which  I  preferred  for  the  exterior  of  a  monu- 
mental building — a  row  of  incandescent  lamps  or  flood  lighting. 
How  can  I  say?  I  do  know  that  the  answer  depends  on  what  the 
architect  is  trying  to  emphasize  and  on  what  he  is  trying  to  hide. 

Consider  flood  lighting  for  a  moment.  It  is  rarely  so  done  as  to 
do  justice  to  the  architecture.  It  was  all  right  at  the  San  Francisco 
fair.  It  was  wonderful  in  fact  because  like  on  the  stage  spoken  of 
a  few  minutes  ago  it  served  to  bring  out  color  and  not  form.  But  as 
used  on  the  occasional  building  it  is  hardly  so  successful,  for  instance, 
the  Boston  State  House  last  fall,  or  the  new  Technology  buildings 
last  spring.  In  the  case  of  the  last  two  buildings  the  architect  had 
worked  out  all  his  contrasts  of  opening  with  wall  space,  all  his  con- 
trasts of  supporting  column  with  its  lintel,  to  be  seen  by  daylight, 
and  by  daylight  the  light  comes  from  above.  How  can  one  expect 
flood  lighting  to  do  anything  bat  invert  the  architectural  effect  if  it 
is  thrown  up  from  a  lower  building  onto  a  higher  one?  And  how  can 
you  expect  the  interior  of  a  building  to  be  artistically  satisfactory 


262  ILLUMINATING    ENGINEERING    PRACTICE 

if  the  light  that  comes  from  the  windows  by  <day  produces  a  composi- 
tion that  is  entirely  inverted  by  the  change  in  direction  of  the  arti- 
ficial lighting  at  night. 

How  do  these  theories  work  out  in  our  homes?  There  can  be  no 
one  accepted  set  of  rules  for  the  guidance  of  the  man  who  lays  out 
the  electric  lighting  in  a  private  house.  I  personally  like  as  few 
light  sources  as  possible,  and  I  certainly  only  put  in  wall  brackets, 
because  handsome,  skilfully  designed  electric  brackets  are  of  value 
decoratively  on  the  walls,  but  as  I  have  already  said  I  never  light 
these  in  the  living  rooms  in  my  own  house. 

I  remember  that  when  I  first  built  a  big  room  for  the  furniture  which 
I  had  collected  while  living  abroad,  we  all  gathered  one  evening 
to  discuss  the  position  of  the  various  pieces  and  to  move  them  around 
till  we  thought  all  were  perfectly  placed.  The  room  was  lighted  by 
electric  wall  brackets,  some  old  French  ones  I  had  had  wired,  and 
by  lamps  on  the  tables — they  were  connected  to  floor  receptacles. 
Nothing  around  looked  right!  We  almost  broke  our  backs  for  two 
hours,  moving  the  furniture,  but  each  change  seemed  only  to  make 
the  composition  worse.  I  turned  the  switch  which  extinguished  all 
the  wall  brackets.  The  room  suddenly  felt  " right."  What  I  found 
was  successful  in  my  own  case  is  what  I  now  try  to  apply  for.  my 
clients.  I  get  along  without  light  from  the  wall  brackets. 

So  when  it  comes  to  the  varying  treatment  of  the  different  rooms 
of  the  house  I  don't  for  a  minute  concede  that  efficiency  alone 
should  be  the  keynote;  what  we  should  have  above  all  is  a  beautiful 
effect.  Yet,  we  should  have  enough  light  for  the  average  person. 
One's  old  Jacobean  oak  staircase  should  be  light  enough  so  that  a 
short-sighted  person  will  not  fall  down  and  break  his  neck,  but  there 
might  be  some  of  the  dim  mysteriousness  of  the  ancient  staircase 
arranged  for  in  its  lighting. 

I  want  to  lay  great  emphasis  here  on  the  difference  there  is  between 
rooms  in  public  buildings  and  similar  rooms  in  private  buildings. 
Take  for  instance  the  reading  rooms  in  a  public  library  and  the  li- 
brary (reading  room)  in  a  private  house.  It  is  obvious  that  the 
public  reading  rooms  should  be  so  lighted  that  a  maximum  number  of 
workers  should  be  provided  for,  every  reader  in  fact  should  have  the 
light  fall  with  the  correct  intensity  and  from  the  correct  angle.  But 
the  private  library  is  quite  different.  Half  the  time  the  people  sitting 
in  it  may  not  be  reading;  at  any  rate  it  surely  needs  to  accommo- 
date not  more  than  one  or  two  actual  readers  at  a  time.  There 
hould  be  light  enough  to  enable  one  to  see  the  people  around  the  room 


LOWELL:  ASPECTS  OF  LIGHTING  263 

but  in  my  opinion  the  rest  of  the  room  should  be  only  so  lit  as  to  make 
the  most  artistic  background.  In  one  corner  you  may  see  a  lamp  on 
a  table  throwing  its  light  onto  the  richly  bound  books  in  the  bookcase 
— and  there  is  nothing  more  decorative  for  a  background  than  hand- 
some bindings;  in  another  you  may  see  a  well  placed  picture  lamp  or 
the  worker's  desk  lamp.  But  there  should  be  no  attempt  to  make 
the  room  look  like  a  picture  gallery.  The  use  of  picture  lighting  in 
a  private  home  is  a  matter  of  personal  taste.  So  many  of  the  large 
homes  to-day  make  claims  to  have  valuable  collections  of  paintings 
stored  in  them  that  the  owners  feel  that  they  should  be  shown  off 
to  the  greatest  advantage  at  all  times;  hence,  that  abomination  of  the 
architect  the  modern  picture  lamps,  with  their  reflectors  bracketed 
out  from  the  wall  or  ceiling  about  the  picture. 

Again  let  me  say  that  there  must  be  a  difference  between  the 
lighting  of  the  room  of  a  public  building  and  a  private  one.  Put 
your  fine  pictures,  if  you  must  show  them  off  and  not  enjoy  them 
quietly  and  separately,  in  a  gallery  with  all  the  advantages  of  a  care- 
fully, skilfully  worked  out  lighting  scheme,  shown  where  they  can  be 
seen  to  the  best  advantage,  but  don't  force  your  guests  to  eat  in  a- 
picture  gallery  because  the  dining  room  has  some  of  the  owner's 
choicest  paintings  in  it,  brilliantly  lighted  by  reflectors.  I  have  in 
mind  one  beautiful  walnut  panelled  room  where  each  picture  on  the 
wall  at  night  is  so  brilliantly  lighted  that  every  picture  with  its  frame, 
seems  like  a  window  cut  in  the  panelling  looking  into  the  landscape 
beyond,  and  there  is  no  sense  of  support  in  the  wall  surface. 

Let  us  make  another  comparison — the  dining  room  in  a  public 
restaurant  and  the  private  dining  room.  When  I  go  to  a  public 
restaurant  I  find  in  America  that  the  desire  is*  to  have  a  brilliantly 
lighted  room,  profusely  lighted  by  different  sources.  My  own  dining 
room  however  is  a  Jacobean  oak  room  taken  from  an  old  house  in 
England.  Near  the  pantry  door  there  is  a  hidden  receptacle  which  is 
used  with  a  lamp  and  a  reflector  when  the  table  is  being  set.  When 
all  is  ready  for  the  guests  the  lamp  is  removed  and  the  room  is  lighted 
only  by  candles.  No  system  of  lighting  that  I  know  makes  a  more 
becoming  light  for  the  guests,  or  sets  off  the  linen,  the  china,  and  the 
silver  to  better  advantage.  Each  source  of  light  too  is  so  low  in 
intensity  that  its  contrast  with  the  dark  background  is  not  un- 
pleasant. I  like  to  see  a  similar  method  of  lighting  applied  to  the 
restaurant  dining  room  because  I  personally,  though  I  go  to  a  public 
dining  room,  like  to  confine  my  attention  to  the  people  at  the  same 
table  with  me.  I  like  the  sense  of  privacy  which  the  French  like 


264  ILLUMINATING   ENGINEERING    PRACTICE 

even  in  a  public  restaurant.  Sherry's  in  New  York  is  arranged  that 
way.  There  are,  however,  many  people  who  go  to  a  restaurant  to  see 
or  to  be  seen  throughout  the  room ;  for  them  some  system  of  indirect 
lighting  may  seem  more  practical. 

The  private  house,  should  as  you  pass  from  one  room  to  another, 
provide  a  series  of  pictures,  where  the  furniture,  the  light  sources,  the 
people  are  all  so  combined  as  to  make  an  artistic  and  picturesque 
composition.  Like  the  landscape  painting  we  already  considered 
it  will  have  individuality  in  which  the  lighting  will  be  the  controlling 
factor.  You  can  perhaps  now  understand  my  personal  objection 
to  indirect  lighting.  It  makes  it  so  hard  to  compose  the  picture. 

You  must  not  assume,  however,  that  indirect  lighting  according  to 
my  opinion  has  no  place  in  a  well  arranged  home.  For  there  are 
many  cases  where  it  is  advantageous  to  obtain  our  light  from  a  large 
surface  with  a  low  intensity  rather  than  single  sources  of  concen- 
trated light.  I  have  the  evil  habit  for  instance  of  reading  in  bed. 
My  bedroom  is  sanitarily  painted  in  white.  I  throw  the  light  with 
two  strong  reflectors  onto  the  wall  behind  me  and  obtain  a  wonderful 
'light  on  my  book  or  over  the  large  pages  of  a  newspaper.  Scientif- 
ically correct  but  artistically  vile.  But  then  I  don't  receive  visitors 
in  my  bedroom. 

The  transition  from  the  private  dwelling  to  the  art  museum  is 
direct. 

It  has  always  been  my  contention  that  the  ideal  public  art  museum 
should  have  many  of  the  characteristics  of  a  private  house.  Most  of 
the  objects  in  it  were  originally  made  for  everyday  use.  Pictures 
were  to  be  hung  on  the  walls  of  the  living  rooms.  China,  silver,  fur- 
niture, fabrics,  carvings  were  all  used  in  rooms  that  were  human  in 
scale.  There  are  a  few  colossal  paintings,  a  few  heroic  statues  to  be 
provided  for,  but  surely  the  keenest  artistic  pleasures  come  from 
seeing  works  of  art  in  the  surroundings  for  which  they  were  intended. 
Modern  tendencies  in  museums  have  been  in  the  other  direction, 
however.  The  tendency  was  to  group  all  the  paintings  together,  to 
put  all  the  furniture  in  another  group,  and  the  objects  d'art  off  by 
themselves.  This  has  been,  I  think,  largely  due  to  the  fact  that  the 
administrative  staff  of  many  museums  have  thought  too  much  of  the 
executive  part  of  their  work  and  too  little  of  the  artistic  part. 

The  art  museum  is  for  the  student,  but  it  is  quite  as  much  for  the 
occasional  visitor  as  for  those  who  frequent  the  galleries  day  after 
day.  Now  the  constant  passing  through  the  galleries,  the  constant 
work  in  the  exhibition  rooms  of  the  museum  staff  is  apt  to  make  them 


LOWELL:  ASPECTS  OF  LIGHTING  265 

consider  the  visual  comfort  of  the  constant  worker  even  more  than 
the  aesthetic  satisfaction  to  the  occasional  visitor  who  comes  to  get 
the  artistic  stimulus  of  seing  beautiful  things  skilfully  shown  in  pleas- 
ant groupings. 

This  all  creates  a  two-fold  problem,  for  the  objects  must  be  clearly 
lighted  with  a  considerable  intensity  of  light  and  yet  it  is  of  para- 
mount importance  that  the  ultimate  effect  of  the  galleries  as  a 
whole  be  harmonious.  It  is  at  once  a  practical  and  an  artistic  prob- 
lem and  the  method  adopted  in  arranging  for  the  lighting  of  such  a 
building  offers  a  good  example  of  the  way  the  artist  must  always 
attempt  to  solve  the  problem  of  lighting  both  by  daylight  and  by 
artificial  light. 


COLOR  IN  LIGHTING 

BY   M.    LUCKIESH 
INTRODUCTION 

It  appears  desirable  from  an  analytical  viewpoint  to  divide  the 
problem  of  lighting  into  two  parts,  namely,  that  which  involves  light 
and  shade  or  brightness  distribution,  and  that  which  involves  color. 
In  dealing  with  the  first  part,  the  lighting  expert  is  concerned  with 
the  distribution  of  light  and  with  the  second  part,  with  the  quality 
or  spectral  character  of  th§  illuminant.  Sometimes  these  two  prob- 
lems are  intricately  interwoven  but  there  is  much  advantage  in  gen- 
eral in  considering  the  problems  separately  especially  if  it  be  granted 
that  lighting  should  be  considered  from  the  standpoint  of  the  appear- 
ances of  objects  either  singly  or  as  a  group. 

The  subject  of  color  in  lighting  is  complicated  by  the  fact  that 
the  eye  is  not  analytic  but  synthetic  in  its  operation.  For  instance, 
a  color  sensation  in  general  is  the  result  of  the  integral  effects  of 
radiant  energy  of  many  wave-lengths  or  frequencies.  Often  it  is 
necessary  to  know  the  luminous  or  energy  intensities  of  these  various 
components  yet  sometimes  merely  the  subjective  or  resultant  color 
is  of  interest.  Spectrum  analysis  yields  the  desired  data  in  the 
former  cases  while  such  instruments  as  the  monochromatic  color- 
imeter furnish  satisfactory  data  in  the  latter  cases  depending  upon 
the  problem  at  hand.  One  of  these  two  viewpoints  must  be  chosen 
by  the  lighting  expert  for  a  given  problem. 

It  is  not  the  intention  to  ask  the  lighting  expert  to  become  a  color 
specialist  and  it  is  impossible  to  present  a  complete  treatment  of 
the  subject  of  color  in  lighting  in  a  single  lecture.  However,  it  will 
be  the  aim  to  present  a  sufficient  amount  of  the  science  of  color  to 
enable  the  lighting  expert  to  diagnose  his  problems  and  a  sufficiently 
varied  number  of  applications  of  color  in  lighting  to  show  the  trend 
of  progress  and  to  impress  him  with  the  extent  of  the  field  if  neces- 
sary. Notwithstanding  the  brevity  with  which  the  theory  of  color 
will  be  presented  it  may  appear  to  some  that  it  is  unnecessary  to  be 
acquainted  with  some  of  the  aspects  discussed.  However,  it  cannot 

267 


268  ILLUMINATING   ENGINEERING   PRACTICE 

be  too  strongly  emphasized  that  an  art  is  an  applied  science;  that 
is,  science  is  the  tool. 

In  the  practice  of  color  science  it  is  desirable  that  definite  termi- 
nology be  used  consistently.  The  measurement  of  color  is  necessary 
for  specifying  installations  and  for  recording  results.  Many  appli- 
cations of  color  in  lighting  are  directly  dependent,  for  successful 
results,  upon  a  knowledge  of  the  principles  of  color  mixture.  Obvi- 
ously color  and  vision  are  closely  related  in  the  applications  of  color 
in  lighting  but  unfortunately  many  of  these  relations  cannot  be 
discussed  here.  The  psychology  of  color  is  of  extreme  importance 
but  many  of  the  questions  raised  by  the  lighting  expert  are  at  pres- 
ent unanswered.  The  color  of  surroundings  is  of  much  greater  im- 
portance than  has  been  generally  recognized  in  practice.  The  sur- 
roundings influence  the  visual  impression  and  also  the  color  of  the 
useful  light.  It  should  be  recognized  that  the  color  exhibited  by 
the  lighting  unit  very  often  plays  a  dominating  part  in  the  impression 
of  a  lighting  condition  even  in  those  cases  when  the  color  of  the  useful 
light  is  far  different  from  the  color  of  the  visible  portion  of  the  unit. 
Wrong  conclusions  have  been  arrived  at  by  failing  to  recognize  such 
facts  as  the  foregoing  or,  in  other  words,  by  not  applying  a  searching 
analysis  to  the  conditions.  Artificial  daylight  has  been  demanded 
for  many  places  and  it  is  now  practicable  owing  to  the  relative 
high  efficiencies  of  modern  illuminants.  Many  arts  have  been 
standardized  in  daylight  and  the  eye  has  been  evolved  under  day- 
light conditions.  For  these  reasons,  and  others,  artificial  daylight 
finds  many  fields  of  application.  Artificial  daylight  units  have  been 
available  for  some  time  and  it  is  now  possible  to  record  some  experi- 
ences gained  from  a  great  many  installations.  On  the  other  hand, 
aesthetic  taste  sometimes  demands  that  the  early  illuminants  be  simu- 
lated in  color.  This  provides  an  interesting  aspect  although  the 
problems  are  not  difficult  because  only  the  subjective  color  is  usually 
of  interest.  The  means  for  obtaining  various  color  effects  are  gradu- 
ally being  developed  although  the  lighting  expert  must  yet  provide 
colored  media  for  many  special  applications.  It  has  been  thought 
desirable  to  conclude  this  lecture  with  brief  descriptions  of  a  number 
of  applications  of  the  science  of  color  in  lighting  and  the  appended 
bibliography  will  be  depended  upon  to  cover  much  that  cannot  be 
incorporated  in  a  single  lecture.  It  appears  best  not  to  attempt  to 
incorporate  much  numerical  data  obtained  from  experiments  in  the 
various  fields  treated  in  this  lecture  because  these  data  are  available 
elsewhere  (see  Bibliography).  The  method  of  treatment,  therefore, 


LUCKIESH:  COLOR  IN  LIGHTING  269 

will  be  general,  the  more  important  viewpoints  will  be  discussed  and 
an  attempt  will  be  made  to  present  a  broad  discussion  of  an  extensive 
subject. 

COLOR    TERMINOLOGY 

There  is  a  great  need  for  the  standardization  of  color  terminology 
and  for  the  development  of  a  practicable  system  of  color  notation. 
Many  terms  are  in  use  for  describing  a  few  color  qualities  and  there 
is  great  confusion  owing  to  the  fact  that  the  same  term  is  used  by 
different  persons  to  describe  different  factors  or  various  terms  are 
applied  to  the  same  factor.  It  appears  unnecessary  to  recount  this 
confused  state  but  it  is  advisable  to  propose  terminology  to  be  stand- 
ardized by  this  society.  The  proposals  which  follow  appear  to  the 
author  to  be  the  most  satisfactory  and  the  most  consistently  used. 

Color  can  be  considered  from  two  broad  standpoints.  Spectrum 
analysis  provides  data  which  are  the  most  generally  useful  in  the 
science  of  color.  On  the  other  hand,  interest  in  color  is  often  merely 
in  regard  to  its  appearance.  The  two  viewpoints  are,  therefore, 
objective  and  subjective  and,  while  both  must  necessarily  be  inter- 
woven into  a  complete  system  of  terminology,  the  latter  dominates 
in  the  present  consideration.  Nevertheless  it  must  be  understood 
that  analytical  data,  which  will  be  discussed  later,  supply  the  solid 
foundation  of  color  science  and  art. 

Hue. — This  is  the  visual  quality  of  a  color  which  is  correlated 
on  the  physical  side  with  the  length  or  frequency  of  the  predominat- 
ing lightwaves  with  the  exception  of  a  large  class  of  colors,  called 
purple,  which  includes  also  such  colors  as  pink  and  rose.  The 
purples  are  mixtures  of  red  with  blue  or  violet  and  have  no  spectral 
match  in  hue.  It  is  customary  to  designate  the  spectral  hue  of  the 
complementary  to  the  purple.  In  many  cases  the  hue  is  directly 
apparent  in  the  name  of  a  color;  however,  there  are  a  great  many 
color  names  in  daily  use  which  are  burdensome  owing  to  the  lack 
of  any  suggestion  of  the  hue.  The  hue  of  a  color  is  determined  by 
comparing  the  color  directly  with  spectral  colors.  If  a  match  in 
hue  be  made  between  a  given  color  and  a  spectral  hue  at  equal 
brightnesses,  in  general  it  will  be  found  that  the  two  colors  do  not 
yet  appear  alike.  The  difference  is  accounted  for  by  the  next 
quality  to  be  considered. 

Saturation. — In  general,  in  order  to  make  the  foregoing  match 
perfect,  it  will  be  necessary  to  add  a  certain  amount  of  white  light 


270  ILLUMINATING   ENGINEERING    PRACTICE 

to  the  comparison  spectral  hue.  The  percentage  of  light  of  spectral 
hue  in  the  total  mixture  of  white  and  spectral  hue  is  a  measure  of 
the  saturation  or  purity.  The  term  "purity"  is  misleading  to  many. 
For  example,  if  a  perfect  black  be  added  to  a  given  pigment,  the 
saturation  or  purity  is  unaltered;  however,  black  naturally  suggests 
impurity  to  many.  A  spectral  hue  represents  complete  saturation 
(zero  "per  cent,  white")  or  a  color  of  highest  purity  as  considered 
from  the  physical  side.  The  usual  procedure  of  using  the  term, 
saturation  or  purity,  in  discussions  and  in  measurements  of  determin- 
ing the  "per  cent,  white"  is  confusing  to  many  persons.  It  is  sug- 
gested that  the  term,  saturation  or  purity,  be  always  used  instead 
of  "per  cent,  white"  and  denoted  by  100  per  cent,  minus  the  per 
cent,  white.  Spectral  colors  would  then  be  represented  physically 
by  unity  or  100  per  cent.  A  color,  which  is  matched  by  a  mixture 
of  two  luminous  intensities  corresponding  to  8  parts  of  white  light 
and  2  parts  of  a  certain  spectral  hue,  would  then  have  a  saturation 
of  0.2  or  20  per  cent. 

In  making  determinations  of  saturation  two  chief  difficulties  are 
encountered,  namely,  a  standard  white  light  and  a  standard  method 
of  color  photometry.  Average  daylight,  which  is  considered  by  some 
to  be  clear  noon  sunlight,  corresponding  in  spectral  distribution  of 
energy  to  that  of  a  black  body  at  a  temperature  of  5ooo°C.,  can  be 
accepted  as  a  standard  white.  This  can  be  accurately  matched  by 
means  of  an  artificial  light-source  equipped  with  a  proper  colored 
screen.  However,  a  true  physiological  white  is  considered  by  some 
to  meet  certain  requirements  which  are  not  necessarily  met  by  clear 
noon  sunlight.  The  flicker  photometer  provides  a  method  of  color 
photometry  which  at  least  gives  consistent  results  although  the 
method  has  yet  to  receive  the  approval  of  standardizing  laboratories 
for  the  photometry  of  extreme  color  differences. 

Brightness. — The  third  quality  of  a  color  is  defined  by  the  term 
brightness.  The  measurement  of  this  requires  a  standard  method 
of  color  photometry  as  discussed  in  the  preceding  paragraph.  This 
quality  of  a  color  is  of  interest  both  as  relative  and  absolute  bright- 
ness. In  general  the  relative  brightness  of  a  color,  that  is,  its  re- 
flection or  transmission  factor,  is  of  chief  interest.  However,  it 
must  be  noted  that  the  reflection  or  transmission  factors  of  colors 
are  not  constant  as  in  the  case  of  neutral  colors  but  depend  upon 
the  spectral  character  of  the  illuminant. 

From  the  standpoint  of  describing,  and  recording  appearances  of 
colors  the  three  factors  hue,  saturation  and  brightness  are  sufficient; 


LUCKIESH:  COLOR  IN  LIGHTING  271 

however,  the  two  following  terms  are  quite  useful  and  complete  a 
terminology  of  large  descriptive  power. 

Tint. — If  the  hue  and  brightness  of  a  color  be  maintained  constant 
and  the  saturation  be  changed  throughout  a  complete  range  from 
zero  to  unity,  a  series  of  tints  of  constant  hue  and  brightness  will 
result.  If  to  a  given  color  of  high  saturation,  white  light  be  added  in 
gradually  increasing  amounts  a  series  of  tints  of  constant  hue  and 
increasing  brightness  will  result.  Tints,  then,  are  colors  of  partial 
saturation. 

Shade. — If  the  hue  and  saturation  of  a  color  be  maintained  con- 
stant and  the  brightness  be  varied  by  varying  the  intensity  of  illumi- 
nation, a  series  of  shades  will  result.  In  the  case  of  pigments  the 
addition  of  various  quantities  of  perfect  black  results  in  the  pro- 
duction of  different  shades  of  a  color  of  constant  hue  and  saturation. 

A  system  of  notation  is  one  of  the  great  needs  in  the  art  and  science 
of  color  but  the  problem  is  too  complicated  to  discuss  extensively 
here,  especially  inasmuch  as  there  are  more  vital  problems  to  deal 
with.  It  is  unlikely  that  a  single  system  of  color  notation  will  be 
developed  to  satisfy  all  the  requirements  of  the  applications  of  color 
but  the  most  promising  system  appears  to  be  one  which  includes  an 
accurate  description  of  the  three  qualities,  hue,  saturation,  and 
brightness.  The  scientist  can  supply  himself  with  the  more  analyt- 
ical data  necessary  for  his  purposes. 

COLOR  MEASUREMENTS 

It  is  quite  outside  the  scope  of  this  lecture  to  enter  deeply  into  a 
discussion  of  color  measurements,  for  such  information  can  be  found 
elsewhere;  however,  it  appears  advisable  to  describe  the  various 
methods  briefly  and  to  point  out  a  few  applications  and  limitations 
of  the  results. 

Photometry. — Only  two  methods  are  of  sufficient  importance  here  to 
be  treated.  These  are  the  direct  comparison  and  flicker  methods  of 
photometry.  For  color  photometry,  the  latter  method  appears  to 
be  the  more  desirable  owing  to  the  greater  consistency  of  the  results. 
It  has  not  yet  been  proved  that  this  method  provides  a  true  measure 
of  brightness  in  the  case  of  great  differences  in  color,  nevertheless  it 
measures,  with  a  high  degree  of  consistency,  a  factor  which  very 
likely  is  the  brightness  quality  of  color.  It  does  not  eliminate 
differences  due  to  normal  variations  in  color  vision  among  various 
persons. 


272  ILLUMINATING   ENGINEERING   PRACTICE 

Spectro photometry. — The  spectrophotometer,  by  means  of  which 
are  obtained  the  relative  luminous  or  energy  intensities  throughout 
the  spectrum  of  an  illuminant  or  of  the  light  reflected  or  transmitted 
by  a  colored  medium,  furnishes  the  most  analytical  data.  The  ap- 
plications of  color  in  lighting  are  often  dependent  for  success  upon  the 
spectral  character  of  the  illuminants  and  of  the  colored  media  used. 
Unfortunately  such  data  cannot  be  expressed  simply  and  cannot  be 
readily  interpreted  without  considerable  experience,  nevertheless, 
the  lighting  expert  is  working  blindly  in  many  cases  without  the 
aid  of  such  data.  The  eye  is  incapable  of  determining  the  spectral 
character  of  light  without  the  aid  of  proper  instruments  and  many 
instances  are  encountered  where  the  lighting  expert  has  stumbled 
into  pitfalls  owing  to  the  absence  of  the  information  provided  by 
analytical  data  in  such  cases. 

Colorinketry. — There  are  available  many  types  of  colorimeters 
which  provide  data  varying  considerably  in  analytical  nature,  but 
each  has  fields  of  application  in  which  it  is  quite  satisfactory.  The 
simplest  forms  might  be  termed  tintometers.  These  instruments 
are  generally  used  for  such  purposes  as  maintaining  a  product  within 
certain  limits  or  in  classifying  a  product  according  to  color.  A 
series  of  colors  of  the  same  hue  but  varying  in  saturation  or  slightly 
varying  in  hue  or  brightness,  are  provided  as  a  series  of  comparison 
standards.  Such  instruments  have  few  applications  in  lighting 
practice.  Other  instruments  employ  a  mixture  of  two  colors  for 
limited  ranges  of  color  comparison.  The  resulting  data  are  of  a 
slightly  greater  analytical  nature. 

It  is  well-known  that  any  color  can  be  matched  by  a  mixture  of 
three  colors,  red,  green,  and  blue,  in  proper  proportions.  By  means 
of  this  tri-chromatic  instrument  an  illuminant  or  colored  medium 
can  be  analyzed  in  terms  of  the  three  arbitrary  colors.  There  being 
an  infinite  number  of  sets  of  three  colors  which  fulfill  the  foregoing 
requirements  for  most  practical  purposes,  it  is  seen  that  the  data 
which  are  obtained  are  restricted  to  the  three  colors  used  unless 
properly  reduced  to  a  standard  system.  These  can  be  transformed 
into  other  systems  but  in  the  present  state  of  knowledge  the  author 
has  little  confidence  in  the  value  of  data  after  being  subjected  to 
such  transformations.  Extreme  caution  must  be  exercised  in 
interpreting  such  data  because  the  color-matches  are  merely  sub- 
jective and  therefore  furnish  little  information  regarding  the  spec- 
tral character  of  the  colors  examined.  For  instance,  with  such  an 
instrument  the  color  of  the  light  from  a  quartz-tube  mercury-arc  is 


LUCKIESH:  COLOR  IN  LIGHTING  273 

specified  in  practically  the  same  numerical  terms  as  average  day- 
light, although  the  spectral  character  of  the  two  illuminants  are 
very  different. 

The  monochromatic  colorimeter  is  a  very  satisfactory  colorimeter 
because  by  means  of  it  the  three  qualities  of  a  color,  namely,  hue, 
saturation,  and  brightness,  are  determined.  It  has  a  further 
advantage  in  referring  color  measurements  to  a  reproducible  stand- 
ard— the  spectrum — although  the  lack  of  a  standard  and  constant 
white  causes  difficulty.  It  should  be  borne  in  mind  that  those 
measurements  are  made  by  subjective  color-matches  so  that  the 
data  do  not  provide  a  spectral  analysis  of  the  color  which  is  examined. 

Spectrophotography. — Photography  of  the  spectrum  provides  a 
means  of  analyzing  colors  spectrally.  The  application  of  spectrum 
photography  is  usually  confined  to  those  requirements  which  are 
not  so  exacting  although  by  careful  procedure  fairly  accurate  analyses 
may  be  consummated.  Many  variables  enter,  such  as  exposure, 
development,  and  non-uniformity  of  emulsion  but  of  the  greatest 
importance  is  the  non-uniform  spectral  sensibility  of  photographic 
emulsions.  Various  means  can  be  resorted  to  in  order  to  eliminate 
the  effects  of  non-uniform  dispersion  in  the  case  of  a  prism  spectro- 
graph  and  the  non-uniform  spectral  sensibility  of  the  emulsion. 

COLOR  MIXTURE 

In  many  applications  of  color  in  lighting  the  principles  of  color 
mixture  may  be  used.  The  greatest  difficulties  have  been  encoun- 
tered perhaps  through  the  confusion  of  the  primary  colors.  There 
are  three  general  methods  of  color  mixture,  namely,  the  additive,  the 
subtractive,  and  the  juxtapositional,  although  the  first  two  are  of 
chief  importance  here.  Many  applications  of  color  mixture  involve 
both  methods. 

As  previously  stated  any  color  can  be  matched  in  hue  by  a  proper 
mixture  of  three  primary  colors,  namely,  red,  green,  and  blue.  This 
method  is  termed  additive.  Many  sets  of  primary  colors  can  be 
used  and  a  satisfactory  set  can  be  determined  by  experiment.  In 
order  to  obtain  these  primary  colors  it  is  generally  necessary  in 
practice  to  subtract  certain  colored  rays  from  the  illuminant  usually 
by  colored  screens.  The  latter  is  an  example  of  the  subtractive 
method  and  is  the  one  employed  in  the  mixture  of  pigments.  The 
subtractive  primaries  are  usually  considered  to  be  red,  yellow  and 
blue.  The  specification  of  three  primaries  depends  upon  the  object 

18 


274  ILLUMINATING   ENGINEERING   PRACTICE 

to  be  attained  but  it  does  not  appear  that  there  is  sufficient  justi- 
fication for  considering  red,  yellow  and  blue  to  be  the  true  sub- 
tractive  primaries.  Purple,  yellow,  and  blue-green  appear  to  have 
a  greater  claim  as  the  sub  tractive  primaries  because  by  mixture  of 
these  a  greater  range  of  hues  is  obtainable.  For  instance,  from  the 
former  set  a  purple  color  cannot  be  obtained,  yet  in  using  the  latter 
primaries  nothing  is  sacrificed  because  purple  is  available  and  red 
can  be  obtained  by  a  mixture  of  yellow  and  purple.  As  a  matter  of 
fact,  the  red  used  as  a  primary  in  the  mixture  of  pigments  is  in  reality 
a  purple  so  that  the  confusion  appears  to  arise  from  a  misnomer 
applied  to  this  pigment. 

As  an  illustration  of  the  difference  between  the  two  methods  the 
mixture  of  yellow  and  blue  provides  an  excellent  example.  On 
mixing  these  additively  in  proper  proportions,  white  light  is  obtained 
but  on  mixing  them  subtractively  green  is  obtained.  In  producing 
colored  screens  for  lighting  purposes,  the  spectral  characters  of  the 
illuminant  to  be  used  and  of  the  colored  media  available  are  invalu- 
able guides  in  obtaining  the  desired  results.  Likewise  this  is  true 
in  many  cases  of  the  additive  mixture  of  colored  light.  The  juxta- 
positional  method  is  well  exemplified  in  some  processes  of  color 
photography  where  minute  colored  filters,  red,  green  and  blue  in 
color,  are  used  in  the  form  of  rulings  or  starch  granules.  This  method 
is  of  little  importance  in  the  general  practice  of  color  in  lighting  al- 
though there  are  instances  where  it  can  be  used  to  great  advantage. 

COLOR  AND  VISION 

The  visual  phenomena  of  color  have  been  very  extensively  studied 
yet  there  remains  a  vast  unexplored  unknown.  Many  of  the  prob- 
lems pertaining  to  color  which  arise  in  lighting  practice  can  be  solved, 
or  at  least  can  be  better  understood,  by  applying  present  knowledge 
pertaining  to  color  and  vision.  A  few  of  the  most  important  phe- 
nomena are  briefly  described  below. 

Simultaneous  Contrast. — Colors  mutually  affect  each  other  when 
viewed  simultaneously,  the  magnitude  of  the  influence  being  great- 
est when  the  colors  are  in  juxtaposition.  The  phenomena  may  be 
divided  into  two  general  parts,  namely,  hue  contrast  and  brightness 
contrast.  These  two  influences  are  usually  at  work  simultaneously 
so  that  it  requires  keen  analysis  to  diagnose  a  particular  case.  This 
phenomenon  is  perhaps  the  most  important  in  the  viewing  of  colored 
objects  and  must  be  credited  with  supplying  a  great  deal  of  beauty 
to  all  vari-colored  objects. 


LUCKIESH:  COLOR  IN  LIGHTING  275 

Growth  and  Decay  of  Color  Sensation. — The  various  color  sensations 
do  not  rise  to  full  value  immediately  upon  presentation  of  the  stimuli 
and  likewise  they  do  not  decay  to  zero  immediately  upon  cessation 
of  the  stimuli.  Further,  the  different  color  sensations  rise  and  fall 
at  different  rates.  Of  the  red,  green,  and  blue  sensations  the  green 
is  the  most  sluggish  and  the  blue  the  most  active. 

After-images. — After  a  stimulus  of  a  color  sensation  is  removed  the 
sensation  persists  for  some  time  depending  upon  the  color.  This 
persistence  of  the  sensation  is  termed  an  after-image.  During  its 
decay  its  appearance  continually  changes.  If  immediately  after 
the  stimulus  has  ceased,  the  retina  be  stimulated  with  a  moderate 
intensity  of  white  light  the  after-image  due  to  the  first  stimulus  will 
usually  be  approximately  complementary  in  color  to  the  original 
sensation.  Obviously  the  results  will  usually  be  very  complicated. 
No  attempt  will  be  made  to  explain  these  here  except  by  the  indefi- 
nite fatigue,  because  the  leading  color  theories  seriously  differ  in 
their  explanations. 

Retinal  Color  Sensitivity. — The  retina  varies  over  its  surface  in  its 
sensitivity  to  color.  The  central  region  is  relatively  less  sensi- 
tive to  light  of  short  wave-lengths  due  perhaps  to  the  yellowish 
pigmentation  which  has  resulted  in  defining  this  region  as  the 
"  yellow-spot."  The  extreme  peripheral  retina  is  relatively  in- 
sensitive to  color.  The  sensitive  area  varies  for  different  colors  and 
also  is  dependent  upon  the  size  and  brightness  of  the  colored  patch 
which  is  viewed.  The  minimum  perceptible  brightness-difference  is 
approximately  constant  for  all  colors  at  high  intensities  but  differs 
considerably  at  low  intensities.  In  general  it  decreases  as  the  wave- 
length decreases. 

Purkmje  Effect. — The  eye  is  relatively  more  sensitive  to  short- 
wave energy  than  to  long-wave  energy  at  low  intensities  than  it  is 
at  high  intensities.  In  other  words,  if  blue  and  red  colors  appear  of 
the  same  brightness  under  ordinary  intensities  of  illumination,  the 
blue  will  appear  much  brighter  than  the  red  when  the  intensity  of 
illumination  is  reduced  to  a  very  low  value.  The  intensity  at 
which  the  effect  begins  to  be  noticeable  depends  upon  many  condi- 
tions but  an  approximate  average  is  at  a  brightness  of  a  white  sur- 
face illuminated  to  an  intensity  of  one-tenth  foot-candle  or  of  one 
meter-candle.  This  effect  is  best  described  in  a  few  words  by  stating 
that,  in  general,  at  low  illuminations  the  spectral  sensibility  curve  of 
the  eye  shifts  toward  the  shorter  wave-lengths. 

Visual  Acuity. — It  has  been  proved  that  visual  acuity,  or  the 


276  ILLUMINATING   ENGINEERING    PRACTICE 

ability  to  distinguish  fine  detail,  is  better  in  monochromatic  light 
than  in  light  of  extended  spectral  character.  The  effect  is  not  as 
marked  for  ordinary  seeing  yet  details,  such  as  letters  on  an  ordi- 
nary printed  page,  do  appear  better  defined  under  monochromatic 
light.  In  other  words,  for  equal  discrimination  or  clearness  of  a 
page  of  type,  lower  intensities  of  illumination  are  required  with  light 
approaching  monochromatism  than  with  light  having  a  more 
extended  spectral  character.  Results  obtained  by  the  author  using  a 
yellow  light  whose  spectral  character  could  be  so  altered  as  to 
approach  more  and  more  toward  monochromatism  indicate  that  the 
increase  in  defining  power  in  this  case  approximately  offsets  the 
opposite  effect  due  to  the  attendant  decreasing  illumination. 

Color  Vision. — No  hypothesis  of  color  vision  at  the  present  time  is 
in  complete  accord  with  the  experimental  data  available.  The  fault 
obviously  may  lie  with  either  the  hypothesis  or  with  the  data. 
However,  the  fact  is  mentioned  because  of  the  tendency  of  those  not 
fully  informed  on  the  subject  to  accept  one  hypothesis  and  to  attempt 
to  explain  everything  pertaining  to  color-vision  on  this  basis.  There 
are  two  hypotheses  whose  proponents  have  been  serious  opponents 
to  each  other.  One  of  these  is  called  the  Young-Helmholtz  theory. 
It  was  enunciated  by  Young  and  given  considerable  experimental 
foundation  by  the  great  work  of  Helmholtz.  This  hypothesis  was 
builded  largely  from  the  side  of  physics  and  is,  therefore,  based  largely 
upon  the  observed  facts  of  color  mixture.  Three  substances  or  sets 
of  nerves  which  are  responsible  for  three  color  sensations,  respec- 
tively, red,  green,  and  blue,  have  been.,  assumed  to  exist,  and  to 
account  for  all  color  sensations  by  their  varying  degrees  of  response 
to  different  stimuli.  Anatomical  research  has  not  verified  the  exis- 
tence of  those  assumed  substances. 

The  other  hypothesis  which  ranks  with  the  foregoing  in  importance 
is  known  as  the  Hering  theory.  This  hypothesis  has  been  builded 
largely  from  the  side  of  psychology  on  the  basis  that  four  distinc- 
tive colors  are  seen  in  the  visible  spectrum,  namely,  red,  yellow, 
green,  and  blue.  Three  substances  are  assumed  to  exist,  the  break- 
ing down  of  a  substance  being  effected  by  one  color  of  a  pair  and  the 
building  up  of  the  same  substance  being  attributed  to  the  other  color 
of  the  pair.  Black  and  white  are  assumed  to  be  distinct  sensations 
unconnected  with  color  sensations  with  the  result  that  the  three  pairs 
are  considered  to  be  red  and  green,  blue  and  yellow,  black  and  white. 
There  is  no  anatomical  evidence  of  the  existence  of  these  three  sub- 
stances or  processes. 


LUCKIESH:  COLOR  IN  LIGHTING  277 

Von  Kries  is  largely  responsible  for  injecting  the  "rod  and  cone" 
hypothesis  in  color-vision  theory.  The  cones,  which  exist  practi- 
cally alone  in  the  center  of  the  retina  (the  fovea)  and  become  less 
dense  toward  the  periphery,  are  assumed  to  be  responsible  for  both 
achromatic  and  chromatic  sensation.  The  rods,  which  predominate 
in  the  peripheral  regions  and  are  absent  in  the  central  region,  are 
assumed  to  be  responsible  for  achromatic  sensations.  The  rods  are 
supposed  to  be  largely  responsible  for  light  sensation  at  low  inten- 
sities and  are  in  general  more  responsive  to  rays  of  shorter  wave- 
lengths. The  cones  are  supposed  not  to  be  rendered  very  much  more 
sensitive  be  dark  adaptation.  The  rods  and  cones  actually  exist  in 
the  retina  as  revealed  by  anatomical  research.  Many  experimental 
facts  have  been  beautifully  woven  into  this  ''duplicity"  hypothesis. 

Many  interesting  modifications  of  these  hypotheses  have  been 
made  and  hypotheses  based  upon  other  principles  are  also  worthy  of 
attention.  It  is  quite  beyond  the  scope  of  this  lecture  to  discuss  the 
many  proposed  hypotheses;  however,  adequate  treatments  are  avail- 
able elsewhere. 

PSYCHOLOGY  OF  COLOR 

The  great  unknowns  in  lighting  are  chiefly  those  involving  psy- 
chology which,  as  an  experimental  science  is  in  a  primary  stage  of 
development.  The  foregoing  applies  equally  to  the  subject  of  color  in 
lighting.  The  definite  data  on  the  psychology  of  color  are  so  meager 
that  it  is  difficult  to  treat  the  subject  briefly,  therefore,  in  order  not 
to  stray  too  far  afield,  only  a  few  general  statements  will  be  incor- 
porated here.  It  appears  quite  probable  that  at  some  future  time 
the  language  of  color  will  be  understood.  Occasionally  glimmerings 
of  understanding  appear  among  the  chaos  of  color  experience  yet, 
on  the  whole,  there  is  no  great  amount  of  data  to  aid  the  lighting 
expert. 

There  is  general  agreement  in  classifying  colors  into  warm,  neutral 
and  cold  groups.  Spectrally  these  attributes  are  found  to  lie  in 
regular  succession.  Yellow,  orange,  and  red  are  the  regions  to 
which  the  attribute  of  warmth  is  given.  The  cold  colors  are  found 
at  and  near  the  blue  region.  The  neutral  colors  are  found  in  the 
central  region,  namely,  the  greens  and  adjacent  colors  and  neu- 
trality is  again  approached  at  the  very  extremes  of  the  spectrum. 
Fairly  neutral  colors  also  usually  result  from  an  additive  mixture  of 
the  colors  near  the  extreme  limits  of  the  spectrum.  Intelligent  use 
of  this  knowledge  can  be  applied  to  many  lighting  problems.  How- 


278  ILLUMINATING   ENGINEERING    PRACTICE 

ever,  it  is  necessary  at  this  point  to  insert  a  word  of  caution.  The 
lighting  expert  should  carefully  discriminate  between  that  portion 
of  a  given  condition  which  is  predominantly  responsible  for  the 
impression  arising  from  color.  In  general,  if  the  light  source  is 
visible  (it  may  be  either  a  primary  or  a  secondary  light-source)  its 
color  plays  a  dominating  part  in  the  impression  upon  the  ordinary 
observer.  If  the  primary  light-sources  are  concealed  the  color  of  the 
surroundings  are  more  effective  in  producing  the  impression  than 
the  actual,  color  of  the  important  surface  such  as  a  book  which  the 
observer  may  be  reading  or  goods  on  display  in  a  show-window. 
Specific  examples  may  make  the  point  clear.  If  a  semi-indirect  light- 
source  bowl  be  of  a  warm  color,  such  as  orange-yellow,  the  observer 
whose  aesthetic  sense  demands  the  warm  color  will  often  neglect  to 
inquire  further.  In  other  words,  the  lighting  will  usually  be  satis- 
factory to  him  notwithstanding  the  light  which  constitutes  the  pre- 
dominant part  of  the  useful  illumination  may  be  the  much  whiter 
light  emitted  by  a  gas  mantle  or  tungsten  filament  located  in  the 
semi-indirect  bowl.  Another  example  can  be  drawn  from  many 
installations  of  artificial  daylight  which  have  recently  been  made. 
Notwithstanding  that  a  quality  of  daylight  closely  approaching  day- 
light is,  in  many  cases,  not  only  desirable,  but  proper,  tradition  or 
habit  requires  that  the  artificial  light  must  be  of  a  yellowish  color. 
If  the  surroundings,  such  as  the  background  in  a  show-window  or  the 
walls  and  ceiling  of  a  paintings  gallery,  be  covered  with  warm  colors, 
the  white  light  from  the  artificial  daylight  units  can  be  directed  upon 
the  objects  to  be  displayed  and  yet  the  warm  appearance  of  the 
whole  will  be  largely  maintained. 

A  room  with  southern  exposure,  which  in  this  zone  of  latitude 
receives  much  direct  sunlight  can  be  " cooled"  to  some  extent  by 
the  employment  of  cool  colors  in  the  furnishings.  Conversely  a 
room  with  northern  exposure  can  be  "  warmed"  considerably  by  the 
employment  of  warm  colors  in  'the  surroundings.  It  is  true  that 
the  light  is  somewhat  altered  by  selective  reflection  from  the  colored 
surroundings  but  the  major  portion  of  the  effect  is  often  apparently 
purely  psychological. 

At  this  point  it  is  well  to  emphasize  the  apparent  existence  of 
two  distinct  mental  attitudes  in  regard  to  color  in  lighting.  Rooms 
are  generally  decorated  for  daylight  conditions  and  are  presumably 
satisfactory  when  completed.  However,  notwithstanding  all  illu- 
minants  ordinarily  used  for  general  interior  lighting  are  quite  yellow 
in  integral  color  in  comparison  with  daylight,  complaint  is  sometimes 


LUCKIESH:  COLOR  IN  LIGHTING  279 

heard  of  the  garish  whiteness  of  the  unaltered  light  emitted  by 
modern  gas  and  electric  filament  lamps.  The  correction  resorted 
to  is  usually  the  application  of  yellow  screens  of  glass,  gelatine,  or 
silk  fabric.  Why,  if  the  daylight  condition  is  satisfactory,  is  the 
artificial  lighting  too  cold?  Obviously  the  question  is  answered  by 
admitting  the  existence  of  day  and  night  criteria  which  are  widely 
different.  The  reason  for  the  existence  of  these  two  very  different 
criteria  possibly  may  be  traced  to  phenomena  of  vision  but  probably 
may  be  correctly  attributed  to  tradition.  Artificial  light  for  ages 
was  quite  yellow  and  only  recently  have  the  illuminants  become  con- 
siderably whiter.  Perhaps  the  demand  for  yellow  artificial  light 
arising  from  some  aesthetic  senses  is  largely  due  to  the  insistence  of 
habit.  It  is  difficult  to  account  for  the  foregoing  in  any  other  man- 
ner considering  the  tremendous  difference  in  color  still  existing 
between  artificial  illuminants  and  natural  daylight.  That  the 
double  standard  can  be  partially  eliminated  at  least,  the  author  can 
testify  from  experience.  It  is  not  the  desire  here  to  condemn  this 
double  requirement  but  to  diagnose  it.  It  is  a  condition  which  the 
lighting  expert  must  meet  and  one  which  involves  many  of  the  facts 
and  applications  of  color  science. 

Colors  have  been  characterized  according  to  their  emotional 
effect  by  such  words  as  exciting,  soothing,  gay,  somber,  serene,  and 
many  others.  In  studying  the  attributes  applied  to  colors  by  poets 
and  painters  it  is  found  that  there  is  apparently  a  general  agreement 
in  usage.  However,  a  treatment  of  the  subject  is  beyond  the  scope 
of  this  lecture.  The  emotional  value  of  colors  has  been  mentioned 
in  passing  with  the  hope  that  the  lighting  expert  will  avail  himself, 
by  study  and  observation,  of  the  possibilities  of  expression  through 
the  language  of  color. 

There  are  available  some  data  on  color  preference,  but  such  data 
must  be  carefully  interpreted  or  difficulties  will  be  encountered. 
In  obtaining  data  on  color  preference  the  observer  is  concerned  with 
nothing  except  the  colors  being  compared.  Other  considerations 
enter  into  lighting  problems  which  call  for  a  modification  of  data  on 
color  preference  before  it  can  be  applied.  For  instance,  pure  colors 
are  more  frequently  preferred  than  tints  and  shades,  a  fact  estab- 
lished by  various  investigators,  yet  this  does  not  apply  to  the  decora- 
tion and  lighting  of  an  interior.  Of  the  pure  colors  the  reds  and 
blues  are  the  more  often  preferred  of  a  group  of  pigments  representing 
the  entire  range  of  spectral  colors  as  well  as  the  purples.  Yellow 
usually  ranks  quite  low  in  the  preference  order.  Strangely  enough, 


280  ILLUMINATING   ENGINEERING   PRACTICE 

the  colors  more  commonly  encountered  in  interior  decoration 
(cream,  yellow,  orange,  buff,  brown)  generally  rank  low  in  such 
color-preference  investigations.  Perhaps,  in  such  investigations, 
the  momentary  delight  in  the  less  common  color  sways  the  judg- 
ment oppositely  to  that  resulting  from  prolonged  association  with 
the  color.  Certainly  the  warmer  tints  and  shades  predominate  in 
interiors  and  usually  these  correspond  in  hue  to  the  yellow-orange 
region  of  the  spectrum. 

The  distribution  of  light,  shade,  and  color  in  an  interior  deter- 
mines the  mood  of  the  setting  as  a  whole  and  often  should  be  con- 
sidered the  chief  factor  to  be  studied;  however,  too  often  it  is  not 
pre-visualized  but  incidentally  results  from  a  certain  arrangement 
of  outlets  equipped  with  a  unit  that  is  merely  popular.  Great 
opportunities  are  open  to  the  lighting  expert  who  learns  to  apply  the 
language  of  light,  shade,  and  color. 

SURROUNDINGS 

As  previously  stated  the  surroundings  are  very  important  in 
molding  the  mental  impression  of  a  lighting  condition.  The  distribu- 
tion of  light  and  shade  is  largely  controlled  by  the  reflection  coeffi- 
cients of  the  surroundings.  Color  is  intricately  interwoven  into  the 
whole,  but,  inasmuch  as  the  psychological  importance  of  the  color 
of  the  surroundings  has  been  touched  upon,  the  discussion  here  will 
be  confined  to  the  modification  of  light  by  selective  reflection  from 
the  colored  surroundings. 

A  colored  surface  appears  colored  by  reflected  light  because  it  has 
the  property  of  reflecting  light  of  certain  wave-lengths  and  of  absorb- 
ing others,  thereby  altering  the  incident  light.  A  yellow  wall 
paper  reflects  the  blue  rays  only  slightly,  the  result  of  subtracting 
blue  rays  from  white  light  being  a  yellow  light.  A  red  fabric  appears 
red  under  daylight  because  it  reflects  only  the  red  rays  in  daylight. 
It  appears  a  relatively  brighter  red  under  tungsten  or  gas  light  than 
under  daylight  for  equal  illuminations  owing  to  the  relatively 
greater  amount  of  red  rays  present  in  the  light  from  the  artificial 
illuminants  per  unit  of  light  flux.  Under  the  light  from  a  mercury 
arc  lamp  the  red  fabric  appears  almost  black  because  there  are 
present  in  the  light  from  the  mercury  arc  practically  no  rays  which 
the  red  fabric  is  able  to  reflect.  This  shows  that  the  relative  bright- 
nesses of  colored  objects  varies  with  the  spectral  character  of  the 
illuminant  and  that  selective  reflection  from  the  surroundings  is 


LUCKIESH:  COLOR  IN  LIGHTING  281 

responsible  for  a  change  in  the  color  of  the  incident  light.  Daylight 
entering  interiors  usually  has  been  altered  by  reflection  from  many 
colored  objects,  such  as  buildings,  foliage,  pavements,  lawns,  and 
earth,  with  the  result  that  daylight  in  interiors  is  quite  variable  in 
quality.  This  variation  causes  difficulty  in  accurate  color  work  from 
day  to  day  and  from  season  to  season.  Skylight  is  much  more 
bluish  in  color  than  sunlight  so  that  tremendous  variations  in 
quality  are  apparent  as  the  relative  amounts  of  sunlight  and  sky- 
light vary.  Moreover,  the  variation  in  the  relative  amounts  of 
skylight  and  sunlight  entering  windows  or  other  openings  is  generally 
continuous. 

The  magnitude  of  the  change  due  to  reflection  from  colored  sur- 
roundings has  been  measured  in  a  miniature  interior  for  various  color 
combinations  of  the  walls  and  ceiling  and  for  different  systems  of 
lighting.  Obviously  the  influence  of  the  surroundings  upon  the  color 
of  the  useful  light  at  a  given  point  such  as  a  desk-top,  depends  upon 
the  relative  amounts  of  light  reaching  the  point  directly  and  indi- 
rectly. For  ordinary  direct-lighting  systems  the  alteration  due  to 
colored  surroundings  is  usually  appreciable  although  not  as  great  as 
for  indirect-lighting  systems.  In  a  representative  case  it  was  found 
that  the  light  from  tungsten  lamps  in  an  indirect  lighting  fixture  was 
altered  to  a  color  far  yellower  than  the  old  carbon  lamps  when  the 
colors  of  the  cream-tinted  ceiling  and  brownish  yellow  walls  were 
of  a  very  common  combination.  The  effect  is  of  considerable  mag- 
nitude in  semi-indirect  installations  depending,  of  course,  upon  the 
relative  values  of  the  direct  and  indirect  components. 

If  in  a  given  case  of  indirect  lighting  the  artificial  illuminant  is 
too  cold,  it  is  possible  to  obtain  the  identical  results  by  two  expedi- 
ents. In  one  case  the  walls  and  ceiling  would  be  refinished  with 
coverings  of  a  warmer  or  yellower  tint,  in  the  other  case  a  yellowish 
screen  would  be  placed  over  the  lighting  unit  so  as  to  alter  the  light 
by  selective  absorption.  If  artificial  illuminants  have  become  too 
cold  in  color  to  suit  the  aesthetic  sense,  why  not  in  many  cases,  resort 
to  the  use  of  warmer  colors  for  the  surroundings  such  as  walls  and 
ceiling?  This  method  would  also  tend  to  warm  up  daylight  in  color 
which  is  very  much  colder  than  the  common  artificial  illuminants. 
But  here  the  question  of  the  double  standard  enters  again. 

In  an  installation  of  artificial  daylight  it  was  desirable  to  have 
the  lighting  units  appear  as  yellowish  as  possible  to  harmonize  with 
the  general  color  scheme  of  the  room  and  it  was  also  essential  to 
have  the  lamps  enclosed  by  a  glass  ball.  This  was  achieved  by 


282  ILLUMINATING   ENGINEERING   PRACTICE 

etching  the  ball  inside  and  tinting  with  a  very  unsaturated  yellow 
with  the  result  that  the  ball  appeared  quite  yellowish  in  color  while 
the  light  which  was  approximately  directly  transmitted  was  only 
slightly  altered  in  color.  There  was  a  slight  loss  of  light  due  to  the 
coloring  but  this  was  negligible  in  comparison  with  the  satisfactori- 
ness  of  the  results.  There  are  many  important  and  interesting  con- 
siderations which  are  beyond  the  scope  of  this  treatment.  An 
analysis  of  a  given  condition  will  reveal  them. 

In  closing  this  discussion  it  appears  profitable  to  enunciate  a  few 
simple  but  pertinent  facts.  A  yellowish  surface  under  daylight 
illumination  may  appear  exactly  like  a  neutral  surface  under  an 
ordinary  yellowish  artificial  illuminant.  Surroundings  consisting 
chiefly  of  such  colors  as  brown,  buff,  yellow  or  orange  shades,  which 
are  neutral  or  warm  in  appearance  under  daylight  appear  relatively 
much  warmer  by  ordinary  artificial  light.  In  indirect  and  many 
semi-indirect  systems  of  lighting  the  alteration  of  the  light  by  colored 
surroundings  is  so  great  as  to  produce  in  many  cases  an  effect  with  a 
modern  illuminant  similar  to  that  obtained  with  the  old  illuminants 
in  ordinary  direct  fixtures. 

ARTIFICIAL  DAYLIGHT 

For  the  production  and  appreciation  of  colored  objects,  daylight 
is  the  generally  accepted  standard.  The  arts  as  well  as  the  eye  have 
been  evolved  under  natural  light  with  the  result  that  the  demand  for 
light  approaching  daylight  in  quality  for  many  purposes  is  deeply 
and  permanently  rooted.  Daylight  varies  tremendously  in  spectral 
character  so  that  it  is  necessary  to  determine  the  standards.  Meas- 
urements of  intensity  and  quality  of  north  skylight  on  a  clear  day 
reveal  a  fair  constancy  which  doubtless  accounts  for  the  dependence 
upon  north  skylight  for  accurate  color-discrimination.  However, 
north  skylight  varies  from  clear  to  cloudy  days  but  not  as  much  as 
the  light  from  other  points  of  the  compass  in  northern  latitudes. 
Clear  noon  sunlight  is  quite  constant  and  although  not  always 
available  represents  a  fair  average  daylight  outdoors.  Noon  sun- 
light and  north  skylight  have,  therefore,  been  accepted  as  two 
distinct  standard  daylights. 

There  are  three  possible  methods  of  producing  artificial  daylight: 
namely,  (i)  directly  from  the  light  source,  (2)  by  adding  comple- 
mentary light  in  proper  proportions,  (3)  by  altering  the  light  from 
an  illuminant  by  means  of  a  selective  screen.  The  only  available 
illuminant  which  fills  directly  the  requirements  of  accurate  color 


LUCKIESH:  COLOR  IN  LIGHTING  283 

work  is  the  Moore  carbon-dioxide  tube  lamp.  Some  arcs  emit  light 
roughly  approximating  sunlight  in  color  but  the  variations  in  in- 
tensity, and  usually  in  quality,  have  discouraged  their  use  for  refined 
color  work.  The  Moore  tube  lamp  emits  light  approximating  sky- 
light in  quality  closely  enough  for  the  most  exacting  color-matching. 

Some  years  ago  the  light  from  the  tungsten  lamp  was  combined 
with  that  from  the  mercury  arc  in  such  proportions  as  to  give  a 
subjective  white  light.  This  combination  met  some  requirements 
but  could  not  possibly  approximate  daylight  in  spectral  character 
owing  to  the  discontinuous  spectrum  of  the  mercury  arc.  The 
spectrum  of  the  light  from  the  Moore  carbon-dioxide  tube  is  dis- 
continuous but  only  for  small  intervals.  On  various  occasions 
colored  lights  have  been  combined  with  the  light  from  ordinary 
artificial  illuminants  to  produce  an  approximate  daylight  effect. 
However,  the  only  method  of  producing  artificial  daylight  which  up 
to  the  present  has  been  extensively  applied  is  that  which  involves 
the  use  of  colored  screens.  These  have  included  the  use  of  gas 
mantles,  arc  lamps,  and  tungsten  filament  lamps. 

The  historical  development  of  such  units,  which  has  been  treated 
elsewhere,  will  not  be  repeated  here  in  order  to  preserve  the  limited 
space  for  a  discussion  of  the  more  practical  aspects  of  the  subject. 
Inasmuch  as  the  author  is  unaware  of  any  other  extensive  and  diver- 
sified installations  of  artificial  daylight,  the  remaining  discussion  of 
this  subject  will  be  largely  drawn  from  records  of  hundreds  of  in- 
stallations in  which  many  tungsten  filament  '" daylight"  units  of 
various  types  have  been  used.  Excepting  for  the  accurate  north 
skylight  units,  practical  considerations  and  requirements  have 
played  an  important  part  in  determining  the  final  units  developed. 
The  colored  screens  have  been  made  for  several  years  entirely  of 
glass,  to  comply  with  the  requirement  of  a  satisfactory  unit. 

In  imitating  north  skylight  it  has  proved  most  satisfactory  to 
press  the  colored  glass  in  the  form  of  a  plate  or  shallow  dish  in  order 
to  insure  uniformity.  In  such  cases  where  accurate  color-matching 
is  required,  efficiency  should  be  a  minor  consideration  and  experi- 
ence has  proved  this  to  be  very  generally  true.  Using  modern  gas- 
filled  tungsten  lamps,  north  skylight  of  satisfactory  quality  is 
reproduced  by  this  subtract! ve  method  at  losses  of  from  75  to  85 
per  cent,  of  the  original  light.  It  has  been  found  that  the  colored 
screens  can  be  produced  inexpensively  and  with  sufficient  accuracy 
to  meet  the  requirements.  A  brief  resume  of  the  fields  in  which  such 
units  are  operating  at  the  present  time  is  presented  later. 


284  ILLUMINATING    ENGINEERING   PRACTICE 

Experience  has  shown  that,  for  the  less  refined  color  work  and  for 
the  layman's  eye,  untrained  in  accurate  color  discrimination,  little 
or  no  advantage  is  gained  in  correcting  the  light  further  than  to  an 
approximation  to  clear  noon  sunlight.  For  this  reason  practical 
artificial  sunlight  units  have  been  developed.  These  units,  whose 
important  part  consists  of  an  enclosing  colored  glass  envelope,  have 
been  installed  for  general  lighting  purposes  in  many  different  fields. 
The  absorption  losses  of  these  units,  using  gas-filled  lamps  operating 
in  the  neighborhood  of  18  lumens  per  watt,  is  approximately  50 
per  cent.  A  brief  resume  of  the  fields  in  which  sun  units  are  operat- 
ing at  the  present  time  is  presented  later. 

Besides  the  preceding  considerations  other  reasons  have  led  to 
the  development  of  a  gas-filled  "daylight"  lamp.  In  the  general 
practice  of  lighting  a  daylight  lamp  has  usually  been  preferred  to  a 
daylight  unit  whose  design  conformed  to  the  ideas  of  the  manu- 
facturer and  whose  types  were  limited  by  manufacturing  expediency. 
There  has  been  a  constant  demand  for  an  illuminant  approaching 
daylight  in  quality  but  of  sufficiently  high  efficiency  for  general 
lighting  purposes.  For  some  time  the  daylight  efficiency  of  arti- 
ficial illuminants  has  been  rapidly  increasing.  Obviously  the  time 
has  been  approaching  when  this  demand  could  be  supplied  and  the 
experiment  has  been  tried  with  successful  results.  Other  considera- 
tions hastened  the  culmination  of  this  event.  For  example,  artificial 
daylight  is  cold  in  appearance,  although  there  are  many  applications 
where  this  "coldness"  is  a  delightful  part  of  the  illusion.  However, 
experience  appeared  to  indicate  at  the  present  time  a  limit  to  the 
"coldness"  which  would  be  acceptable  for  general  lighting  at  night 
in  many  fields.  Furthermore,  luminous  efficiency  is  of  some  im- 
portance when  illuminants  are  used  for  general  lighting  purposes. 
Therefore,  there  has  been  developed  a  gas-filled  tungsten  "daylight " 
lamp  which  corrects  the  light  well  toward  average  daylight,  the 
resulting  light  approximating  black-body  radiation  at  a  temperature 
somewhat  below  the  apparent  black-body  temperature  of  the  sun. 
It  appears  quite  legitimate  and  desirable  to  increase  the  apparent 
temperature  of  the  tungsten  filament  hundreds  of  degrees  above  its 
melting  point  by  means  of  a  proper  colored-glass  bulb.  The  color 
of  the  resulting  light  blends  well  with  daylight  entering  interiors 
and  has  proved  satisfactory  in  hundreds  of  installations.  A  brief 
summary  of  the  fields  in  which  this  quality  of  light  is  at  present  used 
is  presented  later. 

Various  developments  of  artificial  daylight  units  have  been  dis- 


LUCKIESH:  COLOR  IN  LIGHTING  285 

cussed  herewith  to  illustrate  the  practical  requirements.  The  dis- 
cussion would  apply  in  general  as  well  to  other  illuminants  besides 
the  tungsten  lamp  but  it  has  been  necessary  to  confine  the  discus- 
sion to  developments  in  connection  with  the  tungsten  lamp  because 
these  represent  the  first,  and  at  present,  the  only  developments  on 
a  large  commercial  scale.  For  the  same  reason  the  installations 
described  later  must  be  largely  confined  to  the  tungsten  lamp. 
Various  other  light  sources  have  been  used  on  a  small  scale  and 
usually  for  a  single  kind  of  artificial  daylight  unit.  In  such  de- 
velopments the  demands,  opinions,  and  tastes  of  consumers  are  very 
influential,  and  hence  experiences  have  been  incorporated  with  the 
hope  that  they  will  aid  in  future  developments. 

Data  on  the  light  absorption  of  daylight  illuminants  are  available 
elsewhere.  There  is  a  lack  of  agreement  in  the  results  by  different 
investigators  due  doubtless  chiefly  to  the  variation  of  the  daylight 
standard.  The  absorptions  presented  by  the  author  in  the  fore- 
going are  those  obtained  by  actual  measurement  upon  units  which 
satisfactorily  fulfilled  their  missions. 

SIMULATING  OLD  ILLUMINANTS 

The  development  of  artificial  daylight  units  has  been  for  the 
purpose  of  satisfying  a  requirement  which  is  not  generally  or  promi- 
nently influenced  by  individual  taste.  It  has  had  for  its  object  the 
extension  of  day;  that  is,  by  its  use  many  arts  can  be  pursued  and 
appreciated  at  night  as  well  as  during  the  day.  It  provides  in- 
surance against  the  failure  of  natural  daylight  even  during  day- 
light working  hours  which  occurs  often  especially  in  the  crowded 
cities.  Esthetic  taste  has  been  neither  a  factor  for  nor  against  this 
development;  however,  the  aesthetic  taste  does  enter  prominently 
into  many  problems  of  lighting.  A  director  of  an  art  museum  in 
fairness  to  the  artist  and  to  the  general  public  should  make  use  of 
artificial  daylight  if  economical  considerations  are  favorable,  but, 
an  individual  in  his  home  may  satisfy  his  taste  without  being  justly 
criticized.  Some  of  these  tastes  demand,  for  many  purposes  in 
the  home,  a  light  of  a  warm  quality — a  quality  simulating  that  of 
the  old  illuminants.  It  is  not  a  question  here  whether  this  demand 
is  a  result  of  tradition  or  the  insistence  of  habit.  The  problem  of 
the  lighting  expert  is  to  appease  the  desire  if  only  the  individual 
taste  is  important.  The  increasing  efficiency  in  light  production 
makes  it  possible  to  alter  artificial  light  to  meet  any  requirements. 


286  ILLUMINATING   ENGINEERING   PRACTICE 

The  problem  of  simulating  old  illuminants  is  relatively  simple 
compared  with  the  exacting  requirements  in  the  production  of  arti- 
ficial daylight:  In  the  former  case  only  an  approximate  subjective 
color-match  is  necessary  while  in  the  latter  case  a  close  approxima- 
tion in  spectral  character  is  required.  Usually  yellow  fabric,  dyes, 
or  amber  glass  are  used  to  produce  the  warmer  color.  However, 
the  available  colors  if  used  singly  are  usually  greenish  yellow  when 
sufficiently  unsaturated  as  in  the  case  of  amber  glass. 

If  a  kerosene  flame,  or  carbon  incandescent  lamp  be  concealed  in 
a  diffusing  glass  accessory,  the  color  is  seldom  noticed.  The  un- 
saturated yellow  of  these  old  illuminants  has  been  termed  an 
''aesthetic  yellow"  in  order  to  distinguish  it  from  other  yellows  in 
the  vocabulary  of  the  lighting  expert.  Data  have  been  obtained  by 
many  observers  with  various  units  of  this  character  especially  one 
unit  containing  tungsten  lamps  tinted  with  ordinary  amber  and 
another  containing  lamps  colored  with  the  "aesthetic  yellow"  color 
which  produced  a  close  match  to  the  kerosene  flame.  In  general 
the  amber  color  was  considered  obtrusive  while  the  other  color  was 
apparently  unnoticed.  This  point  is  worthy  of  consideration  in  any 
extensive  application  of  the  foregoing. 

Unfortunately  no  single  dye  is  available  having  the  desirable 
characteristics  of  high  transparency,  but,  it  is  a  simple  matter  to 
correct  any  of  the  common  yellows  of  greenish  tinge.  This  is  readily 
eliminated  by  the  use  of  a  slight  amount  of  pink  coloring.  Many  of 
the  so-called  red  coloring  materials,  when  of  light  density,  afford  a 
satisfactory  pink.  The  ordinary  lamp  dyes  can  be  readily  mixed 
to  provide  the  proper  color  but  these  are  usually  more  or  less  fugitive. 
If  possible  it  is  well  to  color  a  glass  plate  and  place  this  at  sufficient 
distance  from  the  light  unit.  Coloring  media  which  are  very  fugitive 
when  subjected  to  Considerable  heat  are  often  quite  permanent  to 
light  when  kept  reasonably  cool.  A  metal  screen  placed  in  contact 
with  the  colored  screen,  will  often  keep  the  latter  sufficiently  cool. 
Coloring  media  are  available  for  incorporating  into  glass,  which 
produce  a  proper  unsaturated  yellow  color  which  simulates  the 
yellow  of  the  older  illuminants,  but  the  difficulties  in  obtaining  a 
glass  of  the  proper  color  and  high  transparency  are  very  great. 

Amber  glass  when  very  dense  loses  its  greenish  tinge  but  then  the 
color  is  too  saturated  for  the  present  purpose.  However,  a  satis- 
factory approximation  to  a  subjective  match  with  the  old  illuminants 
can  readily  be  obtained  by  combining  a  small  percentage  of  the  light 
passing  through  a  dense  amber  glass  with  a  large  amount  of  unaltered 


LUCKIESH:  COJ.OR  IN  LIGHTING  287 

light.  Such  a  procedure  is  not  always  practicable  but  is  a  highly 
satisfactory  solution  in  suitable  cases.  A  satisfactory  coloring 
material,  disregarding  its  opacity,  for  use  where  the  temperature 
is  high  as  on  the  bulb  of  a  gas-filled  tungsten  lamp  is  a  yellow-orange 
pigment  used  in  ordinary  oil  painting.  This  can  be  incorporated 
in  a  suitable  binder  after  being  thoroughly  ground  and  sifted. 
Good  results  have  been  obtained  with  yellow  shellac  in  alcohol  in 
which  the  yellow-orange  pigment  is  thoroughly  stirred  although 
some  other  binders  are  more  satisfactory.  On  standing,  the  pig- 
ment settles  out  but  can  be  brought  readily  into  complete  suspen- 
sion by  slight  stirring  or  shaking.  Gas-filled  tungsten  lamps  covered 
with  this  pigment  have  been  in  continuous  service  for  many  days 
without  showing  any  sign  of  discoloring.  The  lamps  can  be  dipped 
although  it  has  also  been  found  satisfactory  to  apply  the  coloring 
on  a  rotating  lamp  by  means  of  a  camel-hair  brush. 

It  may  be  of  interest  to  know  the  theoretical  efficiencies  of  modern 
illuminants  when  screened  to  simulate  the  old  illuminants  exactly 
in  spectral  character.  Results  of  computations  have  been  pub- 
lished elsewhere  for  the  vacuum  and  gas-filled  tungsten  lamps  oper- 
ating at  various  efficiencies.  From  these  data  an  idea  of  the  amount 
of  light  absorbed  can  be  obtained.  The  resulting  specific  outputs 
of  the  vacuum  tungsten  lamp  operating  at  7.9  lumens  per  watt 
when  screened  to  match  a  kerosene  flame  and  a  carbon  incandescent 
lamp  in  color  are  respectively  4.5  and  6.3  lumens  per  watt.  Similar 
specific  outputs  for  the  gas-filled  tungsten  lamp  operating  at  16 
lumens  per  watt  are  respectively  7.4  and  n  lumens  per  watt.  For 
the  gas-filled  lamp  operating  at  12  lumens  per  watt  the  corresponding 
outputs  are  6.3  and  9.3  lumens  per  watt. 

COLORED  MEDIA 

Essential  tools  in  applying  color  in  lighting  are  colored  media  and 
a  knowledge  of  the  fundamental  principles  of  the  science  of  color. 
The  latter  have  been  briefly  discussed  in  preceding  paragraphs  and 
a  few  suggestions  regarding  colored  media  are  presented  below. 
Illuminants  differing  in  color  have  been  harmoniously  blended  in 
many  instances  but  the  greater  possibilities  of  such  applications 
naturally  are  found  in  installations  of  great  magnitude.  In  the 
general  practice  of  color  in  lighting  an  acquaintance  with  colored 
media  is  essential.  Among  the  chief  colored  media  are  glasses,  silk 
fabrics,  gelatines,  lacquers,  pigments,  aniline  dyes,  and  chemical 


288  ILLUMINATING   ENGINEERING    PRACTICE 

salts.  Often  a  problem  can  be  solved  very  readily  through  an  ac- 
quaintance with  the  availability  of  colored  media. 

Colored  glasses  can  be  obtained  from  a  number  of  jobbing  houses 
as  well  as  glass  factories.  Fairly  pure  colors  can  be  obtained 
from  manufacturers  of  signal  glasses.  With  little  or  no  correction, 
these  often  afford  excellent  primary  colors  for  applications  of  color 
mixture. 

Colored  lacquers  can  be  obtained  very  readily.  These  are  usually 
of  two  classes,  one  for  indoor  applications  and  the  other  for  outdoor 
uses.  The  latter  are  usually  colored  varnishes  which  resist  the 
action  of  moisture.  Unfortunately  colored  lacquers  are  not,  in 
general,  very  permanent  under  the  combined  effects  of  light,  heat, 
moisture,  and  gases.  To  insure  permanency  it  is  well  to  flow  these 
lacquers  upon  plane  sheets  of  glass  and,  after  drying,  to  protect  them 
with  glass  covers.  If  these  are  installed  in  such  a  manner  as  to 
prevent  undue  heating,  many  ordinary  lacquers  will  be  fairly  per- 
manent. Ventilation  is  quite  essential  and  sometimes  by  placing 
a  metal  screen  of  coarse  mesh  in  contact  with  the  colored  screen 
the  life  of  the  latter  can  be  prolonged.  Lacquers  can  be  colored 
with  aniline  dyes  and  other  materials  providing  a  proper  solvent  is 
employed. 

Often  an  insoluble  pigment  or  dye  can  be  suspended  in  a  binding 
medium  to  a  sufficient  degree  to  enable  lamps  or  glassware  or  other 
media  to  be  colored  by  immersion.  An  air  brush  can  be  used  success- 
fully in  such  work  with  the  advantage  that  it  is  necessary  to  prepare 
only  small  amounts  of  the  colored  solution.  Colored  gelatines  can 
be  obtained  from  theatrical  supply  houses,  a  wide  range  of  colors 
being  available.  For  special  purposes,  and  in  cases  of  emergency, 
gelatine  can  be  dyed  and  flowed  upon  sheets  of  glass.  These  are 
not  permanent  but  their  life  can  be  prolonged  considerably  if  kept 
reasonably  cool. 

Colored  fabrics  such  as  silk  lend  themselves  to  many  applications 
of  interior  lighting.  Colored  solutions  find  uses  especially  in  tem- 
porary lighting  installations  and  in  demonstrations. 

The  method  of  using  these  materials  obviously  varies  with  the 
problem  at  hand.  If  colored  glasses  of  proper  spectral  characteris- 
tics are  available  they  can  be  placed  in  such  a  position  as  to  inter- 
cept the  light  emitted  by  the  illuminant.  However,  if  the  correct 
tint  is  not  at  hand,  it  is  often  possible  to  obtain  the  desired  result 
by  combining  colors  according  to  the  various  methods  of  color- 
mixture.  For  instance  if  a  pink  glass  be  not  available,  a  pink  tinge 


LUCKIESH:  COLOR  IN  LIGHTING  289 

can  be  obtained  by  adding  red  light  and  a  slight  amount  of  blue  or 
violet  to  the  unaltered  light"  emitted  by  an  ordinary  illuminant. 
This  combination  may  be  obtained  by  using  three  light  sources  or 
by  using  a  single  one.  In  the  latter  case  a  checkerboard  pattern 
can  be  built  up  by  means  of  red,  blue,  and  clear  glass;  however,  the 
light  must  be  emitted  from  an  extended  area  so  that  the  colors  are 
well  blended.  Lacquers  can  be  used  in  quite  the  same  manner. 
If  only  one  lighting  unit  is  to  be  used,  the  screen  can  be  made  by 
daubing  on  a  clear  glass  various  spots  of  the  proper  colors.  How- 
ever, lacquers  can  be  readily  mixed  or  diluted  to  obtain  the  desired 
color.  This  can  be  done  by  following  the  principles  of  color  mixture. 
In  general  it  should  be  noted  that  the  eye  cannot  always  be  trusted 
to  judge  the  satisfactoriness  of  a  color  for  a  specific  purpose  because 
it  operates  synthetically  in  regard  to  light-waves. 

APPLICATIONS  OF  COLOR  IN  LIGHTING 

It  has  appeared  advisable  to  supplement  the  broad  general  treat- 
ment of  color  in  lighting  with  brief  descriptions  of  applications. 
Some  of  these  will  be  representative  of  many  similar  cases  but  all 
have  been  chosen  as  useful  illustrations  of  the  great  possibilities  of 
color  in  aiding  and  pleasing  mankind.  This  aspect  of  lighting  is 
endless  in  extent  and  its  ramifications  are  numberless.  No  strict 
classification  will  be  adhered  to  but  in  general  those  having  a  more 
scientific  basis  for  existence  will  be  treated  first  while  those  involving 
chiefly  aesthetic  taste  will  be  discussed  later. 

Artificial  North  Skylight. — This  quality  of  light  is  the  most  generally 
acceptable  for  accurate  color  work,  such  as  in  matching  and  in 
inspecting  colors.  There  is  no  need  to  discuss  it  further  than  to 
record  the  classes  of  work  for  which  it  is  being  used  at  present. 
It  is  not  used  for  lighting  large  areas  in  the  sense  of  general  lighting 
although  there  are  some  rather  extensive  installations  in  existence. 
Artificial  north  skylight  is  at  present  used  in  the  following  places 
and  occupations  and  perhaps  in  others:  department  stores,  color 
printing,  textile  mills,  dye  houses,  laboratories,  cigar  factories 
and  stores,  haberdasheries,  chiropody,  hair  dressing,  sugar  testing, 
dentistry,  painting,  paint  and  wall  paper  stores,  millinery  shops, 
button  factories,  diamond  and  jewelry  shops,  medical  examinations, 
surgical  operations,  microscopy,  chemical  analysis,  flour  mills,  paper 
mills,  garment  factories,  cotton  mills. 

Artificial  Noon  Sunlight. — This  quality  of  light  is  at  present  quite 
19 


2  QO  ILLUMINATING    ENGINEERING   PRACTICE 

extensively  used  for  general  lighting  in  many  cases  where  the  require- 
ments are  not  as  refined  as  in  the  previous  cases,  and  where  eyes  un- 
trained in  refined  color-discrimination  are  involved.  Among  the 
places  and  occupations  in  which  this  quality  of  daylight  is  at  present 
being  used  are  the  following:  lithographing,  paint  shops,  and  stores, 
tailor  shops,  wall-paper  stores,  in  green  houses,  artists'  studios,  art 
galleries,  operating  rooms  in  hospitals,  paper  mills,  flour  mills, 
garment  factories,  shoe  stores,  textile  mills,  florist  shops,  dry  clean- 
ing, laundries,  furniture  stores,  undertaking,  millinery  shops, 
haberdasheries,  art  schools,  and  in  illuminating  color  photographs. 

1  Approximate  Artificial  Daylight. — Under  this  head  will  be  discussed 
briefly  the  applications  that  have  been  made  of  "artificial  daylight'' 
which  is  only  an  approximation  to  average  daylight  being  in  reality 
a  compromise  between  quality  and  efficiency.  This  discussion  of 
the  application  of  this  quality  of  light,  which  in  these  particular  cases 
is  obtained  from  a  tungsten  lamp  corrected  by  means  of  a  colored 
glass  bulb  so  that  its  visible  spectrum  closely  approximates  that  of  a 
black  body  operating  at  a  temperature  midway  between  the  melting 
point  of  tungsten  and  the  apparent  temperature  of  the  sun,  applies 
to  any  other  artificial  light-source  similarly  corrected.  The  effi- 
ciency of  light  production  has  reached  a  point  where  it  has  proved 
expedient  to  obtain  a  better  quality  of  light  by  sacrificing  some  of  the 
light.  Data  are  available  from  installations  involving  many  thou- 
sands of  lamps  but  only  a  few  points  will  be  discussed  for  the  purpose 
of  showing  the  trend  in  this  aspect  of  artificial  daylighting.  A  record 
of  the  applications  of  this  approximate  daylight  includes  all  of  the 
fields  included  under  the  preceding  two  paragraphs  with  the  excep- 
tion of  the  cases  where  very  accurate  color  discrimination  is  required. 
Investigation  shows  that  such  a  quality  of  light  is  in  use  for  general 
lighting  in  the  following  places  and  occupations  and  perhaps  in 
others:  department  stores,  haberdasheries,  cigar  stores,  art  galleries, 
clothing  stores,  millinery  shops,  tailor  shops,  shoe  stores,  jewelry 
shops,  paint  and  wall  paper  stores,  furniture  stores,  undertaking, 
laundries,  dry  cleaning,  medicine  and  surgery,  hospitals,  color  print- 
ing, hardware  stores,  libraries,  grist  mills,  florist  shops,  automobile 
display  rooms,  textile  plants,  illumination  of  color  photographs, 
photographic  studios,  offices,  drug  stores,  hair  goods  shops,  stationary 
stores,  barbor  shops,  laboratories,  microscopy,  grocery  stores, 
confectionary  stores,  upholstering  shops,  breweries,  hair  dressing, 
show  windows,  fur  stores,  and  in  a  number  of  isolated  places.  The 

application  of  this  illuminant  to  art  museums  is  especially  worthy 


LUCKIESH:  COLOR  IN  LIGHTING  291 

of  attention  owing  to  the  exacting  requirements.  A  number  of 
museums  are  at  present  equipped  with  this  illuminant,  notably  the 
Cleveland  Museum  of  Art.  One  interesting  result  has  been  the 
popularity  of  the  museum  at  night.  These  applications  of  artificial 
daylight  are  sufficiently  numerous  and  diversified  to  indicate  that 
consumers  are  not  universally  satisfied  to  accept  the  accidental 
quality  of  light  emitted  by  various  artificial  illuminants  providing  a 
much  better  quality  of  light  can  be  obtained  without  a  prohibitive 
loss  in  luminous  efficiency.  This  is  a  natural  result  of  the  education 
of  the  public  resulting  from  the  activities  of  this  society. 

No  further  discussion  of  the  applications  of  artificial  daylight 
appears  necessary  in  those  fields  which  prominently  involve  the 
appearance  of  colors;  however,  artificial  daylight  has  found  its  way 
into  fields  not  generally  expected.  For  instance,  there  has  always 
existed  a  feeling  of  unsatisfactoriness  in  the  lighting  during  the  period 
of  the  day  when  daylight  must  be  reinforced  by  artificial  light. 
This  is  perhaps  partially  due  to  a  difference  in  the  distribution  of 
light  in  the  two  cases.  However,  the  difficulty  is  also  partially,  if 
not  largely,  due  to  the  difference  in  color.  Experiments  with  arti- 
ficial daylight  for  desk-lighting  have  been  quite  convincing  to  many 
persons.  A  number  of  installations  of  approximate  artificial  day- 
light units  have  indicated  that  this  is  probably  a  large  field  for 
future  development.  Physiological  and  psychological  research  has 
yet  to  explore  this  field.  Many  other  unique  applications  could  be 
discussed  to  advantage  but  it  is  believed  that  sufficient  space  has  been 
given  to  this  subject  at  present.  However,  it  has  been  considered 
profitable  in  this  lecture  to  devote  considerable  space  to  this  develop- 
ment in  lighting  because  it  represents  perhaps  the  stride  of  greatest 
magnitude  and  portend  in  the  application  of  the  science  of  color  in 
lighting  that  has  been  made  recently. 

Applications  of  Color  Mixture. — Many  diversified  applications  of 
the  principles  of  color  mixture  are  open  to  the  lighting  expert.  The 
stage  offers  the  greatest  possibilities  although  ordinary  specifications 
of  stage-lighting  often  provide  only  clear,  red,  and  blue  lamps.  It 
is  obvious  that  the  range  of  colors  resulting  from  mixtures  of  these  is 
quite  limited.  When  it  is  considered  that  the  lighting  effects  are 
valuable  tools  in  the  hands  of  the  stage  director  it  is  wondered  why 
facilities  are  npt  provided  for  using  at  least  the  three  primary  colors, 
red,  green,  and  blue,  and  also  clear  lamps.  If  space  permits  it  would 
be  desirable  to  add  yellow  lamps.  Of  course,  yellow  could  be  ob- 
tained by  mixing  red  and  green  but  inasmuch  as  it  is  an  important 


2Q2  ILLUMINATING   ENGINEERING   PRACTICE 

stage-lighting  color  it  appears  undesirable  to  sacrifice  it  in  obtaining 
the  red  and  green  originally  and  then  to  produce  it  again  by  mixture 
at  a  greatly  reduced  efficiency. 

The  primary  colors  have  been  used  in  show  windows  and  for  many 
special  effects.  One  unique  installation  is  found  in  a  pretentious 
residence.  Red,  green,  and  blue  lamps  are  installed  above  a  large 
oval  panel  of  opal  glass  set  in  the  ceiling  of  a  dining  room.  Any 
quality  of  light  could  be  obtained  by  controlling  various  lamps 
by  means  of  three  rheostats  located  in  a  cabinet  in  the  wall.  A 
number  of  installations  on  a  larger  scale  have  been  placed  in  ball- 
rooms and  restaurants.  Such  applications  should  be  more  numerous 
considering  the  pleasure  obtainable.  A  few  cases  have  been  noted 
where  colored  lights  have  been  mixed  for  the  general  illumination  of 
theatres,  bill  boards,  special  displays,  ball  rooms,  etc.  Flashers  have 
usually  been  used  but  rheostats  can  be  readily  designed  to  be  mechan- 
ically operated  so  as  to  vary  the  intensity  of  the  various  components 
by  imperceptible  increments.  Beautiful  effects  have  been  obtained 
by  illuminating  clothing  models  with  mixtures  of  the  primary  colors, 
accentuating  the  effects  occasionally  by  directed  unaltered  light. 
The  latter  effect  is  intensely  beautified  by  the  colored  shadows  which 
remain  due  to  a  flood  of  colored  light  of  a  lower  intensity  than  the 
clear  directed  light.  Incidentally  this  brings  out  the  point  that 
colored  shadows  can  be  used  in  many  lighting  effects  with  wonderful 
success.  Many  possibilities  of  the  use  of  color  in  lighting  are  found 
in  interiors.  Colored  lights  obtained  by  mixture  provide  pleasing 
variety  and  deal  harshly  with  the  monotony  of  ordinary  lighting 
installations.  In  ordinary  lighting  tints  are  more  satisfying  to  the 
aesthetic  sense  than  saturated  colors  and  these  tints  are  readily 
obtained  by  adding  lights,  fairly  saturated  in  color,  to  the  ordinary 
unaltered  light.  In  general  it  is  necessary  to  conceal  the  sources. 
In  the  home  the  tint  can  easily  be  adapte'd  to  fit  the  place,  the  occa- 
sion, or  the  mood.  Various  possibilities  can  be  provided  in  different 
rooms  or  in  the  same  room.  Moonlight,  sunlight,  candle-light, 
fire-light,  etc.,  can  be  provided  with  ease. 

Recently  a  moving  picture  theatre  has  been  provided  with  a 
yellowish  light  of  low  intensity  for  use  ordinarily  during  the  projec- 
tion of  pictures  and  a  bluish  light  for  use  when  night  scenes  are  on 
the  screen.  This  is  an  example  of  the  many  possibilities  of  using 
colored  light  in  illusory  presentations. 

Colored  light  has  been  used  successfully  in  the  flood-lighting  of 
monuments,  buildings,  and  pageants. 


LUCKIESH:  COLOR  IN  LIGHTING  293 

Special  Color  Effects. — In  a  few  rare  instances  colored  light  has  been 
applied  to  billboards  and  other  displays  and  doubtless  this  field  for 
colored  light  will  be  developed  eventually.  The  play  of  colored  light 
on  properly  painted  displays  is  attractive  and  when  the  efficiency  of 
light  production  has  sufficiently  increased  these  applications  should 
increase  in  number.  Special  color  effects  have  been  proposed  in 
which  complete  changes  are  produced  by  properly  associating  the 
colored  pigments  used  in  painting  the  scene,  or  advertising  material, 
with  the  colored  illuminants.  These  should  eventually  find  a  wide 
field  on  the  stage  and  in  displays.  A  few  applications  have  been 
made  but  the  difficulty  at  present  lies  in  the  necessity  of  a  complete 
grasp  of  color  science  in  order  to  accomplish  the  desired  results. 

Notable  Installations  of  Colored  Light. — A  notable  installation  of 
luminants  of  different  color  and  brilliancy  is  found  in  the  Allegheny 
County  Soldier's  Memorial  Building,  Pittsburgh,  in  which  mercury 
arc,  Moore  tube,  flame  arc,  and  tungsten  lamps  are  woven  into 
harmonious  effect. 

The  applications  of  light  and  color  at  the  Panama-Pacific  Exposi- 
tion are  well  known.  This  installation  represents  one  of  the  greatest 
undertakings  in  lighting  ever  attempted  and  also  stands  as  an 
example  of  the  achievements  that  can  be  attained  by  the  lighting 
expert  who  has  the  hearty  cooperation  of  architects  and  other 
responsible  authorities. 

Simulating  Old  Illuminants. — A  few  applications  of  this  char- 
acter have  been  made  but  it  is  difficult  to  discuss  this  subject 
analytically  because  the  requirements  are  not  sufficiently  exacting 
to  demand  uniformity  in  the  developments.  The  results  have 
been  obtained  by  the  use  of  color  in  ornamental  glassware,  of  colored 
screens  over  the  aperture  of  indirect  units,  of  colored  fabrics,  and 
of  colored  lamps.  Many  interior  lighting  units  approach  this 
result  by  the  unconscious  application  of  warm  tints  to  the  lighting 
accessories.  In  those  cases  where  the  aim  has  been  specifically  to 
simulate  older  illuminants  a  common  error  has  been  made  in  em- 
ploying an  amber  color  instead  of  an  unsaturated  yellow  as  discussed 
earlier  in  this  text. 

Modifying  Daylight. — A  few  installations  of  this  character  have 
been  noted,  the  object  usually  being  to  eliminate  the  cold  appear- 
ance of  daylight  by  using  ceiling  or  side  windows  glazed  with  an 
unsaturated  yellow  glass.  Several  notable  installations  are  found 
in  pretentious  buildings.  A  satisfactory  glass  has  been  obtainable 
in  the  market.  Such  applications  have  their  best  field  in  open- 


2Q4  ILLUMINATING   ENGINEERING   PRACTICE 

ings  where  only  skylight  enters.  A  specific  instance  was  observed 
in  an  elaborate  hotel  where  a  ceiling  window  at  the  bottom  of  a 
lighting  court  was  glazed  with  a  yellowish  glass.  Other  instances 
have  been  found  in  residences.  In  one  case  the  windows  in  the 
dining  room  received  little  sunlight  and  the  windows  were  glazed 
with  a  transparent  yellow  glass.  The  effect  of  the  unobtrusive, 
unsaturated  yellow  glass  was  always  pleasing  and  extremely  so  on 
dismal  rainy  days.  Stained  glass  windows  are  colored  chiefly  for 
decorative  effect  but  the  modification  of  the  light  which  passes 
through  them  often  adds  variety  and  interest  to  the  interior. 

Bibliography 

In  this  general  lecture  it  has  been  thought  best  to  exclude  references  to  various 
investigators  and  practitioners  who  have  contributed  to  the  progress  of  the  art 
because  historical  treatment  would  lead  the  discussion  far  afield;  however,  a 
bibliography  of  the  representative  work  on  the  subject  has  been  appended. 
No  pretense  to  completeness  in  the  bibliography  is  entertained,  although  the 
following  references  have  been  selected  with  this  lecture  in  mind.  Preference 
has  been  given  to  published  work  of  recent  years,  to  those  works  which  include 
extensive  bibliographies  of  the  available  material,  to  discussions  treated  from 
practical  viewpoints,  and  to  the  availability  of  the  publication.  As  a  result  of 
such  a  procedure  and  of  the  desire  to  be  concise,  many  worthy  papers  have  not 
been  directly  mentioned.  However,  by  referring  to  the  various  bibliographies 
found  in  the  publications  actually  referred  to,  a  comprehensive  view  of  the 
various  subjects  can  be  obtained. 

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PAUL  F.  BAUDER.— "  Reflection  Coefficients."    Trans.  I.  E.  S.,  6,  1911,  p.  85. 

Louis  BELL. — "Monochromatic Light  and  Visual  Acuity."  Elec.  World,  57, 
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E.  J.  BRADY.— "Daylight  Glass."    Trans.  I.  E.  S.,  9,  1914,  p.  937. 

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LUCKIESH:-  COLOR  IN  LIGHTING  295 

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H.  E.  IVES  and  E.  J.  BRADY.—  "A  Gas  Artificial  Daylight."  Light,  Jour. 
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H.  E.  IVES  and  E.  F.  KINGSBURY.—  "  Color  Photometry."  Trans.  I.  E.  S., 
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light." Elec.  World,  May  4,  1911;  Lond.  Blum.  Engr.  4,  1911,  p.  394. 

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"Light  and  Shade  and  Their  Applications."     New  York,  1916. 

"The  Language  of  Color"  (in  preparation). 

"  Color  Photometry."     Elec.  World,  May  16,  1914;  April  19,  1913;  Mar.  22, 


"  Monochromatic  Light  and  Visual  Acuity."  Elec.  World,  Aug.  19,  1911; 
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296  ILLUMINATING   ENGINEERING   PRACTICE 

M.  LUCKIESH  and  F.  E.  CADY. — "Artificial  Daylight — Its  Production  and 
Use."     Trans.  I.  E.  S.,  9,  1914,  p.  839. 
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1916,  p.  122. 

A.  J.  MARSHALL. — "Use  of  Tungsten  Lamps  with  Mercury  Arcs."  Trans. 
I.  E.  S.,  4,  1909,  p.  251. 

G.  S.  MERRILL. — "Tungsten  Lamps."     Proc.  A.  I.  E.  E.,  1910,  p.  1709. 

D.  MCFARLANE  MOORE.— "The  White  Moore  Light."     Trans.  I.  E.  S.,  5, 
1910,  p.  209,  n,  1916,  p.  162. 

HUGO  MUNSTERBERG. — "The  Problem  of  Beauty."  Philos.  Rev.,  28,  p.  121; 
Abs.  in  Psych.  Bull.,  7,  1910,  p.  233. 

E.  L.  NICHOLS.— "  Daylight  and  Artificial  Light."     Trans.  I.  E.  S.,  3,  1908, 
p.  301. 

P.  G.  NUTTING. — "Monochromatic  Colorimetry."  Bull.  Bur.  Stds.,  9,  1913, 
No.  187. 

R.  FF.  PIERCE. — "Artificial  Daylight  for  Color-matching."  Amer.  Gas 
Ltg.  Jour.,  99,  1913,  p.  68. 

T.  E.  RITCHIE. — "Color  Discrimination  by  Artificial  Light."  Lon.  Ilium. 
Engr.,  5,  1912,  p.  64. 

W.  D'A.  RYAN.— "Lighting  the  Panama-Pacific  Exposition."  Trans.  I.  E.  S., 
n,  1916,  p.  629. 

C.  H.  SHARP.— "  Daylight  Units."     Trans.  I.  E.  S.,  10,  1915,  p.  219. 

C.  H.  SHARP  and  P.  S.  MILLAR. — "An  Example  of  Use  of  Tungsten  Lamps 
to  Produce  Daylight  Effect."  Trans.  I.  E.  S.,  7,  1912,  p.  57. 

P.  T.  WALDRAM.— "Yellow  Glass  to  Produce  Sunlight  from  Blue  Skylight." 
Lon.  Ilium.  Engr.,  2,  1909,  p.  472. 

M.  F.  WASHBURN,  D.  CLARK  and  M.  S.  GOODELL. — "Effect  of  Area  on  Pleas- 
antness of  Colors."  Amer.  Jour,  of  Psych.,  22,  1914,  p.  578. 

N.  A.  WELLS. — "Affective  Character  of  Colors  of  Spectrum."  Psycho.  Bull., 
7,  1910,  p.  181. 

R.  S.  WOODWORTH.— "The  Psychology  of  Light."     Trans.  I.  E.  S.,  6,  1911,  p. 

437. 

"Color  Photometry"  (Research  Com.  Rep.).     Trans.  I.  E.  S.,  9,  1914,  p.  505. 

"Lighting  of  Cleveland  Art  Loan  Exposition."  Elec.  World,  Dec.  27,  1913, 
P.  1332. 

"The  Lighting  of  Pictures."  Elec.  Rev.  and  W.   E.,  Jan.  17,  1914,  p.  137. 

Report  on  the  Lighting  of  the  Cleveland  Museum  of  Art,  Trans.  I.  E.  S., 
u,  1916,  p.  1014. 


CHURCH  LIGHTING  REQUIREMENTS 

9  BY   EMILE   G.   PERROT 

From  the  very  beginning  light  has  played  a  most  important  part 
in  the  life  of  the  world;  shut  out  light  from  any  living  thing — plant, 
brute  or  man,  and  part  of  life  itself  is  taken  away.  As  light  is 
necessary  to  the  fullness  of  physical  life,  in  like  manner  the  spiritual 
life  of  man  craves  as  its  perfection,  spiritual  light. 

The  old  law  prescribed  a  seven-branch  candlestick  as  part  of  the 
sacred  treasures  to  be  kept  before  the  eyes  of  the  people;  when 
Christ  came  he  voiced  the  need  of  men's  souls  when  he  proclaimed: 
"I  am  the  Light  of  the  World."  As  a  symbol  of  Him,  the  Light  of 
the  World,  the  early  Christians  lit  candles  in  the  dark  chambers  of 
the  catacombs;  symbols  these  lights  were  indeed,  but  they  served 
the  added  purpose  of  illumination. 

So  then,  the  architect,  whether  designer  of  lofty  cathedral  or 
lowly  church,  must  consider  light  both  symbolic  and  illuminant. 

In  the  early  centuries  of  Christianity,  the  use  of  a  multitude  of 
candles  and  lamps  was  undoubtedly  a  prominent  feature  of  the  cele- 
bration of  the  Easter  vigil,  dating,  we  may  believe,  almost  from 
Apostolic  times.  Eusebius  speaks  of  the  "pillars  of  wax"  with 
which  Constantine  transformed  night  into  day,  and  other  authors 
have  left  eloquent  descriptions  of  the  brilliance  within  the  churches. 
The  number  of  lamps  which  Constantine  destined  for  the  Lateran 
Basilica  has  been  estimated  at  8730.  The  practice  of  providing 
immense  hanging  coronoe  to  be  lighted  on  the  great  festivals  seems 
to  have  lasted  throughout  the  Middle  Ages  and  to  have  extended 
to  every  part  of  Christendom. 

We,  in  these  days  of  brilliant  artificial  light,  cannot  easily  realize 
what  unwonted  splendor  such  displays  imparted  to  worship  in  a 
comparatively  rude  and  barbarous  age.  To  these  magnificent 
chandeliers  various  names  are  given,  for  example,  cantharus,  corona, 
stantareum,  pharus,  etc.  Such  works  of  art  were  often  presented 
by  emperors  or  royal  personages  to  the  basilicas  of  Rome. 

Much  more  remarkable,  however,  are  the  remains  of  some  mag- 
nificent metal  work  on  a  vast  scale.  The  great  candelabrum  of 

297 


298  ILLUMINATING   ENGINEERING   PRACTICE 

Reims  was  preserved  until  the  French  Revolution.  It  was  no  doubt 
meant  to  stand  before  the  high  altar  in  imitation  of  the  great  seven- 
branch  candlestick  of  the  temple  of  Jerusalem.  Its  height  was 
over  eighteen  feet  and  its  width  fifteen. 

No  less  wonderful,  and  happily  still  entire,  is  the  great  candela- 
brum of  Milan,  commonly  known  as  "The  Virgin  Tree."  This 
chef-d'oeuvre  of  twelfth-century  art  is  also  a  seven-branch  candle- 
stick and  over  eighteen  feet  in  height.  With  such  great  standing 
candelabra  as  those  of  Reims  and  Milan,  we  may  associate  certain 
large  chandeliers  still  preserved  from  the  eleventh,  twelfth,  and 
thirteenth  centuries.  Those  of  Reims  and  Toul  perished  in  the 
French  Revolution.  But  at  Hildesheim  we  have  a  circular  corona 
of  gilt  copper  suspended  from  the  roof  and  dating  from  1050,  twenty 
feet  in  circumference  and  bearing  seventy-two  candles.  That  at 
Aix-la-Chapelle  is  still  larger  and  still  more  remarkable  for  the 
artistic  beauty  of  its  details.  While  as  a  splendid  specimen  of  later 
medieval  work  is  that  still  preserved  in  the  church  of  Aerschot, 
Belgium,  at  least  until  recently. 

As  an  example  of  a  beautiful  and  at  the  same  time  unique  candela- 
brum, that  in  the  church  of  Leau,  Belgium,  is  extremely  interesting, 
combining  a  lectern  with  the  candelabrum.  The  chapel  of  the 
Hotel  des  Invalides,  Paris,  represents  a  very  fine  type  of  candle 
lighting,  with  two  rows  of  chandeliers.  The  Madeleine  at  Paris  is 
also  a  specimen  of  the  same  method  of  lighting. 

In  the  early  days  the  candle  was  the  only  illuminant,  and  in 
Ronian  Catholic  Churches  it  is  still  required  by  the  rubrics  to  be 
burned  on  the  altar  during  Mass  and  other  ceremonies.  In  the 
advance  of  science,  however,  religion  caught  the  benefit  and  flooded 
its  churches  with  the  imprisoned  sunlight  let  free  from  oil  and  coal. 
When  later  electricity  was  employed,  religion  seized  the  new  light 
to  serve  its  purpose. 

That  we  may  understand  the  "raison  d'etre"  so  to  speak,  of  sym- 
bolism in  the  church,  it  will  be  well  to  consider  briefly  the  subject 
of  symbolism  in  art  and  the  principles  which  underlie  it,  and  which 
give  it  the  importance  it  deserves.  Art  does  not  produce  the  real; 
it  merely  implies  or  suggests  the  real  by  the  use  of  certain  signs  and 
symbols  which  have  been  recognized  as  equivalent.  If,  for  example, 
we  wish  to  bring  to  the  mind  of  another  the  thought  of  water,  we 
do  not  bring  a  glassful  and  place  it  before  the  person;  we  simply  use 
the  word  "water,"  a  word  of  five  letters,  which  bears  no  resemblance 
or  likeness  to  the  real  article,  yet  brings  the  original  to  mind  at  once. 


Fig.  3. — Evangelical  church.     An  excellent  example  of  concealed  chancel  illumination. 


Fig.  4. — Evangelical  church.     Good  example  of  semi-concealed  lighting.     Lamps,  with 
proper  reflectors,  are  placed  on  the  chancel  side  of  the  hammer  beams  and  in  the  rear  of  the 


PERROT:  CHURCH  LIGHTING  REQUIREMENTS  299 

This  is  the  linguistic  sign  for  water.  The  chemical  sign  for  it,  H2O, 
is  quite  as  arbitrary,  but  to  the  chemist  represents  the  original  as 
clearly  as  the  word  does  to  the  mind  of  another.  And  only  a  little 
less  arbitrary  are  the  artistic  signs  for  it.  The  old  Egyptians  con- 
veyed their  meaning  by  drawing  a  zigzag  line  up  and  down  the  wall. 
Turner,  in  England,  often  made  a  few  horizontal  scratches  from  a 
lead  pencil  to  do  duty  for  it,  and  in  modern  painting  we  have  some 
blue  or  green  paint  touched  with  high  lights  to  represent  the  same 
thing.  None  of  these  symbols  attempt  to  reproduce  the  original, 
or  have  any  other  meaning  than  to  suggest  it.  They  are  signs  which 
have  meaning  because  we  agree  beforehand  thus  to  understand 
them. 

Now,  the  agreement  to  understand  the  sign  is  what  might  be 
called  the  recognition  of  the  convention.  All  art  is  in  a  measure 
conventional,  arbitrary,  unreal,  if  you  please.  Everyone  knows 
that  Hamlet  in  real  life  would  not  talk  blank  verse  in  his  latest 
breath. 

The  drama,  and  all  poetry,  for  that  matter,  is  an  absurdity  if  one 
insists  upon  asking,  "  Is  it  natural?  "  It  is  not  natural;  it  is  artificial, 
and  unless  the  artificial  be  accepted  as  symbolizing  the  natural, 
unless  the  convention  of  metre  and  rhyme  be  recognized,  one  is  not  in 
a  position  to  appreciate  verse.  This  is  equally  true  of  music.  The 
opera  is  a  most  palpable  convention,  and  the  flow  of  music  which  so 
beautifully  suggests  the  depths  of  passion  and  the  heights  of  romance, 
is  merely  an  arbitrary  symbol  of  reality.  Recognize  this  and  you 
have  taken  the  first  step  forward  toward  the  understanding  of  art; 
fail  to  recognize  this,  and  art  must  remain  a  closed  book  to  you. 

Furthermore,  the  principle  of  indirectly  representing  by  a  sign  the 
Godhead  or  the  truths  which  He  came  to  establish,  had  its  sanction 
in  the  Divine  Master  Himself,  for  in  His  own  public  life  He  con- 
tinually makes  use  of  parables  and  indirect  means  to  convey  to 
His  followers  the  divine  lessons  He  wished  to  teach. 

It  is  for  this  reason  that  we  find  so  much  use  of  signs,  emblems,  and 
symbolic  expressions  in  the  churches  of  centuries  ago  as  well  as  in 
the  ritual  of  the  religion  taught.  Much  might  be  said  on  this  sub- 
ject, but  a  few  examples  will  suffice  to  present  more  clearly  this  phase 
of  the  subject. 

In  the  tenth  chapter  of  St.  John  we  find  Christ  speaking  in  pro- 
verbs and  referring  to  Himself  as  the  "door." 

In  the  third  chapter  of  St.  Matthew,  at  the  baptism  of  Christ  by 
St.  John  we  read  of  the  "Spirit  of  God  descending  as  a  dove  and 


3OO  ILLUMINATING   ENGINEERING   PRACTICE 

coming  upon  Him."  Thus  we  have  Christ  symbolized  as  a  door,  and 
the  Holy  Ghost  as  a  dove.  Many  other  examples  occur  in  holy  writ 
which  could  be  mentioned  in  this  connection,  but  the  foregoing  rep- 
resent in  a  very  striking  manner  the  direct  use  of  symbols  to 
represent  the  persons  of  the  Deity. 

Among  the  symbols  employed  by  the  primitive  Christians  that  of 
the  fish  ranks  probably  first  in  importance.  The  symbol  itself  may 
have  been  suggested  by  the  miraculous  multiplication  of  the  loaves 
and  fishes,  but  its  popularity  among  Christians  was  due  principally, 
it  would  seem,  to  the  famous  acrostic  consisting  of  the  initial  letters 
of  five  Greek  words  forming  the  word  for  fish  ('Txflus)  which  words 
briefly  but  clearly  described  the  character  of  Christ  and  His  claim  to 
the  worship  of  believers,  that  is,  Jesus  Christ,  Son  of  God,  Savior. 

The  word  then,  as  well  as  the  representation  of  a  fish,  held  for 
Christians  a  meaning  of  the  highest  significance.  After  the  fourth 
century  the  symbolism  of  the  fish  gradually  disappeared. 

Referring  now  to  the  specific  subject  of  illumination,  we  can  con- 
sider the  candle  as  the  symbolical  representation  of  Christ,  "The 
Light  of  the  World"  (the  wax  typifying  the  flesh  of  Christ  born  of 
a  Virgin  mother,  because  of  the  supposed  virginity  of  bees.  The 
wick  symbolizes  more  particularly  the  Soul  of  Christ,  and  the 
flame  the  Divinity  which  absorbs  and  dominates  both). 

The  Christian  religion,  as  we  know  it  in  the  twentieth  century, 
has  formed  itself  into  two  great  bodies,  which  we  may  term  the 
evangelical  and  the  ritualistic.  To  light  a  church  so  that  the  lamps 
may  serve  the  practical  purpose  as  illuminant,  and  at  the  same  time 
keep  the  religious  symbolism  in  the  spirit  of  each  of  these  great 
divisions,  is  the  problem  of  church  lighting  that  I  propose  to  discuss. 

THE  EVANGELICAL  CHURCH 

The  evangelical  church  holds  specially  to  the  Scriptures,  and  the 
keynote  of  its  service  is  the  spoken  word  of  the  expounder  of  the 
Holy  Book.  So  light  must  fill  the  auditorium,  must  center  on  the 
preacher,  as  symbol  of  the  Heavenly  Light  that  he  teaches,  filling 
men's  souls. 

In  the  other  great  division,  a  subdued  light  must  envelop  the 
congregation  as  befits  those  attending  on  great  mysteries,  and  the 
light  must  center  on  the  altar,  shining  against  the  darkness  of  the 
background,  appearing  above  all  else  in  the  church,  as  symbol  of 
the  Light  of  Heaven  resting  on  the  mysteries. 


Fig.  7. — Evangelical    church.     Excellent    example    of    direct    lighting    for    dark-ceilinged 

church. 


Fig.  8. — Ritualistic  church.     A  good  example  of  indirect  lighting  by  means  of  lamps  con- 
cealed on  top  of  cornice. 


PERROT:  CHURCH  LIGHTING  REQUIREMENTS  301 

Thus  we  have,  in  general,  the  thought  underlying  the  scheme  of 
lighting  for  churches  of  both  divisions. 

While  it  may  not  be  possible  to  show  practical  examples  of  lighting 
that  exactly  illustrate  the  principles  enumerated  above,  yet  in  the 
main,  we  will  find  these  principles  carried  out  to  a  greater  or  less 
degree  in  all  well-arranged  churches.  Of  course,  the  architectural 
treatment  of  the  design  will  influence  the  scheme  of  lighting,  but  the 
architectural  scheme  should  follow  the  above  principles,  just  as  the 
lighting  is  intended  to  do;  for  instance,  the  plan  of  evangelical 
churches  naturally  takes  a  form  best  calculated  to  permit  everyone 
to  see  and  hear  the  speaker,  hence  there  are  large  auditoriums  so 
designed  as  to  meet  these  requirements.  On  the  other  hand,  the 
plan  of  ritualistic  churches  aims  not  so  much  to  make  a  perfect 
auditorium  as  a  place  first  for  the  altar,  about  which  the  people 
may  gather  to  take  part  in  the  solemn  sacrifice  which  is  offered 
thereon,  the  part  played  by  the  speaker  being  second  in  importance 
to  the  great  mysteries  of  the  sacrifice. 

Further,  the  ritualistic  ceremony  naturally  begets  symbolical 
forms  in  the  architectural  treatment,  so  that  there  are  depicted 
throughout  representations  of  the  great  mysteries  of  religion,  both 
in  the  structural  parts  and  in  the  minutest  details. 

As  the  problem  of  lighting  evangelical  churches  resolves  itself  into 
that  of  general  illumination,  the  treatment  of  such  buildings  can  best 
be  made  to  follow  the  general  rules  recognized  as  a  standard  for  the 
lighting  of  auditoriums. 

THE  RITUALISTIC  CHURCH 

The  problem  of  lighting  ritualistic  churches,  particularly  Roman 
Catholic  churches,  is  one  that  requires  more  study  since  the  predomi- 
nance of  the  symbolical  over  the  practical  is  very  marked.  There 
is  an  added  problem  in  these  churches  of  decorative  lighting  in  addi- 
tion to  the  practical  and  symbolic  lighting.  This  of  late  years,  has 
become  very  marked,  due  to  the  ease  of  obtaining  decorative  effects 
with  the  use  of  the  many  sizes  and  styles  of  electric  lamps.  A  scheme 
of  lighting  for  a  Catholic  Church  which  does  not  include  facilities 
for  decorative  lighting  around  the  sanctuary  where  the  altars  are 
placed  is  incomplete.  While  the  use  of  candles  on  the  altars  is 
required  by  the  rubrics  of  the  church,  and  they  must  be  used,  the 
added  use  of  electric  and  gas  candelabra  makes  it  possible  to 
obtain  decorative  effects  in  light  for  celebrations  far  surpassing  the 
effect  of  the  candle  light. 


302  ILLUMINATING    ENGINEERING   PRACTICE 

The  principal  reason  why  electric  decorative  lighting  has  come  into 
play  in  this  church  is  due  to  the  fact  that  as  the  church  proper  was 
lit  by  electricity,  the  insignificance  of  the  illumination  of  the  altar 
by  candles  alone  became  very  apparent,  and  as  the  altar  is  the  ob- 
ject for  which  the  church  exists,  and  in  its  symbolical  sense,  should 
be  the  richest  part  of  the  church,  it  was  necessary  to  add  electric 
illumination  to  this  part  of  the  edifice  also. 

To  come  now  to  the  actual  working  out  of  these  principles  to 
concrete  problems,  it  would  be  well  to  endeavor  to  establish  rules 
for  guidance  which  can  be  used  in  most  cases.  The  method  of 
lighting  can  generally  be  included  under  one  of  the  three  systems: 
"Direct  general  illumination,"  "semi-indirect"  and  "indirect,"  or 
a  combination  of  any  two  of  them.  In  examining  the  general  form 
of  evangelical  churches,  it  is  found  that  in  plan  they  may  be  grouped 
as  follows:  Square  or  rectangular  plan,  and  Greek  Cross  plan,  all 
usually  consisting  of  one  clear  span.  The  church  may  or  may  not 
have  a  gallery,  but  as  a  rule,  the  floor  area  in  the  center  must  be 
illuminated  from  the  high  ceiling  above.  Usually  it  is  preferable 
to  hang  chandeliers  from  points  each  side  of  the  center  of  the  build- 
ing. The  use  of  central  chandeliers  is,  as  a  rule,  an  unhappy  solu- 
tion, and  should  be  avoided  unless  the  architectural  treatment  of 
the  ceiling  is  such  as  not  to  permit  of  the  use  of  two  rows  of  fixtures; 
then  the  use  of  one  row  or  one  central  fixture  must  be  resorted  to. 

The  lighting  of  the  chancel  should  be  such  that  ample  light 
falls  on  the  preacher.  Should  there  be  a  chancel  arch,  concealed 
lamps  around  the  arch  produce  a  very  impressive  effect. 

Should  a  gallery  be  used,  the  part  of  the  church  under  the  gallery 
can  best  be  lit  by  ceiling  lamps  under  the  gallery,  or  lamps  can  be 
arranged  around  the  columns  near  the  caps.  If  there  is  no  gallery, 
side  lamps  on  the  walls  are  sometimes  necessary  to  supplement  the 
light  from  the  ceiling.  There  is  no  reason,  though,  why  ample  light 
cannot  be  arranged  for  in  the  ceiling.  The  one  point  to  bear  in 
mind  is  to  avoid  the  use  of  naked  lamps  in  line  with  the  vision  of  the 
congregation.  The  use  of  brackets  with  naked  lamps  on  the  wall 
back  of  the  chancel  is  injurious  to  the  eyes  of  the  people,  and  should 
be  avoided. 

Should  there  «be  a  dome  or  window  in  the  center  of  the  ceiling, 
rows  of  lamps  arranged  to  suit  the  architectural  motives  can  be 
used  instead  of  pendants.  Daylight  effect  can  be  produced  by 
placing  lamps  with  suitable  reflectors  back  of  the  glass. 

When  open  truss  work  occurs  the  fixtures  can  be  suspended  from 


PERROT:  CHURCH  LIGHTING  REQUIREMENTS  303 

the  trusses,  or  else  the  lamps  can  be  concealed  from  the  congregation 
by  being  put  on  the  chancel  side  of  the  trusses  or  hammer-beams. 

Very  effective  and  satisfactory  results  are  obtained  by  using  the 
indirect  method  of  lighting.  This  can  be  accomplished  in  either 
of  two  ways:  one  by  concealing  the  lamps  on  top  of  a  cornice,  or 
in  recesses  on  the  tops  of  column  caps  and  projecting  the  rays  upward, 
depending  on  the  reflected  light  from  the  ceiling  for  the  general 
illuminating  effect;  the  other  by  using  indirect  pendant  fixtures  so 
placed  as  to  harmonize  with  the  architectural  treatment  of  the 
ceiling,  and  projecting  the  light  rays  upward. 

Many  fine  examples  of  this  latter  solution  of  the  lighting  of 
evangelical  churches  can  be  seen.  When  the  artificial  lighting 
is  carefully  worked  out  it  follows  closely  the  effects  of  the  natural 
light  in  the  daytime. 

Semi-direct  lighting  has  been  developed  to  a  point  where  high 
lighting  efficiency  coupled  with  artistic  treatment  have  made  this 
method  very  popular,  since  it  combines  in  a  great  measure  the  eye 
comfort  feature  of  indirect  lighting,  and  at  the  same  time  possesses 
the  artistic  effect  attendant  upon  the  use  of  subdued  visible  light 
sources,  for  it  must  not  be  forgotten  that  when  light  is  present,  the 
eye  unconsciously  seeks  to  determine  its  source,  and  when  this  ceases 
to  be  a  part  of  the  decorative  scheme  the  mind  fails  to  get  full  sat- 
isfaction from  the  illumination. 

Turning  next  to  the  lighting  of  ritualistic  churches,  the  problem 
is  more  complex.  As  outlined  above,  symbolism  plays  an  im- 
portant part  in  the  design  of  such  churches,  so  much  so  as  very  fre- 
quently to  determine  the  shape  of  the  floor  plan.  The  cruciform 
plan  is  the  one  most  generally  used  for  large  churches,  consisting 
of  a  nave  and  two  side  aisles  across  the  church,  and  the  nave  tran- 
septs and  apse  for  the  three  divisions  of  the  length  of  the  church. 
Of  course,  all  churches  do  hot  have  side  aisles,  nor  do  they  all  have 
transepts,  but  this  form  of  floor  plan  is  symbolically  correct,  as  it 
represents  the  emblem  of  salvation,  the  cross. 

Formerly,  the  common  method  of  lighting  was  to  arrange  pendants 
from  the  apex  of  the  main  nave  arches,  thus  making  a  row  of  chande- 
liers in  the  middle  of  the  church.  The  side  lamps  were  usually  ar- 
ranged around  the  columns  or  piers,  sometimes  in  the  form  of  a 
corona,  and  sometimes  as  brackets. 

With  the  advent  of  electric  light,  greater  freedom  of  arrange- 
ment of  the  lamps  became  apparent,  hence  marked  progress  was 
made  by  arranging  rows  of  electric  lamps  in  cornices  or  other  archi- 


304  ILLUMINATING   ENGINEERING   PRACTICE 

tectural  features,  doing  away  with  the  need  of  chandeliers.  How- 
ever, a  combination  of  chandeliers  and  cornice  lamps  has  become 
very  common,  due  to  the  marked  decorative  effect  of  outlining  the 
main  architectural  motives  by  means  of  lamps.  This  arrange- 
ment was  even  attempted  in  former  days  with  gas  lighting. 

A  very  remarkable  example  of  semi-indirect  lighting,  using  gas 
as  the  illuminant,  is  that  of  the  Roman  Catholic  Cathedral  of  Phila- 
delphia. The  burners  are  of  the  kinetic  horizontal  type  which  makes 
possible  the  use  of  gas  as  the  illuminant  in  bowls  for  semi-indirect 
lighting,  and  represent  a  step  far  in  advance  in  the  progress  of  gas 
lighting.  Much  study  was  given  to  the  location  and  design  of  the 
fixtures  and  the  results  are  highly  satisfactory. 

The  arrangement  of  lamps  about  the  sanctuary  where  the  altar  is 
placed  requires  the  utmost  care.  In  addition  to  the  local  lighting  for 
the  altar,  it  is  well  to  arrange  concealed  lamps  back  of  the  sanctuary 
arch  and  pilasters  supporting  the  same  which  can  be  lighted  up  at 
certain  parts  of  the  service  to  flood  the  altar  with  light;  moreover, 
provision  must  be  made  for  the  decorative  lighting,  which  changes 
with  the  seasons  of  the  ecclesiastical  year.  For  instance,  in  Catholic 
churches,  there  are  certain  services  and  parts  of  the  service  which 
require  special  lighting  effects,  due  to  the  nature  of  the  service,  and 
whether  the  Blessed  Sacrament  is  exposed  or  not.  For  grand  cele- 
brations, as  at  Easter  or  Christmas,  special  decorative  effects  in 
lighting  and  decoration  with  plants  and  flowers  are  resorted  to: 
In  one  service  of  the  church  on  Good  Friday,  there  is  a  part  where 
total  darkness  reigns  for  a  few  seconds,  and  then  instantly  a  flow 
of  light  fills  the  church.  While  it  is  not  the  desire  of  the  Church 
in  any  way  to  attempt  theatrical  effects,  it  is  the  intention  to  make 
the  exterior  signs  an  expression  of  the  interior  feeling  one  should 
possess  in  attending  the  service.  As  all  of  these  services  are  to 
be  performed  in  a  strictly  liturgical  manner,  it  is  a  very  delicate 
matter  to  introduce  effects  in  lighting  which  will  not  destroy  the  real 
meaning  of  the  service. 

Before  closing,  I  wish  to  call  your  attention  to  a  very  successful 
scheme  of  day-lighting  which  serves  to  illustrate  very  forcibly  the 
proper  method  of  using  light,  by  suggesting  to  the  beholder  the 
sanctity  of  the  place  and  the  feeling  of  awe  which  should  possess  him. 
I  refer  to  the  tomb  of  Napoleon  in  the  Hotel  des  Invalides,  Paris. 
The  altar  is  in  the  apse  and  the  tomb  in  the  rotunda.  The  window 
in  the  rear  of  the  altar  consists  of  tinted  glass  of  a  golden  hue,  so 
that  the  light  filling  this  part  of  the  interior  is  always  bright,  no 


>  0. — Ritualistic  church.     Semi-indirect  lighting;  two  rows  of  fixtures.     Concealed  light- 
ing back  of  sanctuary  arch. 


Fig.  10. — R.  C.  cathedral  of  Philadelphia.     Semi-indirect  lighting  illustrating  the  diffusion 
obtained  by  the  improved  method  of  installation. 

(Facing  page  304.) 


Fig.   ii. — R.  C.  cathedral  of  Philadelphia.     Semi-indirect  gas  fixture  designed  to  conform 
to  the  interior  decorations. 


Fig.   12. — Ritualistic  church.     Decorative  lighting  for  Christmas  celebration.     Floral  and 

electrical  effects. 


PERROT:  CHURCH  LIGHTING  REQUIREMENTS  305 

matter  whether  the  sun  shines  or  not,  thus  symbolizing  the  living 
presence  of  God  on  the  altar.  While  in  the  rotunda,  casting  its  rays 
upon  the  tomb,  the  light  no  longer  suggests  the  brightness  of  heaven, 
but  on  the  contrary,  is  subdued  by  the  blue  tinting  of  the  windows 
of  the  transepts  and  dome,  thus  reminding  one  that  he  is  walking 
in  the  shadow  of  death. 

The  effect  has  been  so  thoroughly  accomplished  that  anyone,  even 
though  claiming  no  pretext  to  the  understanding  of  art,  can  readily 
feel  the  effect  of  the  lighting  as  soon  as  the  building  is  entered. 

In  conclusion  it  may  be  stated  that  that  scheme  of  lighting  a 
church  is  best  which  considers  illumination  in  its  two-fold  aspect: 
First,  eye-comfort  illumination;  secondly,  the  aesthetic,  which  em- 
bodies those  qualities  which  conduce  to  harmony  in  the  general 
architectural  and  symbolical  treatment  of  the  edifice. 


LIGHTING  OF  SCHOOLS,  LIBRARIES  AND 
AUDITORIUMS 

BY   F.     A.    VAUGHN 
INTRODUCTION 

The  Committee  on  Lectures  has  suggested  that  this  lecture  course 
should  be  supplemental  in  character  to  the  Johns-Hopkins  Univer- 
sity course  of  six  years  ago,  at  which  time  the  establishment  of  the 
basic  principles  on  which  the  science  and  art  of  illumination  rests, 
was  the  main  object.  In  the  f ollowihg  discourse,  therefore,  particular 
stesss  will  of  necessity  be  placed  on  the  progress,  during  the  interim, 
of  the  practical  application  of  those  basic  principles  and  on  the 
utilization  of  the  results  and  investigations  and  findings  of  the 
various  committees  of  the  society  appointed  to  coordinate  and  apply 
those  principles  to  the  various  practical  problems,  especially  those 
involving  public  comfort  and  welfare. 

Since,  also,  the  other  lecturers  in  the  present  course  have  been 
assigned  subjects  dealing  with  general  details  of  design  and  applica- 
tion, covering  the  entire  field  of  the  art,  the  following  lecture  will 
necessarily  be  largely  confined  to  an  analysis  of  the  specific  problems 
covered  by  the  title,  the  application  of  already  established  principles 
to  these  problems  and  a  passing  review  of  typical  and  notable  in- 
stallations exemplifying  these  applications. 

The  subject  is  extremely  broad  and  inclusive,  since  the  illumina- 
tion of  the  various  departments  and  divisions  of  schools,  audito- 
riums and  libraries,  embraces  practically  every  known  type  of  illumi- 
nation, for  all  classes,  ages  and  sexes  of  people.  This  discourse  must 
therefore  be  confined  within  limits,  lest  it  overlap  the  subjects  of 
the  other  lecturers;  therefore  it  will  treat  fully  only  those  require- 
ments which  are  more  or  less  uniquely  applicable  to  the  narrower 
interpretation  of  the  subject,  leaving  the  applications  to  the  more 
general  functions  to  others,  or  to  casual  mention.  For  instance, 
in  the  modern  schools  and  colleges  there  are  offices,  catalogue  files, 
dining  rooms,  gymnasiums,  swimming  pools,  shops,  kitchens,  hos- 
pitals, grounds  and  architectural  exteriors,  which  require  practi- 

307 


308  ILLUMINATING   ENGINEERING   PRACTICE 

cally  the  same  treatment  as  the  same  character  of  spaces  in  other 
establishments. 

In  the  formation  of  this  somewhat  narrower  viewpoint  it  will  be 
assumed  that  the  more  unique  functions  of  the  three  divisions 
of  the  subject  will  be  largely  confined  to  education,  edification  and 
amusement,  especially  studying,  reading,  observing  and  listening. 
For  the  purpose  of  this  lecture,  the  principal  function  of  the  schools 
will  therefore  be  considered  to  be  a  public  place  for  study;  that  of  the 
library,  a  public  place  for  reading,  and  that  of  the  auditorium  a 
public  place  for  seeing  and  hearing.  In  each  case  large  masses  of 
the  public  are  involved,  and  each  is  a  factor  in  the  education,  edifica- 
tion and  amusement  of  the  public,  with  a  preponderance  toward  the 
educational  or  amusement  functions  in  the  order  given.  All  are 
fundamentally  of  general  public  benefit,  welfare  and  uplift,  if  properly 
used,  and  all  deal  with  comparatively  large  spaces,  accommodating 
numerous  people. 

Surely,  it  is  tremendously  important  to  pursue  the  study  of  the 
proper  application  of  the  art  of  good  illumination  to  these  spaces  as 
a  means  of  augmenting  the  beneficial  functions  of  these  institutions, 
and  of  avoiding  the  counteracting  influence  of  the  serious  ill  effects 
of  poor  illumination  on  the  physical,  mental  and  moral  develop- 
ment and  well-being  of  the  public. 

Besides  being  useful  and  efficient,  it  is  demanded  of  the  lighting 
installation  in  each  of  the  divisions  that  it  be  artistic  and  harmo- 
nious. This  requirement  is  at  present  perhaps  most  rigid  in  the  case 
of  auditoriums;  less  so  in  the  case  of  libraries,  and  still  less  in  the 
case  of  schools,  with  a  growing  tendency  at  the  present  time  to  intro- 
duce and  extend  this  influence  more  and  more  into  the  schools,  not 
in  the  form  of  elaboration  or  ornateness,  perhaps, — which  is  not 
necessarily  art;  — nor  in  an  expensive  form — which  does  not  always 
indicate  good  taste — but  by  the  harmonious  application  of  the  in- 
stallation to  the  architectural  features  and  the  functions  of  the  room. 
In  each  division  of  this  subject,  the  importance  of  the  physiological 
effects  as  obtained  through  vision  and  the  use  of  the  eye,  is  tremen- 
dous, and  more  and  more  widespread  in  its  scope  as  we  pass  backward 
toward  the  schools,  where  the  development  and  conservation  of  the 
eyesight  of  our  children  and  youths  for  the  future  is  paramount  and 
of  greatest  economic  importance  to  the  nation  and  the  world.  In 
the  other  two  divisions,  however,  the  conservation  of  the  remaining 
eyesight  of  older  persons  is  only  relatively  unimportant. 

In  each  division  it  is  desirable,  for  the  successful  carrying  on  of 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  309 

the  functions  of  that  institution,  to  produce  certain  psychological 
effects,  exemplified  by  the  quietude  of  the  library,  the  attitude 
of  concentration  in  the  schoolroom,  and  the  inviting,  pleasant  and 
comfortable  atmosphere  of  the  theatre.  The  steady,  daily  repeti- 
tion of  concentrated  study  in  the  schools  is  particularly  conducive 
to  eye  fatigue  and  strain,  and  makes  the  application  of  proper,  com- 
fortable illumination  in  schools  more  important  than  does  the  casual 
frequenting  of  the  library  or  amusement  auditorium,  which  are  still 
respectively  important.  The  problems,  therefore,  while  similar  in 
many  respects,  are  quite  different  in  others, — the  similarities  and 
differences,  from  the  standpoint  of  the  following  basic  requirements 
set  forth  by  L.  B.  Marks  in  the  1910  Lectures,  and  their  different 
gradations,  being  more  or  less  apparent. 

These  fundamental  characteristics  of  good  lighting  then  established 
are: 

1.  Sufficient  illumination. 

2.  Low  specific  brightness. 

3.  Freedom  from  glare. 

4.  Diffusion. 

5.  Cost  of  installation. 

6.  Economy  of  operation. 

7.  Simplicity  and  convenience. 

8.  Esthetic  design. 

In  schools,  the  business  of  concentrated  study  for  the  attainment 
of  mental  development  and  education  has  tended  to  minimize  the 
aesthetic  side.  In  the  library,  a  place  for  study  and  reading,  for  all 
classes  of  readers  of  various  ages  and  conditions  of  eyesight,  and 
developed  and  undeveloped  aesthetic  attitudes,  particular  attention 
must  be  paid  to  the  difficult  problems  affecting  eye  efficiency  and 
comfort  in  connection  with  the  use  of  books,  new  and  old,  in  all 
types  and  on  all  grades  and  conditions  of  paper.  Less  of  the  element 
of  slavish  work,  more  of  the  element  of  recreation  and  entertain- 
ment enters  into  the  use  of  libraries,  where  reading  is  done  for  edu- 
cation, information  and  amusement,  and  there  is  more  inclination 
and  opportunity  for  the  observation  of  the  aesthetic  factors  in  the 
installation.  Auditoriums — which  the  dictionary  defines  as  "The 
part  of  a  public  building,  as  a  theatre,  occupied  by  the  audience; 
hence  any  space  so  occupied" — are  principally  used  for  purposes  in 
which  the  recreation  and  amusement  of  the  public  piedominate, 
with  a  still  greater  inclination  and  tendency  to  observe  and  benefit 
by  the  aesthetic  factors.  No  study  and  very  little  reading  is  pursued 


310  ILLUMINATING    ENGINEERING   PRACTICE 

here;  the  main  function  being  that  of  listening  with  the  eyes  open, 
and  therefore  still  subjected  to  the  good  or  bad  effects  of  the  illumina- 
tion. Auditoriums  used  as  concert  halls,  churches,  lodges  or  even 
expositions,  still  retain  a  large  educational  element,  although  in 
varying  degrees,  and  it  is  thus  apparent  how  these  divisions  of  the 
subject  are  inter-related. 

The  illumination  requirements  may  be,  for  good  and  sufficient 
reasons,  lively  or  subdued;  brilliantly  glittering  or  dull;  intensity 
high  or  low;  diffusion  more  or  less;  installation  simple  or  elaborate; 
expenditure  spare  or  lavish;  operation  economical  or  expensive;  but 
the  architect  and  engineer  must  produce  a  harmonious  solution  of 
the  specific  problem,  which  is  compatible  with  the  results  desired. 

Schools,  auditoriums  and  libraries  all  perform  very  important 
general  public  service  in  the  education  and  uplift  of  the  common- 
wealth. Notwithstanding  the  differences  in  the  classes  and  ages 
of  the  users;  in  the  educational  or  amusement  features;  in  artistic, 
aesthetic  or  psychological  aspects — the  physiological  aspects  are 
the  same  in  all,  in  so  far  as  conservation  of  vision  to  the  greatest 
extent  and  for  the  longest  period  is  of  main  importance,  beginning 
with  preventive  measures  in  the  young  and  ending  with  preserva- 
tive measures  in  the  older.  Even  in  schools  for  the  blind,  where 
at  least  some  of  the  inmates,  including  the  teachers,  still  retain  a 
precious  remnant  of  their  vision,  it  is  perfectly  apropos  to  give  most 
careful  consideration  to  the  planning  of  the  illumination. 

Different  intensities  are,  of  course,  required  for  sufficient  illumina- 
tion of  the  different  classes  of  interiors  and  for  the  different  purposes 
for  which  they  are  used.  During  the  daylight  hours,  the  intensities 
are  relatively  high,  although  very  fluctuating,  and  if  carefully  planned, 
the  natural  illumination  is  well  diffused,  though  ofttimes  definitely 
directed.  Different  intensities  may  be  best  for  young  eyes  or  old 
eyes.  In  theatres  and  lodges,  the  intensity  is  under  the  control  of 
an  operator  and  may  be  adjusted  according  to  the  effects  desired  by 
the  stage  director  or  required  by  the  lodge  services.  In  churches, 
there  may  be  differences  in  intensity  desired  by  different  denomina- 
tions, as  well  as  for  different  congregations  and  services,  the  reading 
services  of  the  Christian  Science  Church  having  set  a  relatively  high 
standard  of  intensity  for  church  illumination,  for  instance.  The 
more  subdued  interiors  of  some  of  the  Gothic  churches  require  a 
more  subdued  religious  atmosphere.  An  example  of  a  peculiar 
requirement  in  a  church  during  war  times  is  of  passing  interest, 
where,  because  of  the  Zeppelin  air  raids,  a  London  church  is  required 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  311 

to  darken  the  top  of  its  auditorium  entirely  by  means  of  opaque 
downward  directing  reflectors. 

The  selection  of  fixtures  for  various  interiors  and  requirements  is 
a  most  important  function,  best  performed  through  the  cooperation 
of  the  building  committee,  the  architect  and  the  illuminating 
engineer.  Cooperation  with  the  architect,  which,  since  the  year 
1910,  has  made  great  progress  in  our  profession,  cannot  be  empha- 
sized too  forcibly,  for  where  an  able  architect  is  retained,  the  best 
general  effects  can  be  obtained  with  his  cooperation;  for  the  prob- 
lem of  the  illuminating  engineer  is  not  so  much  the  selection  of  the 
proper  fixture  design  as  the  selection  of  the  proper  system  and  type, 
quality  and  quantity  of  illumination,  although  every  engineer  who 
assumes  to  deal  with  these  problems  should  conscientiously  train 
his  latent  artistic  faculties,  so  that  when  loaded  with  the  responsi- 
bility for  the  aesthetic  features  of  an  installation  he  can  perform  this 
function  in  a  manner  beyond  the  ridicule  of  the  architect  and  with 
credit  to  hiis  profession,  and  so  that  his  engineering  ability  will  not 
be  driven  into  oblivion  in  the  shadow  of  artistic  criticism. 

Given  by  the  architect  certain  architectural  and  aesthetic  features, 
such  as  Gothic  design;  dark  decorations;  dull  colored  ceilings;  cer- 
tain artistic  effects  and  color  schemes,  the  illuminating  engineer 
must  first  select  the  system  of  lighting  and  type  of  installation  best 
suited  to  these  conditions,  and  the  two  collaborators  may  then  work 
out  a  combination  of  art  and  science  in  the  form  of  the  fixtures,  the 
artistic  outer  garment  being  designed  by  the  architect  with  sufficient 
dimensional  and  spacial  features  to  contain  properly  the  utilitarian 
apparatus,  such  as  the  reflectors,  lamps,  etc.  The  selection  of 
fixtures  or  their  design  will  not  be  specifically  treated  in  this  lec- 
ture, as  the  illustrations  will  be  sufficiently  indicative  and  in  the 
majority  of  cases,  the  reason  for  this  selection,  often  confined  to 
designs  obtainable  generally  on  the  market,  will  be  apparent. 

PART  I.— SCHOOL  ILLUMINATION 

Because  of  the  present  general  appreciation  of  the  hygienic  im- 
portance of  sufficient  and  proper  illumination  in  schools,  it  hardly 
seems  necessary,  except  possibly  to  add  emphasis,  to  make  any  state- 
ment regarding  the  tremendous  service  which  the  illuminating  en- 
gineer can  be  to  the  present  as  well  as  future  generations,  by  his  un- 
stinted activities  in  this  phase  of  the  art  of  illumination.  Because 
of  its  preponderant  importance,  the  major  part  of  the  discussion  in 
this  lecture  will  be  devoted  to  a  review  of  accomplishments  in  this 


312  ILLUMINATING   ENGINEERING    PRACTICE 

direction  and  of  suggestions  for  future  accomplishments.  Within 
the  past  five  years,  not  only  has  the  interest  of  the  Illuminating 
Engineering  Society  been  awakened  through  its  individuals  and 
groups  of  members,  but  the  active  and  productive  interest  of  many 
school  committees  having  charge  of  hygiene,  sanitation,  conserva- 
tion of  vision,  selection  of  textbooks,  and  like  important  matters, 
has  been  aroused,  with  the  practical  results  of  the  recommendation 
and  adoption  of  several  codes  and  rules  tending  toward  the  elimina- 
tion of  the  ill  effects  of  bad  lighting  in  our  schools  through  the  im- 
provements in  illumination  and  otherwise.  A  committee  of  the 
society,  having  in  charge  the  preparation  of  a  "Code  of  School 
Lighting,"  is  practically  ready  to  submit  for  general  adoption  a 
splendid  compilation  of  scientific  and  common-sense  suggestions, 
the  general  adoption  and  enforcement  of  which  would  be  of  inestim- 
able benefit  to  mankind.  Mr.  M.  Luckiesh,  the  chairman  of  this 
committee,  last  year  discussed  these  matters  in  his  paper  on  "  Safe- 
guarding the  Eyesight  of  School  Children,"  and  much  of  the  text  and 
some  illustrations  which  follow  in  this  division  are  excerpted  from 
his  presentation,  and  the  tentative  compilation  of  a  code  prepared 
by  the  committee. 

As  exemplifying  the  present  active  interest  in  this  subject,  outside 
of  the  society,  two  typical  references  may  be  made,  one  from  "  Sani- 
tary School  'Surveys  as  a  Health  Protective  Measure,"  by  J.  H. 
Berkowitz,  published  by  the  New  York  Association  for  Improving 
the  Condition  of  Poor;"  and  one  from  "Recommendations  Adopted 
by  the  Board  of  Superintendents  of  New  York  Schools,"  both  dated 
this  year.  The  first  reference  is  as  follows: 

"In  no  other  respect  is  the  teacher's  responsibility  for  the  physical  well- 
being  of  the  pupils  better  defined  than  with  reference  to  the  protection  of 
eyesight.  Posture  is  important,  of  course,  and  the  proper  adjustment  of 
desks  and  seats  is  a  controlling  factor  in  maintaining  it.  Eye-strain  is 
closely  associated  with  incorrect  posture,  and  likewise  caused  by  poor 
seating  arrangements. 

"Height  of  desk  and  seat,  distance  from  each  other,  distance  from 
blackboard,  etc.,  are  some  of  the  factors  to  be  considered  in  relation  to 
eyesight.  The  prevention  of  glare  from  excessive  light  and  reflective  sur- 
faces-is of  the  utmost  importance,  and  yet  perhaps  the  easiest  to  attain. 
The  proper  means  being  provided,  it  rests  entirely  with  the  teacher. 
Perhaps  the  function  of  window  shades  and  their  usefulness  are  not  fully 
appreciated,  but  teachers  should  know  that  glare  and  intense  direct  light 
cause  eye  fatigue.  This  is  particularly  harmful  to  the  immature  and 
highly  susceptible  eyes  of  children. 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  313 

"Unfortunately,  in  most  of  the  classrooms  surveyed  the  testimony 
found  is  against  teachers  and  others  responsible  for  the  welfare  of  the 
pupils.  Torn,  unworkable  window  shades,  particularly  in  classrooms, 
which  give  an  unobstructed  exposure  to  the  rays  of  the  sun,  are  a  menace 
to  children.  Aside  from  the  necessity  of  proper  manipulation  of  shades 
for  the  regulation  of  light,  the  simple  obligation  is  imposed  on  the  teacher 
to  report  promptly  any  damage  which  may  effectively  interfere  with  the 
proper  use  of  such  shades. 

"The  school  authorities  can  be  expected  to  correct  defects  only  if  they 
are  brought  to  their  attention." 

The  second  reference  cites  rules  prepared  by  Dr.  I.  H.  Gold- 
berger,  Assistant  Director  of  Educational  Hygiene,  for  the  Board  of 
Superintendents. 

"There  are  scores  of  classrooms  in  the  public  schools  that  are  poorly 
lighted.  The  women  principals  have  been  working  for  years  to  have  these 
rooms  closed,  but  without  result.  Usually  they  are  in  buildings  which  are 
congested  and  all  available  space  must  be  used.  The  conditions  are 
recognized  as  serious  and  to  counteract  the  ill-effects  of  studying  and 
reciting  in  such  rooms  the  board  of  superintendents  has  had  prepared  the 
following  recommendations  for  the  guidance  of  teachers  in  such  rooms  as 
are  insufficiently  illuminated: 

"i.  Artificial  illumination  should  be  used  whenever  necessary.  No 
rule  can  be  laid  down  to  guide  the  teacher  in  this  matter.  She  must  use 
her  own  discretion  and  judge  when  artificial  light  is  necessary.  It  must 
be  used  at  once  if  pupils  exhibit  any  difficulty  in  reading. 

"2.  Teachers  should  be  alert  to  report  to  the  principal  if  the  windows, 
walls,  or  prismatic  glass  reflectors  are  not  clean. 

"3.  Dark-colored  pictures  should  not  be  hung  on  the  walls,  and  dark- 
colored  charts  should  be  displayed  only  when  necessary,  for  these  diminish 
the  light  in  the  classroom. 

"4.  Teachers  should  refrain  from  placing  curtains  or  any  other  obstruc- 
tions in  the  window. 

"5.  Window  shades  should  be  kept  rolled  up  as  much  as  possible. 
Attention  should  be  paid  to  the  proper  regulation  of  the  shades,  protecting 
the  children's  eyes  from  insufficient  or  excessive  light. 

"6.  To  favor  the  maintenance  of  the  proper  reading  and  working  dis- 
tance, pupils  should  be  seated  so  far  as  possible  at  dfcsks  according  to  their 
size.  Janitors  are  under  the  by-laws  required  to  make  adjustment  of 
furniture  upon  instruction  from  the  principal.  Children  having  defective 
vision  should  be  seated  as  near  as  possible  to  the  front  of  the  room. 

"7.  The  eyes  should  be  raised  occasionally  from  the  work,  and  there 
should  not  be  two  consecutive  periods  of  close  eye  work." 

If  boards  and  individuals  having  the  responsibility  of  the  solution 
of  these  problems  are  making  such  sincere  and  effectual  efforts, 


314 


ILLUMINATING   ENGINEERING   PRACTICE 


surely  the  Illuminating  Engineering  Society  and  its  members  who 
are  specializing  in  this  work  can  be  of  great  assistance  through  the 
adoption  of  a  code  of  school  lighting,  and  by  the  further  promulga- 
tion of  the  correct  basic  principles  through  the  Society,  reciprocal 
meetings  with  other  societies,  and  by  other  cooperative  means. 
The  problem  exists  to-day,  in  several  thousand  schoolrooms,  affect- 
ing millions  of  children.  A  remedy  should  be  speedily  applied.  It 
is  a  matter  for  most  hearty  congratulation  that  it  is  actually  being 
done. 

A  full  consideration  of  the  subject  of  illumination  in  schools  must 
include  illumination  by  daylight,  which,  because  of  its  close  rela- 
tionship to  the  design,  location  and  position  of  the  building,  requires 
the  intimate  cooperation  of  architect  and  engineer. 

The  orientation  of  the  building,  as  well  as  the  arrangement  of  the 
ceiling  and  side  windows,  and  other  openings  for  admitting  day- 
light, should  be  so  planned  that  direct  sunlight,  for  at  least  a  por- 
tion of  the  day,  enters  the  classrooms  and  other  important  portions 
of  the  building.  The  order  of  preference  of  exposures  is  given  by 
the  Code  as  easterly,  southerly,  westerly  and  northerly.  There 
should  be  sufficient  light  in  the  darkest  working  space  and  as  good 
uniformity  as  possible  everywhere.  The  minimum  daylight  in- 
tensities proposed  are  as  follows: 


Illumination  i 

n  foot-candles 

Minimum 

Desirable 

Storage,  passageways,  stairways,  etc.     . 

o.  5 

o.  ^—  =;  .0 

Classrooms,  study  rooms,  libraries  
Auditoriums 

4.0 

•2     O 

4-20 
3—  10 

Sewing,  drafting,  etc  

IO.O 

10—  30 

Blackboards      

40 

4—20 

On  account  of  the  difficulty  of  control  and  regulation  of  day- 
light, actual  or  average  working  intensities  cannot  be  readily  es- 
tablished, but  the  ceiling  and  side  window  shades  and  curtains 
should  be  of  such  optical  and  mechanical  characteristics  and  be  so 
maintained  and  operated  as  always  to  keep  the  illumination  well 
above  the  minimum  set  forth  and  to  protect  the  children's  eyes  as 
much  as  possible  from  glare.  The  daylight  intensities  will,  of  course, 
generally  be  higher  than  by  artificial  illumination.  Daylight 
illumination  should  be  unilateral.  It  should  not  enter  the  classroom 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC. 


from  the  right,  but  should  enter  from  the  left,  with  a  minimum  win- 
dow area,  ordinarily,  of  not  less  than  one-fifth  of  the  floor  area. 
Windows  in  the  rear  may  produce  glare  upon  the  blackboard,  and 
therefore  should  be  regulated  with  care. 

Natural  lighting  through  wired  glass  windows  in  the  ceiling  may  be 
superior,  but  the  architectural  difficulties  usually  prohibit  this 
method  from  adoption  in  as  many  cases  as  would  be  desirable. 
Lighting  from  side  windows  can  be  brought  nearer  to  ceiling  window 
uniformity,  by  attention  to  their  proper  proportion  to  floor  area  and 
effective  visible  sky  area,  and  by  the  scientific  use  of  prismatic  and 
other  forms  of  diffusing  and  directing  window  glass,  and  by  the  con- 
struction of  relatively  narrow  schoolrooms,  with  high  windows 
and  highly  reflective  surroundings. 

With  artificial  illumination,  as  high  intensities  cannot  be  eco- 
nomically obtained  as  with  daylight,  nor  are  they  required,  and  the 
following  average  intensities  have  been  recommended. 


Illumination  i 

n  foot-candles 

Minimum 

Desirable 

Storage  corridors,  stairways,  etc  
Rough  shop  work 

0.25 
I     2S 

0.25-  5.0 
1.2^—    2  .  < 

Fine  shop  work  
Sewing,  drafting,  and  the  like 

3-5 

C    o 

3-5  ~  6.0 
5  .0  —  10.0 

Auditorium  

2.O 

2.O  -   4-O 

Classrooms,  study  rooms  and  libraries.  .  .  . 
Gymnasium 

3-o 

I    O 

3-o  -  5-o 
i  .0  —  <>  .0 

Laboratories  
Blackboards 

3-0 

•i    o 

3-o  -  S-o 
•?  .0  —  «»  .0 

Sufficiency  of  illumination,  which  is  the  prime  factor,  can  be 
secured  by  any  one  of  the  three  general  arbitrary  types  of  systems 
of  illumination  known  as  direct,  semi-direct,  and  indirect.  Some  of 
the  other  important  factors,  however,  may  be  best  produced  only 
by  one  system  or  another.  Greater  diffusion  can  be  obtained  by 
larger  indirect  component,  while  definiteness  of  control  can  be 
obtained  by  the  direct  system  with  properly  selected  reflectors. 
Through  the  exhaustive  work  of  Drs.  Ferree,  Rand  and  Nutting,  J.  R. 
Cravath,  A.  J.  Sweet,  T.  W.  Rolph,  and  others,  the  relative  advantages 
of  the  various  systems  with  relation  to  eye  fatigue  and  ocular  com- 


316  ILLUMINATING   ENGINEERING   PRACTICE 

fort,  have  been  so  thoroughly  investigated  and  discussed  as  to  need 
no  repetition  here.  Since  diffusion,  as  obtained  by  the  indirect  sys- 
tem, tends  to  minimize  the  glare  from  the  light  sources,  and  from  the 
desks  and  furnishings,  as  well  as  from  the  pages  of  the  books,  the 
indirect  is  preferable  to  the  direct  system.  Since  the  semi-indirect 
system  is  intended  to  be  merely  an  intermediate  between,  or  com- 
bination of  the  direct  and  indirect,  it  may  possess  more  or  less  of 
the  advantages  or  disadvantages  of  either  system,  according  as 
the  particular  installation  possesses  more  or  less  of  one  compo- 
nent or  the  other.  Good  results  can  be  obtained  by  the  use  of 
translucent  bowls  if  the  brightness  of  their  surfaces — which  means 
the  direct  component  of  illumination  which  passes  through  the 
glass — be  kept  down,  thus  producing  really  an  indirect  installa- 
tion with  relatively  faintly  luminous  bowls.  In  this  way  the 
semi-indirect  is  merged  into  the  indirect. 

Localized  illumination  has  no  place  in  the  solution  of  this  problem. 
Approximate  uniformity  on  the  working  plane,  is  highly  desirable 
and  could  not  be  obtained  thereby.  Light  sources  must  be  kept  out 
of  the  range  of  vision,  and,  especially  in  the  direct  system,  should 
be  relatively  small  in  size  and  large  in  number  and  should  never 
be  visible  against  a  dark  background.  Bare  lamp  filaments  must 
be  shaded,  screened  or  concealed.  This  is  particularly  necessary 
when  the  modern  extremely  brilliant  lamps  are  used.  Luckiesh 
states  that  the  brightest  permissible  object  visible  from  any  normal 
position  of  the  observer  should  be  not  greater  than  250  milli-lam- 
berts  with  a  maximum  permissible  brightness  contrast  in  the 
normal  visual  field  not  greater  than  20  to  i.  The  Committee  on 
Glare  of  the  Illuminating  Engineering  Society,  in  an  effort  to 
establish  these  and  similar  standards,  compiled  several  reports 
during  the  last  year  treating  very  thoroughly  the  subjects  of 
brightness  and  glare. 

Other  committees  of  the  Illuminating  Engineering  Society  have 
established  a  contrast  ratio  of  100  to  i,  to  apply  over  the  whole 
working  space.  The  above  ratio  of  20  to  i  should  be  interpreted 
as  applicable  to  juxtaposed  surfaces. 

It  should  be  stated  that  all  wall  areas,  ceilings,  and  other  reflect- 
ing surfaces  should  be  matte;  the  ceilings  bright,  especially  for 
indirect  systems,  and  the  walls  only  moderately  so;  the  general 
idea  being  to  avoid  great  contrasts,  mental  depression  and  poor 
efficiency. 

A  matter  of  almost  as  much  importance  as  all  that  has  thus  far 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  317 

been  stated  is  the  character  of  the  surfaces  at  which  the  children 
have  to  look  and  with  which  they  have  to  work,  especially  that  of 
the  paper  used  in  the  school  books.  A  great  amount  of  attention 
has  been  given  to  the  subject  of  glare  from  the  pages  of  school  books 
and  sufficient  interest  has  been  aroused  to  insure  the  future  adop- 
tion of  paper  with  sufficiently  matte  surface  to  minimize  specular 
reflection  and  resultant  glare  and  to  assure  the  selection  and  use  of 
comfortable  reading  type,  form  of  letter,  spacing  of  lines  and  letters, 
and  the  use  of  suitable  qualities-  of  ink. 

Well-diffused  uniform  illumination  from  and  on  matte  surfaces, 
and  produced  by  screened  or  concealed  sources,  minimizes  glare  and 
contrasts  and  is  conducive  to  ocular  efficiency  and  comfort. 

Glare,  as  clearly  denned  and  analyzed  in  the  various  reports  of 
the  Committee  on  Glare  of  this  year,  is  the  omnipresent  bugbear 
particularly  of  school  illumination.  After  its  reduction  by  conceal- 
ing or  screening  the  light  sources;  properly  locating  and  proportion- 
ing the  windows;  using  matte  reflecting  surfaces,  unpolished  desks, 
and  especially  prepared  book  paper,  it  still  lurks  on  the  surfaces  of 
the  blackboard.  Mr.  Luckiesh,  who  treats  this  subject  simply  and 
plainly,  states  that: 

"Glare  due  to  specular  reflection  from  glossy  blackboards  can  be  re- 
duced or  eliminated  by  lighting  them  by  means  of  properly  placed  and  well- 
shielded  artificial  light  sources.  In  Fig.  5  in  Mr.  Luckiesh's  paper 
on  "Safeguarding  the  Eyesight  of  School  Children" — page  181  of  the 
Transactions  of  the  Illuminating  Engineering  Society,  Nov.  2,  1915  are 
shown  simple  graphical  considerations  of  blackboard  lighting.  In  a  is 
shown  a  plain  view  of  a  room  with  windows  on  one  side.  Rays  of  light 
are  indicated  by  A,  B,  and  C  in  a  horizontal  projection.  These  are  sup- 
posed to  come  from  the  bright  sky.  By  the  application  of  the  simple 
optical  law  of  reflection — the  angle  of  incidence  is  equal  to  the  angle  of 
reflection — it  is  seen  that  the  pupils  seated  in  the  shaded  area  will  experi- 
ence glare  from  the  blackboard  on  the  front  wall.  In  b  is  shown  the 
vertical  projection  of  the  foregoing  condition.  It  is  seen  from  this  graph- 
ical illustration  that  by  tilting  the  blackboard  away  from  the  wall  at  the 
top  edge  the  pupils  in  the  back  part  of  the  room  will  be  freed  from  the 
present  glaring  condition.  Whether  or  not  this  tilting  will  remedy  bad 
conditions  can  be  readily  determined  in  a  given  case.  In  b  the  effect  of 
specular  reflection  of  the  image  of  an  artificial  light  source  is  shown  by  D. 
In  c  is  shown  a  proper  method  of  lighting  blackboards  by  means  of  artifi- 
cial light  sources.  This  will  often  remedy  bad  daylight  conditions  whether 
due  to  an  insufficient  illumination  intensity  of  daylight  or  due  to  reflected 
images  of  a  patch  of  sky." 


318  ILLUMINATING    ENGINEERING    PRACTICE 

The  psychological  as  well  as  the  physiological  relation  of  colored 
illumination  to  the  hygienic,  as  well  as  to  the  aesthetic,  conditions  in 
schools  and  other  places  of  intensified  and  concentrated  visual  opera- 
tions is  important  and  has  been  thoroughly  investigated  by  Mr. 
Luckiesh  and  referred  to  by  Messrs.  Black  and  Vaughn,  and  others. 

The  editor  of  the  Illuminating  Engineer  once  said: 

"  Lighting  installations,  like  men,  may  be  notable  for  defects  as  well  as 
virtues.  They  have  this  also  in  common  with  men,  that  they  are  seldom 
hopelessly  bad  or  perfectly  good. 

"Careful  observation,  retentive  memory,  and  a  habit  of  analytical 
reasoning  are  the  basis  upon  which  experience  builds  the  structure  of 
skill  which  is  above  rules  and  formulae. 

"An  impartial  study  of  different  lighting  systems,  with  a  view  to  frank 
criticism  and  comparison  is  one  of  the  best  of  all  methods  of  acquiring 
facility  of  judgment." 

With  the  spirit  of  these  remarks  in  mind,  let  us  now  analytically 
review  several  illumination  installations  exemplifying  the  good  and 
bad  features  discussed  above. 

For  instance,  a  school  room  is  improperly  illuminated  by  daylight 
when  the  light  is  admitted  mainly  from  the  right-hand  side,  as 
shadows  of  the  hand  and  arm  are  thrown  on  the  working  plane. 
This  produces  illumination  which  will  result  in  eye-fatigue.  A  school 
room  with  proper  daylight  illumination  is  one  in  which  the  light  is 
admitted  from  the  left  side  and  the  rear,  as  this  gives  as  nearly  a 
perfect  arrangement  for  maximum  high  efficiency  as  it  is  practical 
to  obtain  without  ceiling  windows. 

In  Fig.  i  (Fig.  7  of  Mr.  Luckiesh's  paper,  "  Safeguarding  the  Eye- 
sight of  School  Children,"  above  referred  to)  is  shown  an  extremely 
bad  condition,  found  in  a  schoolroom  where  drafting  is  taught. 
With  such  localized  type  of  lighting  each  pupil  can  adjust  the  unit 
to  suit  himself,  with  the  result  that  he  is  not  only  injuring  his  own 
eyesight  and  doing  poor  work  on  account  of  misdirected  light,  but 
at  the  same  time  is  subjecting  many  other  pupils  to  bad  light  condi- 
tions, on  account  of  the  position  of  his  lamp.  Actual  inspection  in 
this  case,  Mr.  Luckiesh  says,  showed  that  most  of  the  pupils  were 
using  the  light  improperly  and  were  often  working  in  such  a  way  as  to 
produce  annoying  shadows. 

However,  a  condition  which  is  even  worse  than  that  in  the  draft- 
ing department  just  shown  is  one  where  the  lamps  are  suspended  from 
cords,  as,  with  the  type  of  angle  steel  reflector  in  popular  use,  the 
lamp  is  always  exposed  to  the  view  of  anyone  on  the  opposite  side 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  319 

of  the  reflector.  Due  to  the  fact  that  cords  are  used,  the  lamps  do 
not  stay  in  any  fixed  position,  but  have  a  tendency  to  swing  back  and 
forth  on  account  of  air  currents  or  when  hit  by  the  students,  so  that 
the  light  proves  very  unsatisfactory,  as  is  always  the  case  when 
lighting  equipment  is  used  in  such  a  way  as  to  permit  of  adjustment 
by  the  individual. 

In  Fig.  2  is  shown  a  drafting  or  drawing  room  in  which  use  is  made 
of  the  indirect  system  of  lighting.  It  is  to  be  noted  that  all  of  the 
table  tops  are  brightly  and  yet  uniformly  lighted,  and,  due  to  the 
large  expanse  of  lighted  ceiling,  no  shadows  are  encountered  by  the 
draftsmen  when  working. 

In  the  manual  training  department  of  a  modern  technical  high 
school  (Fig.  3),  localized  lighting  is  used  in  the  wood-working  shop, 
where  angle  reflectors  with  bare  lamps  are  employed.  It  was  noted 
that  one  of  the  lamps  had  been  tied  up  in  order  to  obtain  a  more 
desired  (though  not  necessarily  desirable)  illumination,  and  that  in 
another  cord  a  knot  had  been  tied  evidently  for  a  like  purpose.  A 
student  working  on  any  of  the  benches  would  have  the  glare  from  the 
exposed  lamps  of  all  the  various  units.  A  view  was  taken  in  the  day- 
time, with  the  lamps  used  just  long  enough  to  show  the  location  of 
the  bulbs.  It  would  have  been  impossible  to  have  taken  a  satisfac- 
tory picture  entirely  by  the  light  of  the  units,  since  nothing  would 
have  shown  except  the  lamps  themselves  and  a  small  portion  of  the 
bench  directly  in  front  of  them. 

In  the  wood-working  machine  shop  (Fig.4),  the  illumination  is 
furnished  by  direct  units  with  mirrored  glass  reflectors  with  opaque 
green  enamel  backing.  The  effect  is  to  give  an  even  illumination 
over  all  the  benches;  at  the  same  time  so  screening  the  light 
sources  as  practically  to  eliminate  all  glare  unless  a  person  de- 
liberately looks  up  into  the  reflector.  It  may  be  pointed  out 
here  that  while  it  is  perfectly  plain  and  well  known  that  by  the 
multiplicity  of  units  per  unit  of  area  greater  uniformity  of  inten- 
sity of  illumination  and  greater  dispersion  of  shadows  can  be  secured, 
it  may  not  be  so  apparent  to  all  that  the  same  tendency  obtains  with 
the  indirect  system  and  that  sometimes  numerous  indirect  fixtures 
may  be  used  in  a  given  area  and  sometimes  few,  according  to  the 
results  demanded  and  the  funds  available.  The  higher  grade  dis- 
tribution can  be  secured  with  few  fixtures,  if  proper  selection  of 
reflector,  lamp,  position  and  hanging  length  is  obtained,  so  as  to  dis- 
tribute the  light  as  uniformly  as  possible  over  the  ceiling. 

In  the  sewing  room  of  a  domestic  science  department  in  a  public 


320  ILLUMINATING   ENGINEERING   PRACTICE 

school  (see  Fig.  12,  Luckiesh's  paper  "Safeguarding  the  Eyesight  of 
School  Children"),  steel  reflectors  suspended  on  flexible  cords  have 
been  used  with  the  result  that  bare  lamps  are  constantly  within  the 
direct  range  of  vision  of  the  pupils,  giving  the  worst  possible  condi- 
tions for  working. 

The  domestic  science  room  in  a  high  school  was,  again,  so  furnished 
with  improper  daylight  lighting  that  those  working  on  one  side  are  in 
their  own  light  with  the  windows  at  their  backs;  those  on  the  other 
side  of  the  table  get  the  full  glare  of  the  windows  in  their  eyes;  while 
those  at  the  far  end  of  the  room  have  the  light  coming  in  from  the 
right-hand  side,  and  only  a  few  can  secure  whatever  advantages  there 
may  be  in  this  position — the  whole  arrangement  being  very  unsatis- 
factory. The  tables  should  be  so  arranged  at  right  angles  to  the 
windows  that  at  least  the  largest  possible  number  receive  the  day- 
light from  the  left.  On  the  other  hand,  the  interior  of  a  model 
economics  class  kitchen  in  a  college  was  so  illuminated  artificially  as 
to  result  in  absolutely  uniform  illumination  on  the  stove,  table 
and  kitchen  cabinet,  with  entire  absence  of  glare.  This  was  accom- 
plished by  means  of  the  indirect  system  of  lighting  which  has  the 
advantage  that,  no  matter  where  a  person  is  at  work  in  this  room, 
very  little  shadow  will  be  occasioned  by  his  or  her  position. 

In  contradistinction  to  the  above,  one  usually  finds  in  a  university 
chemical  laboratory,  for  instance,  old  types  of  lighting  in  which  the 
daylight  conditions  are  very  nearly  as  poor  as  those  of  artificial  illumi- 
nation. What  the  effect  in  these  rooms  is  at  night  is  known  to  most 
of  us  from  experience.  The  usual  very  low  mounting  height  and  the 
general  type  of  reflector  used,  its  design  evidently  being  based  on 
uniqueness  of  outline  rather  than  on  the  principles  of  reflection  or  eye 
efficiency  and  comfort,  make  this  usual  plan  almost  unbearable. 

A  notable  grateful  exception  to  the  above  method  of  illuminating 
school  work  rooms  is  found  in  the  chemical  laboratory  in  the  Boy's 
High  School  at  New  Orleans.  The  photograph  of  Fig.  5,  taken  at 
night,  shows  conditions  with  artificial  illumination.  Being  lighted 
by  the  indirect  system,  there  is  uniformity  of  illuminatibn  and 
absence  of  glare  and  shadows  which  is  desirable  for  a  room  of  this 
character. 

It  may  be  interesting,  for  the  sake  of  emphasis,  to  inspect  one  or 
two  examples  of  typically  bad  schoolroom  lighting  as  it  exists  to-day 
in  many  of  our  larger  as  well  as  smaller  cities,  where  the  millions  of 
school  children  are  subjected  to  the  deleterious  influence  of  eye- 
fatiguing  installations. 


Fig.   i. — A  poor  arrangement  with  localized,  adjustable  lighting  equipment  in  a  drafting 

room. 


Fig.  2. — A  drawing  room  properly  illuminated  by  a  totally  indirect  system,  with  simple 

opaque  fixtures. 

(Facing  page  320.) 


Fig.  3- — Manual  training  department  of  a  school  with  very  poor  arrangement  of  localized 

lightirig. 


Fig.  4. — Good  arrangement  of  general  illumination  by  direct  lighting  units  in  wood-working 

shop. 


Fig.  5. — Indirect  lighting  or  chemical  laboratory, 


Fig.  6. — Typical  poor  lighting  in  a  highschool  assembly  room. 

(Facing  Figs.  3  and  4.") 


Fig.  7. — Good  direct  lighting  installed  in  an  old  schoolroom. 


Fig.  8. — Classroom  lighting  with  a  few  indirect  units. 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  321 

The  usual  low-hanging  height  of  units,  as  well  as  the  too  often  total 
absence  of  reflectors  and  shades,  produces  the  worst  possible  lighting 
conditions  for  a  room  which  is  constantly  used  by  a  large  number  of 
students  as  a  place  of  study.  Nevertheless,  this  was  found  to  be  the 
case  in  a  large  assembly  room  of  a  western  high  school  and  can 
be  duplicated  almost  anywhere.  (Fig.  6.)  (See  also  Luckiesh's 
paper  "  Safeguarding  the  Eyesight  of  School  Children,  Fig.  16.) 

Sometimes  an  attempt  is  made  to  improve  the  very  bad  conditions 
produced  by  old-style  gas  fixture  illumination,  by  the  use  of  direct 
prismatic  reflectors  fastened  directly  to  the  old  fixtures;  but  this, 
while  efficient,  was,  in  the  case  under  the  writer's  observation,  im- 
properly installed  as  far  as  avoidance  of  glare  was  concerned  be- 
cause the  old  fixtures  were,  as  they  almost  invariably  are,  too  long 
and  the  glare  therefore  bad.  A  much  more  satisfactory  as  well  as 
cheaper  installation  could  have  been  made  by  eliminating  many  feet 
of  fxture  stems  and  raising  the  units  beyond  the  range  of  vision  of 
the  tudents. 

E\  en  a  newly  planned  school  lighting  system  may  be  faulty,  as 
an  architect's  modern  plan,  which  the  writer  has  in  mind,  called  for 
one  central  direct  lighting  unit  per  classroom,  whereas  a  vast  im- 
provement would  be  obtained  by  four  units  uniformly  spaced  on  the 
ceiling. 

Fig.  7  illustrates  an  old  building  which  has  been  equipped  with  an 
improved  system  of  direct  lighting.  As  will  be  noted,  the  units  are 
hung  'high,  and  the  light  sources  are  shielded  by  means  of  deep 
diffusing  reflectors.  The  reflectors  allow  enough  light  to  pass 
through  to  light  the  ceiling  to  a  low  intensity,  which  adds  to  the 
cheerful  appearance  of  the  room. 

In  Fig.  8  is  shown  a  classroom  in  the  Boy's  High  School  of  New 
Orleans.  This  is  an  installation  of  indirect  lighting,  with  few  units, 
giving  quite  uniform  illumination  and  cutting  down  the  objectionable 
reflection  from  the  varnished  surfaces  of  the  desks,  glass  or  black- 
boards. 

A  satisfactory  and  inexpensive  indirect  system  of  artificial  lighting 
may  also  be  obtained  with  numerous  units  in  a  large  schoolroom,  as 
in  the  study  room  of  the  high  school  at  Lincoln,  Nebraska,  for  in- 
stance. There  is  an  exceedingly  good  uniformity  of  illumination  and 
an  entire  absence  of  glare  from  the  desks  and  seats  in  spite  of  the 
fact  that  it  is  usually  not  easy  to  eliminate  the  undesirable  high 
brightnesses  in  a  large  room. 

A  schoolroom  illuminated  by  means  of  a  satisfactory  and  inexpen- 


322  ILLUMINATING    ENGINEERING    PRACTICE 

sive  semi-direct  system  of  artificial  lighting  is  shown  in  Fig.  9,  where 
inverted  opal  glass  reflectors  of  sufficient  density  are  employed  to 
reduce  the  surface  brightness  of  the  lighting  units  themselves  to  a 
comfortably  low  value. 

In  the  lighting  of  the  auditorium  of  the  J.  W.  Scott  High  School,  at 
Toledo,  Ohio,  use  is  made  of  diffusing  globes  hung  high  on  the  ceiling. 
The  high  mounting  height  would  tend  to  give  good  distribution  with 
reduced  glare  in  the  eyes  of  the  audience. 

The  study-room  of  Lincoln  (Neb.)  High  School,  Fig.  10,  is  illumi- 
nated by  a  type  of  indirect  system  using  numerous  units.  No  bright 
sources  of  light  are  visible  as  one  faces  the  full  length  of  room.  In 
other  words,  there  is  no  glare  from  unconcealed  light  sources. 
Compare  this  with  Fig.  6. 

The  Temple  Speech  Room  of  Rugby  School,  Rugby,  England,  is 
illuminated  entirely  by  indirect  lighting.  The  installation  is  espe- 
cially interesting  because  of  the  use  of  standard  American  fixtures 
installed  in  England,  such  installation  being  made  on  account  of 
the  higher  efficiency  of  the  American  product. 

In  Fig.  ii  is  shown  the  newly  equipped  drafting  room  of  the  Mass. 
Institute  of  Technology,  where  use  is  made  of  inverted  translucent 
reflectors.  The  picture  was  taken  by  daylight  to  show  the  simplicity 
of  detail  of  the  units. 

It  may  be  of  at  least  passing  interest  to  note  the  illumination  of 
some  of  the  special  departments  in  a  school. 

The  cafeteria  in  the  Lincoln  (Nebr.)  High  School  (Fig.  12)  is 
lighted  ,by  the  luminous  bowl  indirect  system,  the  bowls  being  of 
a  low  intrinsic  brilliancy,  which  is  particularly  desirable  in  this 
installation,  as  the  tables  used  have  highly  glazed  tops,  so  that  any 
spot  of  high  brilliancy  on  or  near  the  ceiling  would  cause  annoyance 
by  specular  reflection  from  the  surface  of  the  tables. 

The  Harrison  Park  Gymnasium,  Chicago,  is  illuminated  by  a 
direct  lighting  system,  the  light  sources  being  screened  from  view  by 
deep  mirrored  glass  reflectors  with  opaque  green  enamel  backing, 
placed  high  out  of  the  range  of  vision. 

In  the  Northwestern  University  Gymnasium  use  is  made  of  a  com- 
bination lighting  system.  The  main  illumination  is  entirely  taken 
care  of  by  direct  lighting  units,  the  lamp  being  set  in  a  deep  mirrored 
glass  reflector  with  opaque  green  enamel  backing,  so  as  not  to  be 
visible  under  ordinary  conditions.  Each  unit  is  also  furnished  with 
two  small  reflectors  furnished  with  25  watt  lamps,  which  throw  the 
light  upward,  illuminating  the  ceiling.  Complete  uniformity  of 
illumination  and  absence  of  glare  characterize  this  installation. 


Fig.  9. — Simple  semi-indirect  lighting  installation  in  schoolroom. 


Pig.  10. — Type  of  indirect  system  of  lighting  used   in  study  room. 

(Facing  page  322.) 


Fig.   n. — Simple  semi-indirect  lighting  installation  in  a  college  drafting  room. 


Fig.   12. — Luminous  bowl,  indirect  illumination  in  school   cafeteria. 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  323 

The  Armory  of  the  University  of  Illinois  presents  an  example  of 
good  lighting  of  such  an  area.  Use  is  made  of  units  of  the  direct 
lighting  type,  suspended  from  the  steel  trusses  supporting  the  roof, 
equipped  with  deep  mirrored  glass  reflectors  with  opaque  green 
enamel  backing,  so  as  to  screen  the  lamps  from  the  eye  except  at  an 
angle  from  which  the  reflector  would  not  ordinarily  be  viewed,  with  a 
resulting  satisfactory  clearness  and  uniformity  of  illumination  of 
the  floor.  Indirect  lighting  in  buildings  of  this  nature  is  entirely 
infeasible  due  to  the  type  of  roof  and  the  structural  framework 
beneath,  although  some  upward  light  is  secured  from  the  arrange- 
ment of  units  and  the  construction  thereof. 

PART  H.     LIBRARY  ILLUMINATION 

The  actual  lighting  of  a  library  reading  room,  as  far  as  the  produc- 
tion of  the  required  intensity  is  concerned,  can,  of  course,  be  accom- 
plished by  any  one  of  the  three  systems  of  illumination  already 
spoken  of.  Since  the  character  of  the  work  performed  in  a  library, 
namely,  reading  and  studying,  is  the  same  as  that  performed  in 
schools,  the  arguments  set  forth  above  apply  in  the  case  of  libraries, 
with  the  exceptions  already  noted,  with  respect  to  the  psychological 
and  aesthetic  aspects.  If  localized  direct  lighting  is  used  at  all,  in 
the  library,  care  should  be  used  to  avoid  the  production  of  conditions 
of  glare  and  specular  reflection,  such  as  are  often  obtained  by  direct 
lighting  on  newspaper  racks  or  reading  desks,  often  used  in  libraries. 

If  localized  lighting  is  used  on  the  general  reading  tables,  which  is 
sometimes  quite  a  tempting  type  of  installation  from  the  standpoint 
of  economy  of  operation,  certain  general  principles  must  be  observed, 
so  that  the  light  will  be  as  diffuse  as  is  possible  from  a  direct  lighting 
equipment  and  the  source  of  light  entirely  concealed  from  view  and 
the  intensity  of  the  illumination  from  this  source  only  moderate. 
Sometimes  a  combination  of  a  moderate  amount  of  well-engineered 
localized  illumination  can  be  made  with  the  general  diffused  illumi- 
nation of  an  indirect  system,  as  shown  in  Fig.  13,  but  it  contains,  as 
usually  installed,  too  large  a  component  of  direct  light  to  be 
comfortable. 

In  this  reading  room  it  was  desired  to  continue  to  use  the  table 
lamps  which  had  been  originally  installed,  so  that  a  slightly  lower 
intensity  of  general  illumination  would  be  adequate.  Hence,  the 
reflectors  of  the  table  units  were  replaced  by  spun  metal  casings, 
lowered  on  chain  supports  to  the  proper  screening  height  above  the 
table,  and  a  proper  deep  type  of  aluminized  steel  reflector  was  so 


324  ILLUMINATING   ENGINEERING   PRACTICE 

installed  as  to  completely  hide  the  lamp  from  view.  The  effect 
with  the  lamps  in  service,  is  shown  in  the  accompanying  illustra- 
tion, from  which  it  is  seen  that  there  is  no  glare  from  the  direct 
lighting  units. 

Where  it  is  at  all  possible  in  reading  rooms,  the  position  of  the 
reader  should  be  permanently  determined  and  so  arranged  if  direct 
lighting  is  used,  that  the  illumination  will  fall  on  the  book  from  the 
rear  and  one  side,  and  not  on  his  face,  or  on  the  book  so  as  to  reflect 
directly  into  his  eyes.  In  the  above  illustration  the  reader  could  sit 
and  read  comfortably  with  his  side  to  the  table  but  practically  no  one 
will  do  this,  unless  the  chairs  are  permanently  fastened  in  such  a 
position. 

In  this  reference  room  of  the  above  library,  three  central  units, 
together  with  the  marginal  units  in  the  gallery,  supply  indirect  illumi- 
nation as  well  as  the  lighting  for  the  gallery  book  stacks. 

The  fixtures  in  this  installation  are  intended  to  harmonize  with  the 
architectural  features.  Of  course,  a  simpler  installation  can  be  and 
is  made  in  a  less  pretentious  reading  room  in  the  same  library,  Fig. 
14,  with  due  regard  to  the  utilitarianism  and  harmony  of  design. 
In  this  room  the  illumination  is  augmented  by  certain  sky-lighted 
portions,  with  both  natural  and  artificial  light.  The  ceiling  windows 
are  small,  however.  This  view  shows  the  manner  in  which  the 
book  stacks  on  the  upper  mezzanine  are  illuminated  by  the  same 
opaque  indirect  lighting  units  as  are  used  for  general  illumination, 
and  the  manner  of  lighting  the  book  stacks  on  the  lower  mezzanine 
by  indirect  units  hung  directly  under  the  upper  mezzanine  floor, 
which,  while  made  of  marble,  has  been  painted  matte  white  to 
produce  the  proper  reflecting  surfaces. 

In  this  view  also  is  seen,  to  the  right  and  to  the  left  of  the  picture, 
an  adaption  of  the  indirect  bracket  unit  illuminating  passageways 
into  the  room.  The  steel  half  bowls  are  made  for  utilitarian  purposes 
primarily,  and  are  part  of  the  steel  book  stacks,  special  bracket 
type  mirrored  glass  reflectors  being  used.  At  the  present  time, 
newspaper  racks  have  been  installed  along  the  sides  of  the  passage- 
way under  the  indirect  brackets.  Note  the  entire  absence  of  desk 
lamps. 

In  another  portion  of  the  same  library  book  stacks  are  illuminated 
from  indirect  units,  consisting  of  mirrored  glass  reflectors  with  green 
opaque  enameled  backing  placed  on  top  of  the  book  stacks  and 
throwing  the  light  directly  on  the  ceiling  above,  no  fixtures  of  any 
kind  being  installed.  (See  Fig.  15.) 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  325 

Fig.  1 6  shows  a  newspaper  reading  room  with  newspaper  racks  as 
well  as  tables  for  magazine  use,  illuminated  by  a  simple  indirect 
system. 

Next  to  the  school  room,  perhaps,  conservation  of  children's 
eyesight  can  be  best  accomplished  in  the  Children's  Rooms  of  the 
public  libraries,  and  such  a  room  in  the  Milwaukee  Public  Library 
has  been  indirectly  illuminated,  in  which  library  the  last  four  ex- 
amples also  exist. 

Improvements  have  recently  been  made  in  the  illumination  of  the 
Congressional  Library  at  Washington.  Fig.  17  represents  the  illumi- 
nation of  some  of  the  book  stacks  in  this  library  by  means  of  "scoop- 
ette"  direct  mirrored  glass  reflectors.  In  this  library,  the  room  in 
which  the  paper  racks  are  located  has  also  been  recently  illuminated 
by  the  indirect  systen. 

An  example  of  semi-indirect  illumination  exists  in  the  Toronto 
public  library,  where  the  art  collection  is  illuminated  by  means  of 
medium  density  opal  bowls. 

The  large  reading  room  of  the  John  Hay  Memorial  Library  at 
Providence,  R.  I.,  Fig.  18,  is  lighted  entirely  by  the  opaque  bowl 
indirect  system,  the  fixtures  being  unique  in  their  artistic  design 
and  in  the  fact  that  a  lower  section,  or  sub-structure,  has  been 
worked  into  the  design  so  as  to  contain  light  sources  to  illuminate 
the  outside  of  the  main  bowl  directly  above. 

One  of  the  important  departments  of  the  public  library  is  the 
catalogue  room,. and  the  facility  of  properly  illuminating  the  card 
files  is  of  great  value.  In  the  Milwaukee  public  library  the  catalogue 
card  indexes  are  illuminated  by  an  indirect  system  installed  on  a  very 
elaborate  and  deeply  cut  ceiling,  and  the  diffuse  illumination  from 
this  system  gives  a  most  satisfactory  vertical  component  for  use  on 
the  cards  in  the  files. 

One  of  the  important  and  ever-growing  activities  of  the  public 
library  in  large  cities  is  the  establishment  of  branches  in  the  outer 
portions  of  the  city,  where  people  can  be  reached  by  the  library  if  the 
down  town  library  cannot,  or  will  not,  be  reached  by  the  people.  An 
example  of  the  illumination  of  a  branch  of  this  character  is  illustrated 
in  Fig.  19,  showing  the  Austin  Branch  Library  in  Chicago,  lighted  by 
a  large  number  of  indirect  fixtures,  producing  maximum  diffusion  and 
uniformity  of  illumination. 

Not  all  branches  of  a  library  can  be  as  well  equipped  as  this,  how- 
ever, for  the  primary  object  of  such  branches  is  to  reach  a  certain 
locality  and  class  of  people,  and  it  is  often  more  utilitarian  and 


326  ILLUMINATING    ENGINEERING    PRACTICE 

successful  to  establish  more  branches  of  less  pretentions  in  vacated 
store  buildings  or  other  suitable  quarters.  An  example  of  the  latter 
sort  of  branch  library  may  be  found  in  Milwaukee's  East  Side 
Branch,  located  in  a  store  building,  with  portable  book  stacks  and 
unpretentious,  though  properly  equipped  illumination  system,  con- 
sisting of  three  steel  bowl  type  of  indirect  units,  spaced  down  the 
center  of  the  room. 

PART  HI.    AUDITORIUM  ILLUMINATION 

In  taking  up  the  description  of  the  third  division  of  this  subject, 
namely,  auditoriums,  it  is  thought  best  to  divide  it  into  sections, 
such  as  churches,  lodges,  theatres,  concert  and  lecture  halls.  From 
an  engineering  standpoint,  many  more  large  interiors  for  public 
gatherings  might  be  included,  but  a  narrower  scope  has  been  arbi- 
trarily selected  as  sufficiently  illustrative. 

CHURCHES 

The  present  tendency  toward  liberality  in  religion  seems  to  be  con- 
ducive to  liberality  in  illumination,  and  well-lighted  churches  to-day 
are  so  ordinary  an  institution  as  to  make  it  difficult  to  restrict  the 
number  of  illustrations  exemplifying  this  section. 

Historically,  Fig.  20  represents  one  of  the  first  attempts  to  light  the 
auditorium  of  a  large  church  by  practically  a  single  indirect  fixture, 
and  shows  the  Eighth  Church  of  Christ,  Scientist,  of  Chicago,  in 
which,  except  for  the  alcoves,  the  entire  auditorium  is  so  illuminated. 

Another  historical  indirect  installation  is  the  North  Chicago 
Hebrew  Congregational  Temple,  where  a  combination  of  indirect 
illumination  from  suspended  fixtures  and  from  reflectors  concealed 
in  coves  is  utilized.  In  the  coves  are  reflectors,  lighting  the  arched 
central  portion  of  the  ceiling,  while  suspended  indirect  units  are  hung 
from  the  flat  ceilings  at  the  sides. 

Fig.  2 1  shows  another  early  indirect  church  installation,  the  fixture 
design  of  which  should  be  contrasted  with  some  of  these  following. 

The  church  as  a  structure  lends  itself,  perhaps,  more  readily  to 
architectural  development  than  any  other  of  the  edifices  with  which 
we  are  dealing,  and  for  that  reason  considerably  more  attention 
should  be  paid  to  the  harmonious  design  of  the  lighting  equipment 
installed  than  to  that  used  for  other  purposes.  In  some  of  the 
following  figures,  particular  attention  is  directed  to  the  efforts  made 
to  select  and  design  fixtures  which  will  produce  harmonious  results. 


Fig.   13. — Indirect  lighting  in  library  reference  room  with  specially  designed  direct 

table  lamps. 


"Pior      T 


-rr\r\m     anrl    Kr\r»lr    ctarl^Q    i11iiminatpH    pntirf*1v 


Fig.   15. — Indirect  lighting  of  book  stacks  without  the  use  of  fixtures. 


Fig.   16. — Indirect  illumination  in  newspaper  and  magazine  room  of  a  public  library. 


Fig.   17. — Direct  lighting  of  library  book  stacks. 


Fig.   1 8. — Library  reading  roc 


ited  by  the  indirect  system. 

(Facing  Figs.  15  and  16.) 


Fig.   19. — Indirect  illumination  in  a  branch  library  in  a  large  city. 


Fig.   20. — Church  illuminated  principally  by  means  of  large  central  lighting  unit. 


21. — Early  indirect  church  lighting  installation. 


Fig.  22. — Opaque  indirect  church  fixture  designed  to  harmonize  with  central  ceiling 

ornament. 

(Facing  Figs.  19  and  20.) 


Fig.  27. — Cove  indirect  illumination  in  church. 


Fig.  28. — Installation  of  direct  lighting  to  harmonize  with  Gothic  architecture. 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  327 

Fig.  22  shows  an  indirect  installation  in  the  Everhardt  Memorial 
Church,  Mishawaka,  Ind.,  where  the  main  fixture  is  hung  from  a  cen- 
tral ceiling  ornament  and  with  some  regard  for  the  harmony  between 
ornament  and  fixture;  this  installation  differing  radically  from  the 
Christian  Science  Church  installation,  where  the  fixture  is  suspended 
in  the  center  of  a  large  dome,  as  well  as  from  that  represented  in 
Fig.  21. 

Fig.  23  shows  a  church  auditorium  (Timothy  Eaton  Memorial 
Church  of  Toronto,  Canada)  illuminated  by  specially  designed 
luminous  bowl  indirect  fixtures,  the  translucent  portions  of  the 
bowl  being  illuminated  by  a  small  frosted  lamp,  the  light  for  this 
purpose  seeping  through  tan  silk  panels  in  the  sides  and  bottom  of 
the  bowl. 

An  example  of  the  use  of  a  semi-indirect  installation  in  a  religious 
auditorium  is  shown  in  Fig.  24,  where  medium  density  opal  bowls  are 
suspended  from  the  ceiling  in  two  rows  down  the  auditorium.  As 
stated  earlier,  an  installation  of  this  character  with  bowls  of  suffi- 
ciently moderate  brightness  and  hung  sufficiently  high,  may  be 
made  successful. 

In  certain  cases,  the  architect  may  feel  that  a  large  fixture,  sus- 
pended in  the  center  of  a  dome,  due  to  the  eccentricity  of  view  from 
almost  every  point  in  the  auditorium,  is  aesthetically  wrong  for  his 
structure,  and  yet  a  diffused  type  of  illumination  may  be  desirable. 
A  solution  of  such  a  problem  is  illustrated  in  Fig.  25  where  a  special 
diffusing  glass  ceiling  window  is  constructed  in  such  a  manner  as 
to  give  practically  the  same  diffusion  to  the  light  passing  through 
from  the  lamps  above  as  with  an  indirect  system,  and  this  view  of 
the  Second  Church  of  Christ,  Scientist,  in  Milwaukee,  illustrates  how 
the  architect  and  engineers  obtained  an  artistic  luminous  "sky- 
lighted" dome  with  satisfactory  illumination  results.  By  the  selec- 
tion of  the  glass  and  placing  of  lamps,  almost  perfect  diffusion  com- 
bined with  high  efficiency  was  obtained. 

Fig.  26  shows  the  arrangement  of  the  lamps  above  the  ceiling  win- 
dow and  is  illustrative  of  the  use  of  the  bare  lamps  in  a  diffusely 
reflecting  enclosure,  which  in  this  case,  is  the  attic  structure  of  the 
church.  By  painting  the  attic  the  proper  white,  this  space  is  used  as 
a  huge  diffusing  reflector  impinging  the  light  on  the  window  in  a 
very  diffused  condition,  which,  with  the  high  diffusive  character- 
istics and  low  absorption  of  the  selected  glass,  is  so  distributed  as  to 
produce  practically  perfect  diffusion  in  the  auditorium  without  the 
presence  of  the  light  sources  being  visible  from  below,  thus  producing 


328  ILLUMINATING   ENGINEERING   PRACTICE 

very  closely  the  same  effect  as  would  be  obtained  from  the  illumi- 
nation of  the  ceiling  of  a  dome  beneath  by  an  indirect  fixture,  but 
avoiding  the  presence  of  the  fixture. 

"Sky-lighting"  of  this  character  is  often  done  by  means  of  the 
lamps  above  the  glass  structure  being  placed  in  mirrored  glass  or 
steel-enameled  reflectors,  which  throw  more  or  less  concentrated 
light  upon  the  glass.  This  may  cause  more  difficulty  in  securing 
diffusion  of  the  illumination  beneath,  and  may  also  make  the  light 
sources  more  visible,  especially  if  placed  too  close  to  the  glass. 

Indirect  illumination  can  also  be  obtained  from  lighting  equipment 
and  reflectors  placed  in  coves  at  the  spring  of  the  arched  ceilings,  as 
illustrated  in  Fig.  27,  representing  St.  Cyril's  Roman  Catholic 
Church,  Chicago,  where  mirrored  glass  reflectors  are  thus  installed. 

An  example  of  the  use  of  the  combination  of  direct  and  cove  light- 
ing exists  in  the  illumination  of  the  St.  Helena  Cathedral,  Helena, 
Mont.  Here  are  used  "beehive"  reflectors  in  the  main  ceiling 
arches,  "hood"  reflectors  in  the  upper  column  capitals,  and  "mid- 
get" reflectors  (all  mirrored  glass)  in  the  lower  column  capitals. 

As  intimated  earlier  in  the  discussion,  for  architectural  or  artistic 
reasons,  the  structure  or  the  architect  may  require  that  the  upper 
portions  of  the  auditorium  remain  subdued  in  tone,  and  direct  light- 
*  ing  fixtures  may  therefore  have  to  be  designed  which  will  at  once 
comply  with  the  aesthetic  requirements  and  at  the  same  time  mini- 
mize the  glare  which  would  be  bound  to  be  present,  especially  from 
the  galleries,  if  not  thoroughly  provided  against  in  such  an  installa- 
tion. Fig.  28  shows  the  Plymouth  Church  in  Milwaukee,  where 
the  use  of  thoroughly  diffusing  selected  glass  in  a  lantern  type  of 
fixture  designed  by  the  architect  to  harmonize  with  the  English 
Gothic  architecture  seems  to  suit  the  conditions  by  the  use  of  a  direct 
lighting  system.  In  these  lanterns,  a  downward  directing  prismatic 
reflector  is  used,  and  the  lantern  is  made  luminous  by  an  additional 
small  lamp. 

Another  installation  of  like  character  is  that  of  the  First  M.  E. 
Church,  Evanston,  111.,  with  the  addition  of  side-wall  lanterns.  In 
the  units  in  this  case,  three  mirrored  glass  reflectors  are  used  in 
combination  with  three  prismatic  glass  reflectors,  the  combination 
producing  the  desired  distribution  as  well  as  the  moderate  luminosity 
of  the  sides  of  the  unit. 

If  for  any  reason,  architectural  or  otherwise,  it  is  desirable  to 
illuminate  a  church  from  floor  standards,  it  can  be  done  by  the  in- 
direct system,  providing  the  ceilings  are  suitable,  by  the  use  of  the 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  329 

type  of  floor  standard  shown  in  Fig.  29,  which  indicates  such  an 
installation  in  the  entrance  of  the  First  Church  of  Oak  Park,  HI. 

LODGE  ROOMS 

In  the  lodge  room,  a  semi-religious  function  must  be  considered, 
and  this  division  of  the  subject  therefore  is  closely  allied  with  the 
illumination  of  the  church. 

As  an  example  of  the  lodge  room  recently  lighted  by  the  indirect 
system,  there  is  shown  in  Fig.  30  the  Masonic  Hall  in  the  Auditorium 
Hotel  in  Chicago.  This  hall  is  lighted  by  the  opaque  indirect  units 
on  the  ceiling,  and  also  by  lamps  in  the  cove  under  the  pictures  and 
back  of  the  windows.  Due  to  the  ritual  work  done  in  the  hall,  the 
lamps  are  so  arranged  that  the  light  intensity  may  be  lowered  to  a 
very  low  degree,  or  a  certain  portion  of  the  lamps  may  be  cut  out 
entirely,  as,  for  instance,  the  ceiling  lamps,  leaving  a  dim  illumin- 
ation from  only  the  cove  lamps. 

Fig.  31  shows  the  Boulevard  Masonic  Hall  of  Chicago  which  has 
been  lighted  entirely  by  means  of  side-wall  indirect  brackets,  which 
are  in  the  form  of  decorative  boxes  with  vines,  etc.,  giving  the  effect 
of  growing  plants.  Each  box  contains  a  number  of  mirrored  glass 
reflectors  so  designed  as  to  throw  the  maximum  amount  of  light  onto 
the  ceiling,  with  a  very  small  amount  of  so-called  "wall-splash." 
This  lighting  has  been  so  arranged  that  it  is  possible  to  obtain  three 
different  intensities  besides  a  very  dim  lighting  for  ritual  work. 

Fig.  32  shows  the  lodge  room  of  the  Knights  of  Columbus,  Mil- 
waukee, Wisconsin.  The  lighting  of  this  room  was  accomplished 
entirely  by  luminous  bowl  indirect  lighting  units,  using  bowls  with 
glass  panels,  enough  light  being  permitted  to  pass  through  the  glass 
to  give  them  sufficient  illumination  not  to  appear  dark.  For  ritual 
work  only  the  two  center  units  are  used,  these  being  so  arranged  as 
to  furnish  any  colored  light  desired  for  such  services.  Each  unit 
contains  red,  blue,  green  and  white  colored  light  sources,  each 
color  being  controlled  by  a  separate  dimmer.  By  means  of  this  color 
mixing  facility,  it  is  possible  to  form  any  combination  desired,  and 
to  allow  for  the  changing  of  the  mixture  from  shade  to  shade,  grad- 
ually or  quickly,  as  desirable. 

Fig.  33,  as  an  example  of  semi-indirectly  illuminated  lodge  rooms, 
shows  the  lodge  rooms  of  No.  i  Masonic  Temple,  Washington,  D.  C. 
The  photograph  gives  no  accurate  conception  of  the  actual  illumina- 
tion, since  it  was  made  by  daylight  and  not  by  the  use  of  the  lighting 
units  themselves. 


330  ILLUMINATING    ENGINEERING    PRACTICE 

THEATRES 

Before  inspecting  some  of  the  modernly  illuminated  theatre  audi- 
toriums, it  may  be  of  interest,  again,  to  take  a  backward  step  of  a 
few  hundred  years,  and  review  historically  the  development  of  this 
problem  and  its  solutions. 

Of  historical  interest  is  the  English  theatre  of  300  years  ago, 
where  the  acting  was  done  in  the  open  air  and  the  theatre  was  a 
U-shaped  structure  with  the  audience  on  the  two  sides  in  two  or 
more  storied  amphitheatre  resembling  primitive  "bleachers."  In 
this  type  most  of  the  acting  took  place  under  the  open  sky  where 
the  actors  were  surrounded  on  two  sides  by  the  audience.  These 
performances  all  took  place  in  broad  daylight,  and  hence  required 
no  artificial  illumination. 

In  the  drama  of  the  days  of  Shakespeare's  boyhood,  the  actors 
made  use  of  little  scenery  and  not  much  costuming  or  make-up,  the 
performance  being  given  in  the  courtyard  of  a  village  inn  as  after- 
noon matinees.  The  courtyards  with  their  galleries  formed  auto- 
matically a  theatre  somewhat  similar  to  that  just  described,  per- 
formances again  being  given  by  daylight. 

Coming  now  to  this  country,  and  a  later  date,  the  old  John  Street 
Theatre,  New  York,  which  was  opened  in  1767,  is  of  interest.  The 
auditorium  and  stage  were  of  most  primitive  type  and  the  very 
earliest  type  of  lighting  known  for  an  auditorium  of  this  character 
was  used.  The  view  of  the  interior  of  this  theatre,  which  is  on 
record,  indicates  that  there  were  two  four-arm  chandeliers  near  the 
stage,  each  arm  apparently  carrying  a  kerosene  lamp  or  candle,  the 
whole  design  being  most  primitive  and  simple.  Nevertheless,  con- 
siderable publicity  was  given  at  that  time  to  the  successful  accom- 
plishment of  the  illumination  of  this  theatre. 

The  old  theatres  of  Europe  present  a  few  interesting  features  with 
respect  to  the  illumination  and  their  general  architectural  design. 
Fig.  34  is  typical  of  the  old  type  of  auditorium  lighting,  which  is 
typified  by  a  large  elaborate  central  fixture  supporting  bare  incan- 
descent lamps  in  conjunction  with  ceiling  studded  effects  and  a 
tremendous  number  of  bracket  lamps  lavishly  distributed  over  all 
balconies  and  in  the  direct  range  of  vision. 

Turning  to  our  country,  probably  the  best  known  and  surely  the 
most  deserving  of  fame,  is  the  Metropolitan  Opera  House  in  New 
York  (built  in  1883).  The  lighting  of  this  opera  house  carries  out 
the  same  general  scheme  as  those  in  Europe,  namely,  the  large 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  331 

center  fixture  with  its  semi-protected  lamps  and  its  rows  of  lamps 
around  the  fronts  of  the  balconies  and  boxes — all  of  which  gives  a 
very  hurtful  illumination  for  spectators,  except  for  those  seated  in 
the  body  of  the  house  and  far  enough  forward  to  probably  escape 
the  glare  of  the  side  lamps. 

Later  on,  a  movement,  perhaps  unwittingly  made  in  the  right 
direction,  placed  the  light  sources  directly  on  the  ceiling  and  beams 
in  studded  effects,  which,  if  the  architecture  happened  to  be  appro- 
priate, took  the  sources  out  of  the.  range  of  vision;  but  the  more 
or  less  ignorant  use  of  this  type  of  illumination  produced  in  many 
cases  even  worse  effects  than  the  central  fixture  type. 

The  Auditorium  Theatre  in  Chicago,  shown  in  Fig.  35,  is  typical 
of  the  bad  examples  of  this  type.  This  needs  very  little  description. 
The  bare  lamps  in  the  arches  indicate  for  themselves  the  effect  on 
the  eyes  of  the  spectators. 

Fig.  36  shows  the  Auditorium  in  Milwaukee  taken  by  artificial 
illumination.  The  lighting  underneath  the  top  balcony  is  accom- 
plished by  means  of  lo-in.  sand-blasted  globes,  while  the  main  portion 
of  the  hall  is  lighted  by  rows  of  hundreds  of  bare  lamps,  around  the 
center  ceiling  window  and  sand-blasted  shades  between  the  various 
arches.  The  effect  of  glare  on  the  audience  is  very  bad  indeed, 
as  it  is  almost  impossible  for  a  person  to  sit  in  this  long  hall  without 
having  glaring  lamps  in  his  direct  vision,  the  worst  effect  being  experi- 
enced by  those  in  the  first  and  second  balconies,  and  in  the  boxes, 
in  which  cases  the  eye  receives  the  full  glare  of  the  center  lamps. 
Recently  some  changes  have  been  made  by  the  installation  of  smaller 
and  denser  globes  under  the  balconies,  which  eliminates  the  units 
from  vision.  It  is  also  contemplated  to  modernize  the  entire  light- 
ing scheme  in  this  auditorium  in  the  near  future. 

One  of  the  earliest  scientific  attempts  to  eliminate  light  sources 
from  the  view  of  the  audience  was  in  the  University  of  Wisconsin 
lecture  room,  where  the  lamps  are  placed  in  reflecting  troughs  on  the 
stage  side  of  the  beams,  with  very  satisfactory  results  as  to  glare 
from  the  viewpoint  of  the  audience,  but,  of  course,  with  the  maximum 
of  bad  effect  to  those  on  the  stage  facing  the  audience. 

In  another  theatre  of  national  fame,  the  Hippodrome  of  New 
York,  the  lighting  is  accomplished  by  lamps  carried  along  various% 
construction  members  of  the  ceiling  and  also  over  the  proscenium 
arch.  Here  again  the  glare  is  bad,  but  a  large  portion  of  the  audi- 
ence is  protected  on  account  of  the  immensity  of  the  auditorium. 

Sometimes  in  the  theatrical  audience  there  seems  to  be  consider- 


332  ILLUMINATING   ENGINEERING   PRACTICE 

? 

able  desire  and  possibly  some  excuse  for  ignoring  almost  entirely 
the  engineering  and  economic  side  of  this  question  and  producing 
artistic  results  primarily,  if  not  entirely.  As  an  appropriate  example 
of  a  theatre  where  the  fixtures  have  been  made  highly  artistic  with 
comparatively  little  regard  for  other  factors  in  the  problem,  Fig. 
37  is  presented,  showing  the  Little  Theatre  of  New  York,  with  its 
beautifully  artistic  interior. 

A  somewhat  similar,  foreign  example  of  the  use  of  highly  artistic 
fixtures  is  the  Audience  Room  of  the  Royal  Palace,  Madrid.  Here, 
however,  the  most  interesting  feature  is  the  use  of  the  Moore 
vapor  tube  type  of  system  on  the  ceiling. 

In  later  years  greater  efforts  have  been  made  to  conceal  the 
light  sources  from  the  view  of  the  audience  and  to  produce  soft, 
though  sufficient,  illumination  effects,  at  least,  in  certain  types 
of  theatrical  auditoriums.  This  latter  movement  has  been  toward 
the  indirect  system  of  illumination,  and  the  following  are  some 
examples  of  this  type. 

One  of  the  first,  if  not  the  first,  theatre  to  be  totally  illuminated  by 
the  indirect  system  was  the  Pabst  Theatre  of  Milwaukee.  This 
theatre  was  originally  lighted  by  a  large  central  fixture,  bare  studded 
lamps  around  the  domed  ceiling,  bare  lamps  along  the  fronts  of  the 
balconies,  and  also,  bare  lamps  around  the  proscenium  arch.  Due  to 
the  annoyance  on  account  of  the  large  amount  of  glare  from  the  bare 
lamps,  this  installation  was  finally  superseded  by  a  complete  in- 
direct lighting  system.  Use  was  made  of  a  large  central  unit  hang- 
ing in  the  center  of  the  dome  augmented,  underneath  the  various 
balconies,  by  smaller  units,  mainly  for  artistic  influence.  The 
theatre  walls  being  dark  in  color  and  the  furnishings  of  the  same 
color,  it  was  necessary,  in  order  to  make  indirect  lighting  a  success, 
to  provide  a  means  of  redirecting  the  light  from  the  ceiling  down  on 
the  working  plane.  This  was  accomplished  by  means  of  false 
ceiling,  or  reflecting  discs,  painted  a  very  light  ivory,  which  were 
placed  above  the  various  units.  At  the  same  time  the  dome  was 
redecorated  in  a  lighter  shade.  Fig.  38  shows  the  central  fixture 
and  its  reflecting  disc. 

Fig.  39  shows  an  auditorium  lighted  by  means  of  opaque  indirect 
.fixtures,  typical  of  hundreds  similar  now  in  existence,  where  there  are 
no  bare  lamps  directly  in  the  range  of  vision  of  the  audience. 

In  the  Germania  Theatre  in  Chicago,  an  unconventional  system 
of  indirect  lighting  is  used  by  placing  the  lighting  units  in  specially 
designed  boxes  on  the  side-walls  (Fig.  40),  carrying  out  certain 


Fig.  29. — Indirect  illumination  of  church  lobby  by  means  of  floor  standards. 


Fig.  30. — Lodge  room  lighted  by  opaque  indirect  units  and  auxiliary  special  cove  units. 

(Facing  page  332.) 


Fig.  31. — Lodge  room  illuminated  by  special  design  of  indirect  wall  brackets. 


Fig.   32. — Lodge  room  illuminated  by  luminous-bowl  indirect  units. 


p;g>  33. — Lodge  room  with  semi-indirect  lighting  units. 


Fig.  34.— Early  foreign  type  of  direct  lighting  of  theatres. 

(Facing  Figs.  31  and  32.) 


Fig.  35- — Typical  bare  lamp  studded  effect. 


Fig.  36  — Glaring  lamps  in  auditorium  lighting. 


Fig.  37- — Artistic  arrangement  of  theatre  lighting  installation. 


Fig.  38. — Early  indirect  lighting  installation  in  theatre. 

(Facing  Figs.  35  and  36.) 


Fig.  39. — Auditorium  illumination  by  opaque  indirect  units. 


Fig.  40. — Theatre  illuminated  from  indirewall-bracket  units. 


>  A     '        -N  IT'     — >CJ.        -^V 

AMd 


Fig.  41. — Theatre  illuminated  by  cove-type  of  indirect  installation. 


Fig.  42. — Theatre  illuminated  by  luminous-bowl  type  of  indirect  unit. 

(Facing  Figs.  39  and  40.) 


Fig.  43. — Theatre  illuminated  by  artificial  "skylight." 


Fig.  44. — Theatre  illuminated  without  fixtures;  units  situated  on  top  of  ventilating  registers. 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  333 

artistic  effects  as  well  as  eliminating  the  use  of  fixtures  from  the 
ceiling. 

Another  variation  of  the  indirect  system  is  shown  in  Fig.  41  of  the 
Wilmette  Theatre,  which  is  an  example  of  cove  indirect  lighting  from 
trough  reflectors  place  in  the  cornices  of  the  room. 

Perhaps  the  latest  example  of  the  indirect  illumination  of  a  theatre 
is  the  Palace  Theatre  in  Milwaukee,  which  has  just  been  opened, 
where  the  main  auditorium,  Fig.  42,  is  illuminated  from  a  central 
unit  which  has  the  luminous  bowl  characteristics  through  decorative 
panels  in  the  fixture. 

The  Rialto  Theatre  in  New  York  is  perhaps  the  latest  and  most 
elaborate  example  of  indirect  lighting  effects  in  a  theatre.  Besides 
the  idea  of  lighting  by  means  of  indirect  illumination,  many  colored 
schemes,  decorative  and  flooding,  are  utilized  very  effectively,  and 
it  is  to  be  regretted  that  an  adequate  illustration  of  these  effects 
cannot  be  presented. 

Auditoriums  of  photoplay  theatres  present  a  condition  differing 
somewhat  from  that  in  the  auditoriums  of  the  legitimate  theatre,  in 
that  sufficient  light  has  to  be  furnished  to  permit  the  audience  to  find 
their  way  about,  and  yet  at  the  same  time,  it  must  be  of  a  low 
enough  intensity  so  as  not  to  interfere  with  the  picture  being  shown 
upon  the  screen.  Again,  the  intensities  of  different  parts  of  the 
house  may  be  materially  different,  since  the  surface  most  vitally  im- 
portant is  the  screen  at  the  front  of  the  theatre,  and  it  is  usually 
possible  to  raise  the  illumination  in  the  rear  or  entrance  of  the  the- 
atre, away  from  the  screen,  to  a  much  higher  intensity  of  illumina- 
tion than  toward  the  screen.  In  this  way,  a  person  entering  is  not 
at  first  subjected  to  as  low  an  intensity  of  illumination  as  he  is  after 
having  passed  down  toward  the  front  of  the  theatre,  and  the  .few 
moments'  lapse  between  the  time  of  entering  and  the  time  of  reach- 
ing an  area  of  the  lower  illumination  gives  the  eye  a  certain  amount 
of  time  in  which  it  can  accustom  itself  to  the  lower  illumination.  A 
second  requirement  of  this  type  of  lighting  is  to  have  the  greatest 
amount  of  illumination  thrown  upon  the  horizontal  plane — namely, 
the  seats  and  aisles.  It  is  poor  practice  to  throw  any  amount  of 
light  on  the  side-walls,  due  to  the  effect  upon  the  screen,  and  such 
light  is  practically  wasted,  since  it  serves  no  utilitarian  purpose.  A 
third  point  which  has  to  be  considered  is  the  absence  of  all  sources  of 
light  from  the  field  of  vision,  such  as  bracket  lamps  along  the  side 
walls,  or  lamps  on  either  side  of  the  screen.  Such  lamps  only  tend 
to  disturb  the  eye  and  cause  a  diversion  which  distracts  the  atten- 


334  ILLUMINATING   ENGINEERING   PRACTICE 

tion  from  the  picture  on  the  screen.     Several  of  the  illustrations 
already  shown  were  examples  of  this  type  of  auditorium. 

In  Fig.  43  is  shown  the  auditorium  of  the  Delft  Theatre,  Lscanaba, 
Mich.  This  auditorium  is  used  for  both  the  legitimate  theatre  and 
for  photo-play  productions.  The  lighting  is  effected  solely  by  means 
of  windows  in  the  ceiling,  as  shown.  Above  these  windows  are  long 
boxes,  approximately  18  in.  in  height,  painted  white  inside,  which  act 
as  diffuse  reflectors,  throwing  the  light  through  the  windows  into 
the  auditorium.  The  glass  used  gives  very  good  diffusion  and 
efficiency.  The  lamps  are  arranged  on  three  separate  circuits,  allow- 
ing the  use  of  full  intensity,  a  secondary  intensity,  or  a  very  low  in- 
tensity for  photoplay  work.  The  last,  or  lowest  intensity,  has  been 
so  graded  by  the  use  of  different  size  lamps  as  to  furnish  a  very  luw 
illumination  near  the  front  of  the  theatre,  but  a  higher  illumina 
toward  the  rear. .  This  type  of  lighting  directs  the  greater  percent- 
age of  the  light  directly  to  the  seats  and  aisles. 

In  the  Butterfly  photoplay  theatre  of  Milwaukee  was  made  one  of 
the  first  attempts  at  indirectly  illuminating  an  entire  photoplay 
theatre.  The  light  here,  again,  is  of  two  intensities — either  a  full 
intensity  which  is  used  at  the  end  of  the  program,  or  a  very  low 
graded  intensity,  which  is  on  during  the  presentation  of  the  films. 
The  lighting  is  accomplished  entirely  by  the  opaque  indirect 
units. 

In  Fig.  44  is  shown  a  third  photoplay  auditorium;  this  being  the 
Merrill  Theatre  of  Milwaukee.  The  lighting  system  employed  here 
is  different  from  the  other  two  shown,  in  that  there  are  no  fixtures  on 
the  ceiling,  for  the  indirect  lighting.  The  light  is  thrown  on  the 
ceiling  from  recesses  in  the  side-walls.  Use  is  made  of  mir- 
rored glass  reflectors  set  at  such  an  angle  as  to  give  approximately 
uniform  illumination  on  the  ceiling.  There  are  in  this  installation 
three  different  lighting  schemes — the  first  gives  a  comparatively  high 
intensity  of  illumination,  which  is  used  at  the  end  of  the  program;  the 
second  intensity  is  a  very  low,  graded  intensity,  which  is  used  during 
the  presentation  of  the  films;  the  third  intensity  is  still  lower,  use 
being  made  of  a  blue  light  in  place  of  the  ordinary  light  of  the  vacuum 
lamps,  when  blue  tinted  films  are  used  on  the  screen  for  night  scenes. 
The  lighting  in  the  rear  of  the  auditorium,  under  the  balcony,  is 
accomplished  by  ceiling  windows  of  the  same  general  type  as  those 
of  the  Delft  Theatre  previously  shown — such  lighting  being  con- 
sidered preferable  to  the  effect  produced  by  hanging  fixtures,  which 
would  give  a  cluttered  appearance  under  the  balcony. 


VAUGHN:  LIGHTING  OF  SCHOOLS,  ETC.  335 

CONCLUSION 
t 

-As  stated  at  the  outset,  this  subject  is  so  broad  and  so  much  good 
exemplary  work  has  been  done,  that  one  might  go  on  indefinitely 
describing  interesting  installations,  but  a  sufficient  number,  it  is 
believed,  have  been  shown  to  indicate  the  progress  in  this  branch  of 
the  arfe of  illumination,  and  to  exemplify  the  different  types  of  in- 
stallations which  have  been  developed  to  meet  the  various  con- 
ditions surrounding  the  specific  problems. 


'f 


THE  LIGHTING  OF  FACTORIES,  MILLS  AND  WORKSHOPS 

BY   C.    E.    CLEWELL 

In  May,  1910,  Prof.  Chas.  F.  Scott,  Sheffield  Scientific  School  of 
Yale  University,  in  an  editorial  in  the  Electric  Journal,  made  an 
analysis  of  the  costs  of  factory  lighting  in  terms  of  wages,  thus 
emphasizing  a  new  point  of  view  in  the  consideration  of  industrial 
lighting.  In  the  years  following,  it  has  become  quite  common  to 
evaluate  factory  lighting  costs  to  an  equivalent  proportion  of  the 
wages  of  the  employees  who  use  the  light,  as  one  of  the  best  ways  of 
expressing  the  advantages  of  good  light  in  factory  work. 

RELATIONS  OF  ADEQUATE  LIGHTING  TO  FACTORY  PRODUCTION 

Any  factory  executive  or  manager  should  take  an  interest  in  those 
factors  which  may  influence,  for  good,  the  production  rate  of  his 
plant,  provided  the  matter  is  presented  to  him  in  a  convincing  man- 
ner; and  he  will  be  found,  in  many  cases,  to  accept  as  a  working 
basis  for  the  value  of  the  best  lighting  to  his  plant  the  return  in 
quantity  and  quality  in  production  resulting  directly  or  indirectly 
from  the  expenditure  for  a  modern  system  of  lighting  to  replace  an 
old  and  an  inadequate  system. 

The  value  of  adequate  factory  lighting  may  thus  be  reduced  in  a 
simple  manner  to  such  items  as  the  time  it  saves  the  employees  in 
the  performance  of  their  regular  work,  the  improved  accuracy  it 
makes  possible  in  workmanship,  the  protection  and  safeguarding  of 
the  eyes  of  the  workmen,  the  beneficial  e/ect  of  bright  and  cheerful 
surroundings  on  the  temperament  of  those  affected,  and  the  tendency 
it  has  to  reduce  accident  hazard. 

If,  therefore,  in  summarizing  the  advantages  of  good  factory 
lighting,  in  contrast  to  inferior  lighting  conditions,  the  cost  of  im- 
proved light  be  evaluated  to  the  equivalent  time  saved  the  employees 
in  the  general  run  of  their  work,  it  will  be  found  that  the  wages  thus 
saved  are  usually  materially  greater  than  the  cost  of  the  lighting, 
and  the  net  saving  to  the  plant,  either  through  reduced  wages  for 
the  same  output,  or  in  larger  and  better  output  for  given  wages, 
due  to  improved  lighting,  is  just  as  definite  and  important  an  asset 
22  337 


338  ILLUMINATING    ENGINEERING   PRACTICE 

to  the  plant  as  is  a  new  machine  tool  which,  due  to  its  higher  efficiency 
in  contrast  to  an  older  machine,  is  capable  of  effecting  a  similar 
economy. 

As  a  starting  point,  therefore,  it  is  desirable  to  assume  toward 
adequate  factory  lighting  an  attitude  of  such  a  nature  as  to  class  it 
as  one  of  the  economies  in  industrial  management;  and,  rather  than 
to  place  too  much  emphasis  on  the  cost  of  the  different  available 
types  of  lamps  or  on  the  various  systems  of  lighting,  to  concentrate 
the  major  part  of  the  attention  on  the  improved  quality  and  quan- 
tity of  workmanship  which  may  be  expected  to  accompany  better 
lighting.  In  brief,  it  is  well  to  think  of  lighting  as  an  asset  to  the 
plant,  and,  when  deciding  on  the  type  of  lamp  to  install,  to  consider 
which  type  is  best  suited  to  the  needs  of  the  factory,  rather  than  to 
direct  all  attention,  as  is  so  often  the  case,  on  those  relatively  small 
differences  in  first  cost,  which  sometimes  lead  to  a  selection  of  the 
cheapest  rather  than  the  best. 

As  a  matter  of  fact,  the  past  five  or  ten  years  have  witnessed  wide- 
spread improvements  in  many  factories  where  the  prevailing  former 
conditions  were  very  poor,  and  a  typical  factory  manager  of  to-day, 
whose  sections  are  equipped  with  modern  lighting,  is  able  to  take  a 
certain  pride  in  the  improved  appearance  of  the  surroundings,  and 
at  the  same  time  he  has  the  assurance  that  the  accompanying  im- 
proved workmanship  and  sentiment  of  his  employees,  represent 
material  returns  in  excess  of  any  outlay  he  may  have  been  called 
upon  to  make  for  the  improvements  in  question. 

As  obvious  as  these  indirect  advantages  may  seem  to  be,  they  are 
not  as  satisfying,  nor  are  they  as  useful  in  the  practical  advancement 
of  better  lighting  conditions  in  the  industries,  as  would  be  the  case 
were  there  more  definite  examples  of  cash  returns  available  due  to 
improved  light,  or  were  there  on  record  actual  numerical  percentages 
of  increases  in  output  due  to  the  same  cause.  The  need  for  such 
definite  information  is  made  evident  in  a  statement  by  Dean  A.  J. 
Rowland,  in  a  discussion  on  the  subject  of  factory  lighting  several 
years  ago,  part  of  which  follows:1 

"There  is  one  very  important  detail  of  industrial  lighting  which  seems 
to  have  been  given  but  little  attention  by  anyone;  that  is,  the  accumulation 
of  data  which  will  give  the  answer  to  this  question:  Is  it  or  is  it  not  worth 
while  to  light  rooms  and  machinery  correctly  and  well? 

"Such  questions  are  as  important  as  any  which  can  be  considered  in 
connection  with  industrial  lighting.  The  kind  of  lamps  used,  their 

1  Trans.  I.  E.  S.,  vol.  VIII,  No.  6,  pp.  286  and  287. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  339 

arrangement,  the  kind  of  shades  put  on  them,  are  insignificant  matters 
compared  with  the  money  value  of  good  light  to  the  industries.  This 
will  have  to  be  determined  somehow  if  industrial  lighting  is  to  come  into 
its  own." 

This  quotation  from  Dean  Rowland's  discussion  is  merely  given 
as  typical  of  the  impression  which  prevails  that  such  data  are  badly 
needed,  and  while  this  need  is  generally  recognized,  the  data  desired 
are  very  difficult  to  obtain,  and  several  quotations  from  a  number 
of  authorities  must  be  taken  at  this  time  roughly  to  indicate  the 
available  information  on  this  general  phase  of  the  problem.2  It  will 
be  noted  that  some  of  these  quotations  refer  to  advantages  of  good 
light  and  the  disadvantages  of  bad  light,  based  on  features  other  than 
those  of  economy.  In  a  general  way,  however,  they  bear  directly 
on  the  important  question  as  to  "Why  good  light  is  a  necessity?" 

As  an  example,  the  first  report  of  the  Departmental  Committee  on 
Lighting  in  Factories  and  Workshops  (London,  1915)  contains  several 
comments  as  follows  :3 

"Complaints  of  eye  strain,  headache,  etc.,  attributed  to  insufficient 
lighting  are  common,  and  while  an  exhaustive  medical  inquiry  would  be 
necessary  to  establish  the  connection  between  these  defects  and  inadequate 
lighting,  there  is  a  general  impression  that  unsatisfactory  lighting  is,  in 
various  ways,  prejudicial  to  health.  It  is  also  recognized  that  insufficient 
light  adds  to  the  difficulty  of  the  proper  supervision  of  work,  and  of  the 
maintenance  of  cleanliness  and  sanitary  conditions  generally. 

"Witnesses  gave  specific  instances  of  the  effect  of  improved  lighting  in 
increasing  the  output  and  improving  the  quality  of  work  turned  out." 

Again,  in  the  same  report,  the  following  statement  appears  con- 
cerning the  diminished  output  of  work  due  to  insufficient  light:4 

"The  effect  of  improved  lighting  in  increasing  both  the  quantity  and 
the  quality  of  the  work  is  generally  admitted,  and  specific  instances  are 
quoted  in  the  evidence.  In  one  instance  the  output  was  diminished  12  to 
20  per  cent,  during  the  hours  of  artificial  lighting,  and  in  another  the  earn- 
ings of  the  workers  increased  11.4  per  cent,  after  the  installation  of  a 
better  system  of  lighting." 

A  clause  from  one  of  the  Public  Health  Bulletins  of  the  United 
States  Health  Service5  presents  the  case  from  a  somewhat  different 
point  of  view,  as  follows: 

2  An  experimental  investigation  is  under  way  at  this  time  to  secure  definite  information 
concerning  the  advantages  of  good  factory  lighting,  the  work  being  planned  by  the  Lighting 
Committee  of  the  Commonwealth  Edison  Company  of  Chicago. 

3  Memorandum  of  British  Report,  p.  2. 

4  Main  part  of  British  Report,  p.  xiii. 

*  No.  71,  May,  1915.  p.  105,  J.  W.  Schereschewsky  and  D.  H.  Tuck. 


340  ILLUMINATING   ENGINEERING   PRACTICE 

"In  view  of  the  fact  that  a  large  part  of  the  industrial  operations  in  the 
women's  garment  trades  involve  the  close  and  continuous  use  of  the  eyes, 
the  illuminating  conditions  which  prevail  in  the  workshops  of  the  industry 
become  highly  important  from  the  standpoint  of  industrial  hygiene.  The 
necessity  for  adequate  and  correct  illumination  on  the  various  working 
planes  becomes  the  more  apparent  from  the  consideration  of  the  data  in 
relation  to  the  vision  of  garment  workers  contained  in  the  foregoing  portion 
of  this  report.  These  data  show  that  only  a  little  over  25  per  cent,  of  the 
workers  whose  visual  acuity  was  tested  had  normal  ision  in  both  eyes." 

Turning  now  to  somewhat  more  tangible  wage  equivalents,  several 
good  examples  are  found  in  a  discussion  on  factory  lighting  by  M.  H. 
Flexner  and  A.  O.  Dicker,6  one  of  which  may  be  summarized  as  in 
Table  I. 

TABLE  I 

Under  the  assumption  that  good  factory  lighting  requires  a  loo-watt  tungsten 
lamp  for  each  100  sq.  ft.  of  working  area,  and  that  one  workman  occupies  each 
100  sq.  ft.,  the  following  statements  may  be  made: 

Constants: 

Working  hours  per  annum  (10  X  300) 3,ooo  hours 

Lighting  hours  per  annum  (3^  X  300) 1,000  hours 

Labor  cost  per  hour 35  cents 

Labor  cost: 

3,000  hours  at  35  cents $1,050.00 

Lighting  cost: 

Lamp  (free  renewals) $0.00 

Reflector i .  oo 

Wiring 4 .  oo 


First  cost 


Initial  cost  per  outlet $5 .  oo 

f  Interest  at  6  per  cent $o .  30 

I  Depreciation  at  12^  per  cent 0.63 

Operation  \  Annual  cleaning  at  3  cents 0.36 

Lamp  renewals o.oo 

100  kw-hr.  at  5  cents 5  .  oo 


Annual  operation  cost $6 . 29 

Conclusions: 

Cost  of  light  in  per  cent,  of  wages • o. 60 

Cost  of  light  per  hour $o .  006 

Cost  of  labor  per  hour '  0.35 

Cost  of  light  per  day 0.02 

Cost  of  labor  per  day 3 . 50 

These  data  show  that  the  cost  of  good  lighting  is  a  very  small  proportion  of 
the  value  of  a  man's  time;  in  fact,  if  good  lighting  effects  a  saving  of  five  minutes 
of  a  man's  time  per  day,  a  material  gain  would  be  experienced. 
•  Trans.  I.  E.  S.,  vol.  VIII,  No.  8,  pp.  477  and  478. 


Fig.   i. — Tungsten  direct  lighting  with  opaque  mirrored  glass  reflectors. 


Fig.  2. — Drafting  room  with  a  system  of  semi-indirect  tungsten  lighting. 

(Facing  page  340.) 


Fig.  3. — System  of  industrial  indirect  lighting  with  tungsten  lamps. 


4. — Machine  shop  with  a  system  of  mercury  vapor  lamps. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  341 

A  similar  example  worked  out  in  a  slightly  different  manner  is 
given  in  the  recent  Code  of  Factory  Lighting  of  the  Illuminating 
Engineering  Society,  as  follows:7 

"From  the  manufacturer's  point  of  view,  the  cost  of  the  annual  operation 
and  maintenance  of  the  illumination  of  a  typical  factory  bay  of  640  sq.  ft. 
area,  may  be  taken  at  $50.00  under  certain  assumptions  as  to  energy  cost, 
cleaning,  interest  and  depreciation.  If  five  workmen  are  employed  in 
such  a  bay  at  an  average  of  say  25  cents  per  hour,  the  gross  wages  of  the 
men  in  such  a  bay,  plus  the  cost  of  superintendence  and  indirect  factory 
expense,  may  equal  from  $5000  to  $7000  per  annum. 

"In  a  case  of  this  kind,  therefore,  the  lighting  will  cost  from  0.7  to  i.o  per 
cent,  of  the  wages,  or  the  equivalent  of  less  than  the  wages  for  from  4  to  6 
minutes  per  day.  We -may  roughly  say  that  a  poor  lighting  system  will 
cost  at  least  one-half  this  amount  (sometimes  even  more  through  the  use 
of  inefficient  types  of  and  a  poor  arrangement  of  lamps),  or  the  equivalent 
of  the  wages  for  from  2  to  3  minutes  per  day.  Nearly  all  factories  and 
mills  have  at  least  some  artificial  light,  hence,  in  general,  if  good  light 
enables  a  man  to  do  better  or  more  work  to  the  extent  of  from  2  to  3 
minutes  per  day,  the  installation  of  good  lighting  will  easily  pay  for  the 
difference  between  good  and  bad  light,  through  the  time  saved  for  the 
workmen." 

The  foregoing  discussions  and  quotations  are  typical  of  the  view- 
points which  have  been  assumed  in  recent  years  toward  the  field  of 
industrial  lighting,  on  which  the  following  advantages  of  good  light 
may  be  based: 

1 .  Increased  production  for  the  same  labor  cost. 

2.  Greater  accuracy  in  workmanship. 

3.  Reduced  accident  hazard. 

4.  Avoidance  or  at  least  reduction  of  eye  strain. 

5.  Surroundings  made  more  cheerful. 

6.  Work  performed  with  less  fatigue. 

7.  Order,  neatness  and  sanitation  promoted. 

8.  Superintendence  rendered  more  effective. 

In  other  words,  factory  lighting  is  important  to  production;  its  cost  is 
small  in  comparison  with  its  advantages;  and  when  its  cost  is  interpreted 
or  reduced  into  the  equivalent  wages  saved  through  its  adoption,  the 
expenditure  for  the  best  lighting  is  usually  a  very  small  item  by  contrast. 

SUMMARY  OF  FACTORY  LIGHTING  LEGISLATION 

As  pointed  out  in  a  recent  paper8  by  L.  B.  Marks,  legislation  on 
lighting  is  so  meagre  and  scattered  and  apparently  so  little  called  for 

7  Issued  by  the  I.  E.  S.  in  1915.     Quotation  from  pp.  14  and  15. 
»  Trans.  I.  E.  S.,  No.  i,  1916. 


342 


ILLUMINATING   ENGINEERING   PRACTICE 


in  this  country  that  no  legislative  bureau  has  found  it  worth  while  to 
collate  it.,  Mr.  Marks  points  out  that  five  years  ago  (1911),  based  on 
an  extended  review  of  factory  legislation  under  the  direction  of 
Prof.  John  R.  Commons,9  and  reported  by  E.  L.  Elliot,10  there  were 
only  eleven  states  that  made  any  mention  of  the  subject  of  light  in 
their  general  factory  or  labor  laws,  and  in  not  one  of  these  were 
the  provisions  sufficiently  specific  to  render  them  of  practical  value. 

Since  that  time,  factory  lighting  legislation  has  received  attention 
in  several  states,  the  most  prominent  cases  being  Wisconsin,  New 
York  and  Pennsylvania.  Briefly  stated,  the  legislation  in  both  Wis- 
consin and  New  York,  while  a  step  forward  in  each  case,  has  been 
rather  indefinite,  mainly  because  in  neither  case  are  the  requirements 
specific  with  regard  to  the  illumination  at  the  point  of  work. 

Following  the  legislation  in  these  two  states,  two  important  new 
developments  have  been  made  in  this  direction.  The  one  has  been 
the  publication  in  1915  of  a  Code11  for  the  lighting  of  factories,  mills 
and  other  work  places  by  the  Illuminating  Engineering  Society,  as  a 

TABLE  II 
(Intensity  Values  in  Foot-candles) 


Classification  of  space 

Clewell'* 
(1913) 

6? 
H* 

O 

Wisconsin1* 
(1914) 

~v 
*d 

8£ 
c/5  gi 

w" 

British  report16 
(I9IS) 

Pennsyl- 
vania 14  (1916) 

General  lighting  of  work 
rooms,  irrespective  of  the 
actual  light  required  by 
the  work  

o  80 

O    2  ^ 

Yards.  .  .  . 

o  05 

One 

Stairways,  passages,  stor- 
age, and  the  like  
Foundries  

0.50 

0.50 

3    OO 

I     CQ 

0.25 
I    2"? 

O.  IO 
O    4.O 

0.25 
I    2? 

Rough  manufacturing.  .... 
Fine  manufacturing  

3-oo 
5.00 

2.00 
5  .OO 

0-75 
I  .  "?O 

1.25 
•2    CQ 

1.25 

•2      CQ 

American  Legislative  Review,  vol.  I,  No.  2,  June,  1911. 

Trans.  I.  E.  S.,  1911,  p.  722. 

"Code  of  Lighting,  Factories,  Mills  and  other  Work  Places,"  issued  by  the  Illuminat- 
ing Engineering  Society,  Trans.  I.  E.  S.,  vol.  X,  Nov.  20,  1915,  pp.  605-641. 

"Factory  Lighting,"  N.  Y.,  McGraw-Hill  Book  Co.,  1913. 

"Handbook  on  Incandescent  Lamp  Illumination,"   General  Electric  Co.,  1913. 

Intensities  estimated  on  basis  of  specifications  of  candle-power  per  sq.  ft. 

Minimum  requirements. 

No  recommendation  is  made  for  the  illumination  required  for  the  work.  Intensities 
here  listed  are  specified  as  minimum  values. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  343 

result  of  the  work  of  several  of  its  committees;  and  the  other,  the  first 
report  of  the  Departmental  Committee  on  Lighting  in  Factories  and 
Workshops,  issued  also  in  1915  in  London. 

Following  the  completion  of  the  I.  E.  S.  Code,  representatives  of 
the  labor  departments  in  Pennsylvania  and  New  Jersey  met  with  a 
committee  of  the  I.  E.  S.,  and  on  June  i,  1916,  as  a  result  of  these 
conferences,  the  department  of  labor  and  industry  in  Pennsylvania, 
John  Price  Jackson,  Commissioner,  adopted  the  I.  E.  S.  Code  in 
slightly  modified  form.  Pennsylvania,  by  this  action,  becomes  the 
first  state  in  this  country  on  record  to  adopt  a  factory  lighting  code 
based  on  definite  intensities  of  illumination  on  the  work. 

As  a  basis  for  study  and  comparison,  Table  II  has  been  compiled. 
This  Table  shows  at  a  glance  the  various  illumination  intensity 
requirements  of  certain  states;  those  of  the  I.  E.  S.  Code  and  of  the 
British  Report;  and  the  recommended  intensities  of  certain  authori- 
ties. It  is  to  be  noted  that  the  intensities  placed  under  Wisconsin 
have  been  worked  up  as  the  probable  intensities  equivalent  to  the 
requirements  of  that  state  corresponding  to  candle-power  per  sq.  ft. 
specifications,  and  under  the  assumption  that  an  overhead  tungsten 
system  of  lighting  with  efficient  reflectors  is  used.  No  actual  illu- 
mination intensities  are  specified  in  the  Wisconsin  orders. 

TYPES  OF  LAMPS  AVAILABLE 

The  types  of  lamps  available  for  factory  lighting  at  the  present 
time  include  the  various  electric  filament  lamps,  namely,  the  carbon 
incandescent,  the  metallized  filament,  the  tungsten  or  "Mazda" 
vacuum  and  the  Mazda  gas-filled  units;  the  various  types  of  mantle 
gas  lamps;  the  mercury  vapor  glass- tube  and  quartz- tube  units;  and 
the  various  types  of  electric  arc  lamps,  namely,  the  enclosed  carbon, 
metallic  flame  (or  magnetite),  and  the  flame  units.17 

Of  these,  the  most  widely  used  types  in  modern  lighting  systems, 
and,  in  general,  the  most  practical  types  for  industrial  service,  are  the 
Mazda  and  the  mercury-vapor  electric  lamps,  and  the  mantle  gas 
lamps.  Due  to  the  very  wide  range  of  sizes  in  the  Mazda  units,  there 
is  practically  no  class  of  factory  location  for  which  one  or  another  of 
the  Mazda  lamps  may  not  be  selected  and  used  with  excellent  results. 
This  fact  has  brought  about  a  remarkably  wide  use  of  Mazda  lamps 
in  the  industries  during  the  past  six  or  eight  years,  especially  since 

17  It  has  been  decided  not  to  include  in  this  classification,  those  cases  where  oil,  acetylene, 
or  other  similar  fuels  are  used  for  illumination  purposes. 


344  ILLUMINATING   ENGINEERING   PRACTICE 

their  manufacture  has  been  developed  to  a  point  making  it  possible 
to  use  them  under  rough  factory  surroundings.  In  a  general  way, 
the  mantle  gas  lamps,  likewise,  have  been  developed  in  such  a  variety 
of  types  and  sizes  during  the  past  few  years  that  they  are  also 
suitable  for  a  very  wide  range  of  factory  locations. 

Since  its  initial  application  to  an  industrial  use  in  this  country  in 
1903,  the  mercury- vapor  lamp  has  found  wide  service  in  such  typical 
industries  as  metal  working  plants,  textile  mills,  newspaper  and 
printing  establishments,  and  shipping  and  storage  houses.  Instal- 
lations of  mercury  vapor  lamps  are  on  record  with  as  many  as  2,500 
units  in  a  single  plant. 

The  notable  development  in  the  interest  taken  in  factory  lighting 
has  been  promoted  very  largely  by  the  design  and  marketing  of  these 
modern  types  of  lamps,  and,  physically  speaking,  the  possibilities  in 
this  field  are  due  almost  entirely  to  the  introduction  of  the  many 
new  types  of  lamps  and  auxiliaries,  which,  in  turn,  have  made  it 
possible  to  light  factory  spaces  properly  with  electricity  or  gas, 
whereas,  formerly,  these  same  spaces  could  not  be  lighted  properly 
because  of  a  lack  of  suitable  lamp  types  and  sizes  adapted  to  given 
conditions,  such  as  ceiling  heights,  clearance  between  cranes  and 
ceilings,  and  similar  limitations. 

REQUIREMENTS 

The  principal  requirements  which  should  be  met  fully  in  planning 
factory  lighting  may  be  summarized  as  follows: 

(a)  Sufficient  intensity  of  general  illumination  over  the  floor  area 
to  prevent  accidents  and  to  make  it  possible  to  handle  material  and 
to  get  around  the  machinery  readily. 

(b)  Sufficient  intensity  of  the  illumination  at  the  point  of  work, 
usually  a  higher  intensity  than  in  (a)  although  it  is  practical  in  some 
cases  to  make  the  intensity  of  the  general  illumination  adequate  for 
both  (a)  and  (b). 

(c)  The  use  of  suitable  shades  and  reflectors  with  the  lamps 
mounted  in  such  positions  as  to  avoid  eye-strain. 

(d)  The  electric  circuits  and  gas  mains  of  sufficient  size  to  assure 
normal  working  pressures  of  the  supply  at  all  times. 

(e)  In  addition  to  (d)  the  supply  should  be  adequately  protected 
against  interruption  of  service. 

(/)  The  size  of  the  lamp  should  be  in  accord  with  the  ceiling  height 
of  the  section  where  it  is  employed,  particularly  where  the  entire 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  345 

illumination  is  furnished  from  lamps  overhead,  that  is  to  say,  where 
no  individual  lamps  are  used  close  to  the  work. 

As  a  supplement  to  the  foregoing  list  of  requirements,  certain 
specifications  concerning  artificial  lighting  as  made  by  one  of  the 
Federal  Government  Departments18  are  of  value  in  relation  to  this 
phase  of  the  subject.  These  specifications,  as  abridged  from  the 
bulletin  of  this  department,  are  as  follows: 

General  Illumination. — The  entire  shop  should  have  a  system  of  general 
artificial  lighting  adequate  to  insure  an  illumination  of  not  less  than  i 
foot-candle  over  the  entire  floor  area.19 

Local  Illumination. — At  the  points  of  work,  additional  local  illumination 
should  be  provided,  or  the  general  illumination  increased,  to  meet  the 
specified  intensities  for  given  classes  of  operations. 

Character  of  Lighting  Units  for  General  Illumination. — Satisfactory  units 
for  the  general  illumination  of  shops  would  consist  of  tungsten  or  gas- 
mantle  lamps  provided  with  deep-bowl  reflectors  having  extensive  dis- 
tributing characteristics.  The  units  should  be  suspended  as  nearly  as 
possible  to  the  ceiling  in  such  relative  positions  as  to  insure  a  minimum 
distribution  of  i  effective  lumen  over  each  square  foot  of  floor  area,  that 
is,  a  minimum  of  i  foot-candle  at  each  point  of  the  floor  area. 

Character  of  Lighting  Units  for  Local  Illumination. — The  additional 
local  illumination  of  such  cases  as  machines  and  finishing  table  may  be 
advantageously  secured  by  the  use  of  tungsten  or  gas-mantle  lamps  and 
opaque  reflectors  with  intensive  distributing  characteristics  of  the  deep- 
bowl  or  cone  type.  Fixed  suspension  should  be  used.  The  height  of  sus- 
pension will  depend  upon  the  distribution  characteristics  of  the  reflector 
used. 

For  such  cases  as  cutting,  basting  and  pressing  tables,  the  local  lighting 
units  may  be  made  up  of  tungsten  or  gas-mantle  lamps  with  deep-bowl 
prismatic  reflectors  of  glass  with  intensive  distributing  characteristics,  the 
height  of  suspension  and  the  spacing  being  such  as  to  meet  the  desirable 
intensity  for  the  operation  in  question. 

Glare  Effects. — It  is  important  to  avoid  all  glare  effects,  for  not  only  do 
these  make  seeing  difficult  but  they  are  injurious  to  the  eyes.  Glare  is 
present  from  any  light  source,  under  ordinary  working  conditions,  when 
it  is  in  the  field  of  vision  and  is  of  greater  intrinsic  brilliance  than  the  ob- 
ject to  be  viewed.  It  follows  that  in  the  local  illumination  of  workshops, 
bare  lamps  or  reflectors  of  the  shallow-saucer  type  should  never  be  used. 
Prismatic  reflectors  should  be  of  the  deep-bowl  type  and  suspended  at 
such  heights  as  to  cause  the  units  to  become  practically  concealed  sources. 

18  Public  Health  Bulletin  No.  71,  May,  1915,  J.  W.  Schereschewsky  and  D.  H.  Tuck. 
Treasury  Dept.,  Washington,  D.  C.,  pp.  147  and  148. 

19  These   requirements   apply  'specifically   to   the   workshops  of   the   woman's   garment 
industry. 


346  ILLUMINATING   ENGINEERING   PRACTICE 

Opaque  deep-bowl  or  cone  reflectors  are  always  to  be  used  for  local  illumi- 
nation when  the  height  of  suspension  is  such  that  the  unit  will  be  within 
the  ordinary  field  of  vision. 

All  reflectors  are  made  for  use  with  a  particular  size  of  lamp.  This 
specific  size  should  always  be  used  with  the  reflector.  The  use  of  larger 
lamps  produces  glare  from  the  projecting  portions  and  alters  the  distribu- 
tion characteristics  of  the  combination;  the  use  of  lamps  smaller  than  that 
for  which  the  reflector  is  designed  constitutes  an  uneconomical  unit,  which 
may  produce  inadequate  illumination  and  alter  the  distribution  character- 
istics of  the  reflector. 

SYSTEMS  OF  ILLUMINATION  IN  USE 

In  the  earlier  days,  before  the  introduction  of  the  mercury  vapor 
and  Mazda  lamps,  the  use  of  the  small  carbon  filament  units  and 
the  large  arc  lamps,  usually  resulted  in  a  low  degree  of  general 
illumination  when  some  of  the  lamps  were  mounted  overhead,  thus 
making  it  essential  to  employ  an  individual  or  localized  lamp  close 
to  the  work  of  each  employee. 

With  the  introduction  of  medium-sized  lamps,  that  is  to  say,  the 
mercury  vapor  and  Mazda  lamps,  with  their  wide  range  in  sizes, 
there  has  been  made  possible  a  comparatively  new  system  commonly 
termed  the  overhead  or  general  system  of  illumination,  whereby  a 
large  number  of  medium  (or  even  relatively  small)  units  are  mounted 
well  overhead  in  such  density  of  numbers  as  to  furnish  entirely  ade- 
quate illumination  at  the  work  without  the  addition  of  individual 
hand  or  localized  lamps  mounted  directly  at  the  work. 

Again,  it  is  sometimes  found  advisable  to  carry  out  the  scheme 
of  general  illumination,  but  instead  of  a  uniform  spacing  over  the 
entire  ceiling  area,  to  mount  each  lamp  with  respect  to  some  given 
piece  of  machinery  or  work,  thus  forming  a  system  somewhat  be- 
tween the  general  and  the  strictly  localized  lighting  systems. 

Overhead  lighting  may  be  subdivided  into  three  general  classes, 
namely,  the  direct,  the  semi-indirect  and  the  indirect  systems.  For 
manufacturing  spaces,  the  direct  system  has  been,  and  probably  is 
now  most  widely  used,  partly  because  of  its  higher  efficiency,  and 
partly  because  it  is  usually  better  adapted  to  factory  spaces  and  is 
ordinarily  cheaper  in  first  cost  than  the  other  systems.  Exceptional 
cases  arise,  however,  where  the  semi-indirect  or  even  the  indirect 
systems  may  prove  economical  on  account  of  their  peculiar  advan- 
tages under  certain  circumstances.  For  example,  the  indirect  sys- 
tem is  now  used  with  very  satisfactory  results  in  a  number  of  textile 
mills. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  347 

Furthermore,  in  the  drafting  rooms  and  offices  connected  with 
factories  and  mills,  the  semi-indirect  and  indirect  systems  are  often 
used,  and  it  seems  reasonable  to  expect  that  the  illumination  ad- 
vantages of  these  systems  will  cause  them  to  be  even  more  widely 
used  under  certain  industrial  conditions,  particularly  as  the  efficiency 
of  the  commercial  light  sources  is  further  increased.  Figs,  i  to  6 
inclusive  show  three  overhead  systems  of  tungsten  lighting,  that  is, 
a  direct,  a  semi-indirect  and  an  indirect  system,  and  three  systems 
where  the  mercury  vapor  lamp  is  employed. 

METHODS  OF   CLASSIFYING  LOCATIONS  AND  WORK 

A  classification  of  typical  industrial  locations  and  of  the  various 
kinds  of  work  involved,  is  of  value  when  planning  new  lighting  sys- 
tems where  it  may  be  important  to  know  what  intensity  to  select 
for  a  given  factory  space  in  terms  of  the  experience  of  others  under 
similar  or  somewhat  similar  circumstances,  or  when  the  drafting  or 
enforcement  of  lighting  legislation  is  involved. 

For  the  reason  that  the  industries  cover  an  immense  variety  of 
locations  and  kinds  of  work,  it  is  impracticable  to  attempt  a  compre- 
hensive list  of  all  industries,  and  the  following  cases  are  therefore 
given  merely  as  typical  of  some  of  the  efforts  which  have  been  made 
to  formulate  classifications  of  this  general  nature.  They  are  in- 
tended, for  the  same  reason,  to  serve  rather  as  a  guide,  and  the  refer- 
ences made  to  a  number  of  books  and  pamphlets  will  aid  the  reader 
to  continue  the  study  further  if  these  more  or  less  typical  classifica- 
tions are  found  to  be  inadequate. 

The  Code  of  Factory  Lighting20  issued  by  the  Illuminating  Engi- 
neering Society  attempts  a  broad  classification  under  four  headings, 
as  follows: 

1.  Storage,  passageways,  stairways,  and  the  like. 

2.  Rough  manufacturing  and  other  operations. 

3.  Fine  manufacturing  and  other  operations. 

4.  Special  cases  of  fine  work. 

It  is  obvious  that  in  a  general  summary  of  this  nature,  many  un- 
certain cases  will  naturally  arise  in  the  inspection  of,  or  dealings  with, 
different  factory  buildings.  In  the  code  of  the  Illuminating  Engi- 
neering Society  the  suggestion  is  made  that  this  general  classification 
be  followed  and  that  uncertain  cases  be  left  to  the  judgment  of  a 
lighting  expert.  The  lighting  expert,  if  thus  called  upon  to  make  a 

*•  Trans.  I.  E.  S.,  vol.  X,  Nov.  20,  1915,  pp.  605-641. 


348  ILLUMINATING   ENGINEERING   PRACTICE 

decision  on  such  uncertain  cases,  may  depend  on  a  more  detailed 
subdivision  with  intensities  specified  for  locations  intermediate  be- 
tween the  main  headings  just  listed. 

The  Departmental  Committee  on  Lighting  in  Factories  and  Work- 
shops in  Great  Britain,  in  its  first  report,  which  was  limited  to  in- 
clude the  engineering,  textile  and  clothing  trades,  subdivided  its 
recommendations  under  a  classification  as  follows: 

1.  "Working  areas"  of  workrooms. 

2.  Foundries. 

3.  All  parts  of  factories  and  workshops  not  included  in  i. 

4.  Open  places  in  which  persons  are  employed  and  dangerous  parts  of  the 
regular  road  or  way  over  a  yard  or  other  area  forming  the  approach  to  any 
place  of  work. 

In  its  handbook  on  shop  lighting  for  superintendents  and  elec- 
tricians, issued  in  1914  by  the  Industrial  Commission  of  Wisconsin, 
the  following  subdivisions  are  listed: 

1.  Departments  with  ceilings  n  to  16  feet  in  height,  where  there  is  no  gas  or 
smoke. 

2.  Departments  with  ceilings  9  to  u  feet  in  height,  where  there  is  no  gas  or 
smoke. 

3.  Foundries  and  forge  shops. 

4.  Stairways. 

5.  Platforms. 

6.  Driveways  and  passageways  between  buildings. 

7.  Yards. 

8.  Individual  machines. 

9.  Benches. 

10.  Drafting  tables. 

As  typical  of  some  of  the  more  complete  classifications  in  use,  the 
following  subdivisions21  under  the  general  heading  of  machine  shops, 
indicate  one  way  in  which  some  of  the  operations  carried  on  in  such 
departments  have  been  outlined: 

1.  Bench  work22  (fine). 

2.  Bench  work  (rough). 

3.  Lathes  (fine  work). 

4.  Lathes  (automatic). 

5.  Millers  and  shapers. 

6.  Planers. 

7.  Drills. 

8.  Buffers  and  grinders. 

9.  Saws. 

11  General  Electric  Company's  "Handbook  on  Incandescent  Lamp  Illumination  for 
1916,  p.  109. 

22  Bench  work  is  further  classified  as  follows:  (a)  single  benches  along  the  wall;  (b)  single 
benches  away  from  the  wall;  and  (c)  double  benches  with  operators  on  both  sides. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  349 

10.  Assembling,  erecting  and  inspecting, 
n.  Painting. 

TABULATION  OF  INTENSITIES  COMMONLY  RECOMMENDED 

Any  attempt  to  tabulate  illumination  intensities  commonly  em- 
ployed for  various  industrial  conditions,  is  rendered  impracticable, 
partly  because  of  lack  of  data  on  existing  systems,  and  also  because  a 
table  of  this  kind  would  tend  to  mislead  on  account  of  the  inade- 
quate values  of  the  illumination  so  commonly  found  in  many  existing 
factory  lighting  systems.  It  is  perhaps  better,  therefore,  to  make  up 
a  tabulation  of  this  nature  in  the  form  of  recommended  values,  as 
indicated  in  the  following  paragraphs.  Reference,  in  this  connection, 
should  be  made  also  to  Table  II  in  Section  2,  headed  Summary 
of  Factory  Lighting  Legislation,  where  the  intensities  recommended 
by  various  authorities  are  given  for  a  very  limited  classification  of 
work. 

General  Illumination. — The  United  States  Health  Service23  recom- 
mends for  garment  working  establishments,  i.o  foot-candle  over 
the  entire  floor  area.  The  British  Report  for  1915  recommends  0.25 
foot-candle  without  regard  to  the  needs  of  the  work  itself.  The 
General  Electric  Company24  recommends  i.o  to  2.0  foot-candles  for 
general  illumination  when  supplemented  by  localized  light. 

In  all  of  these  recommendations,  an  effort  is  apparent  to  make  sure 
of  sufficient  illumination  over  every  factory  area  so  that  the  operators 
may  carry  on  their  work  without  risk  of  accident  and  without  loss  of 
time.  From  the  data  available  at  this  time,  an  average  of  about  i.o 
foot-candle  would  appear  to  be  about  the  minimum  with  somewhat 
higher  values  under  particular  circumstances. 

Intensities  for  the  Work. — When  the  general  illumination  is  not 
supplemented  by  localized  light,  then  the  intensities  from  the  over- 
head lamps  should  closely  approximate  those  required  when  localized 
units  are  mounted  close  to  the  work.  The  recommended  intensities 
shown  in  Table  III  may,  therefore,  be  interpreted  as  a  measure  of  the 
illumination  which  should  be  furnished  to  the  work  for  the  various 
operations  listed,  whether  from  localized  or  from  overhead  lamps  as 
the  case  may  be.25  Care  must  obviously  be  taken  in  the  use  of  a 

2J  Public  Health  Bulletin  No.  71,  May,  1915,  p.  147. 

24  "Handbook  on  Incandescent  Lamp  Illumination"  for  1916,  p.  147. 

25  This  table  has  been  compiled  from  the  following  sources:  I.  E.  S.  "Code  of  Lighting 
for  Factories,   Mills  and  other  Work  Places;"  G.  E.  "Handbook  on  Incandescent  Lamp 
Illumination;"  United  States  Public  Health  Bulletin  No.  71;  Clewell's  "Factory  Lighting;" 
and  the  "  Electrical  Salesman's  Handbook  "  issued  by  the  National  Electric  Light  Association. 


350 


ILLUMINATING   ENGINEERING   PRACTICE 


TABLE  III.  —  INTENSITIES 

Bakery  

or    ILLUMIIS 

CLASSES 

Foot- 
candles 

2  .O-    3.O 

rATiON     RECOMMENDED    FOR 
OF  WORK 

Packing  and  shipping: 

VARIOUS 

Foot- 
candles 

2  .  0   -3.0 
2.0   -5-0 

2.0   -4.0 

4.0  -8.0 

0.25-0.5 
4.0  -6  .  o 

I  .  0   -2.0 
2  .  O   -4.0 

0.8  -i  .5 

2.0    -3-5 

2.0   -4.0 
2.0   -3-0 

3-0  -5-0 
6.0  -8.0 

2.0   -4.0 

3.0  -6  .  o 

2.0   -5-0 
5-0  -7-0 

3-0  -5.0 
4.0  -6  .  o 

2.0   -4.0 

0.25-0.5 

0.3  -0.5 
0.3  -0.5 

O.I    -0.3 

o.i  -0.3 

I  .0    -2.0 

2.0  -5.0 

0.8    -2.0 
I  .O    -2  .O 

0.5  -i  .0 
o.i  -0.3 
0.5  -i  .0 

i  .  o  -3.0 

2  .  O   -4.0 

0.25-0.5 

2.0    -4.0 

2.0  -5.0 
4.0  -8  .0 

2  .  O   -4.0 

3-0  -5.0 

2  .  O   -4.0 
2  .  0   -3-O 
3.0  -5-0 
4.0  -0.6 

Bench  work: 
Rough  

I   5—  S   0 

Fine  work  

Paint  shop: 

Fine  

.      3-5-10.0 

2  .  0—   4  .  0 

Box  factory  

Fine  work  (finishing)  
Passageways. 

Book  binding: 
Cutting,  punching,  stitching 
Embossing.  . 

.      3-0-  5.0 
4  o—  6  o 

Pattern  shop  (metal)  

Pottery: 
Grinding  
Pressing 

Folding,  assembling,  pasting 
Candy  factory  

Canning  plants: 
Coffee  roasting  at  tables.  .  .  . 
Filling  tables.  .    . 

.      2.0-  4.0 

2  .O-   4-0 

.      3-0-  4.0 
.0-  i  .5 

.0-    2.0 

.5-  2.5 

Power  house: 
Boiler  room27  

Packing  tables  

Preserving  plant: 
Cleaning  

Packing  tables  (dried  fruits) 

.0-  1.5 

5—    2    5 

Cooking  

Shipping  room 

Printing: 
Presses  

Cotton  mill  weaving2'  
Dairy  or  milk  depot 

.        2.0-   4.0 
2    O—   4    O 

Type-setters  

Sheet  metal  shop: 
Assembling  
Punching 

Drafting  room  . 

7   o 

Electrotyping  

3.0-  6.0 
.      4.0-  7.0 

.        2.0-    4.0 

.      3.0-  6.0 
.      3.0-  5.0 
.      1.25-3-0 

Factory: 

Shoe  shops: 
Bench  work  
Cutting 

Drills.  .  . 

Millers  

Silk  mill: 
Finishing  
Weaving  

Planers.  . 

Rough  manufacturing  ...... 

Special  cases  of  fine  work.  .  . 

Forge  and  blacksmithing: 
Ordinary  anvil  work  
Machine  forging  
Tempering 

.    10.0-15.0 

2  .O-    4-O 

.      2.0-  3.0 
2  o—  4  o 

Winding  forms  
Stairways  

Steel  work: 
Blast  furnace  (cast  house)..  .  . 
Loading  yards  (inspection)  .  .  . 
Mould,  skull  cracker  and  ore 
yards  
Open   hearth    floors    (soaking 
pits  and  cast  house)  
Rolling  mills  
Stamping  and  punching  sheet 
metal  '  

Tool  forging  

Foundry: 
Bench  moulding  

.     3.0-  5.0 

I  .  0-   3.0 
I  .O-    2  .O 

S.o 
7.0 

.      5.0-   6.0 
6  o—  10  o 

Floor  moulding  
Garment  industry: 

Dark  goods 

Stock  room  
Threading  floor  of  pipe  mills  . 
Transfer  and  storage  bays.  .  .  . 
Unloading  yards  
Warehouse 

Glove  factory: 
Cutting  
Sorting 

Hat  factory: 
BlocKing 

4.0-  6  .0 
.      3-0-5.0 

2  .O-    4.0 

-3-0-  8.0 
3.0-  6  .  o 
3-0-  5.0 

4.0-  6.0 
6.0-  8.0 

2.0-  3.0 

2.0-    4.0 
3  .O 

Stock  rooms: 
Rough  materials                      .  . 

Forming 

Fine  materials  
Storage  

Wire  drawing: 
Coarse                                        .  . 

Stiffening  
Jewelry  manufacturing  
Knitting  mill 

Fence  machines  
Fine                                        .... 

Laundry  

Wood  working: 
Rough  
Fine                                        .... 

Leather  working: 
Cutting  
Grading 

Woolen  mill: 
Picking  table  

Meat  packing: 
Cleaning  

Packing  

Offices..  . 

Weaving.  .  . 

24  See  also  G.  E.  "Handbook  on  Incandescent  Lamp  Illumination"  for  1916,  pp.  103-107. 
27  Supplemented  by  individual  lamps  at  the  gauges. 


Fig.  5. — Insulated  wire  department  with  a  system  of  mercury  vapor  lamps. 


Fig.  6. — Hand  press  room  in  bureau  of  engraving  and  printing  showing  use  of  mercury 

vapor  lamps. 

(Facing  page  350.) 


Fig.   ii. — Auxiliary  or  emergency  scheme  of  lighting. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  351 

table  of  this  kind,  because  the  values  of  intensities  listed  .may  some- 
times be  based  on  experience  in  a  limited  set  of  conditions.  Each 
new  location  should,  therefore,  receive  sufficient  study,  at  least,  to 
ascertain  whether  the  conditions  warrant  the  selection  of  an  intensity 
value  as  listed  in  this  table. 

TYPICAL  PLANS 

Perhaps  the  most  apparent  and  important  need  in  planning  factory 
lighting  is  to  be  familiar  with  the  best  methods  for  replacing  a  few 
large  units  for  a  given  floor  area,  with  a  relatively  large  number  of 
medium  sized  or  even  small  units.  Fig.  7  shows  an  excellent  example 
of  an  old  installation  where  a  single  large  lamp28  is  suspended  near  the 
center  of  the  aisle  with  large  separation  distances  between  units  down 
the  aisle.  The  chief  disadvantages  in  a  scheme  of  this  kind  are:  first, 
the  very  poor  distribution  of  the  light,  resulting  in  high  intensity 
values  at  certain  parts  of  the  floor  area  (usually  near  the  lamps  for 
such  low  mounting)  and  very  low  intensity  values  at  points  relatively 
not  very  remote  from  the  lamps;  second,  the  severe  tax  on  the  eye- 
sight, produced  by  the  glare  and  by  the  concentration  of  all  of  the 
light  furnished  such  a  double  bay,  in  one  large  unit  at  its  center;  and 
third,  the  very  objectionable  flicker  of  such  lamps  if  the  voltage  con- 
ditions of  the  supply  circuit  happen  to  be  poor. 

In  Fig.  8  one  typical  solution  of  the  case  shown  in  Fig.  7  is  indi- 
cated. Here  nearly  twenty  smaller  lamps29  replace  the  single  large 
unit,  since  there  is  only  one  lamp  in  Fig.  7  for  each  two  bays.  The 
results  from  an  installation  like  that  of  Fig.  8  are  much  improved  over 
the  older  plan,  the  illumination  being  very  uniform  over  the  entire 
floor  area  and  the  disadvantages  of  the  single  large  lamp  being  almost 
entirely  eliminated. 

In  Fig.  9  the  use  of  one  flaming  arc  lamp  for  every  two  bays  is 
contrasted  with  the  use  of  sixteen  loo-watt  vacuum  tungsten  lamps 
for  the  same  area.  The  first-mentioned  case  is  more  or  less  repre- 
sentative of  many  older  schemes  of  lighting  which  were  dictated  by 
the  lack  of  smaller  or  medium-sized  lamps;  while  the  latter  case  show- 
ing the  many  smaller  units  is  representative,  in  like  manner,  of  the 
application  which  has  been  made  in  many  factory  spaces  of  the 
tungsten  units.  It  is  comparatively  easy  to  see  that  the  distribution 

*•  Actually  an  inclined  electrode,  short  burning,  flame  carbon  arc  lamp. 

19  loo-watt  vacuum  tungsten  lamps  under  the  mezzanine  floor  and  250-watt  lamps  of  the 
same  type  on  the  20  ft.  ceiling  line.  Prismatic  glass  reflectors  are  used  for  both  sizes  of 
lamps  in  Fig.  8. 


352 


ILLUMINATING    ENGINEERING   PRACTICE 


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CLEWELL:  LIGHTING  OF  FACTORIES,  ETC. 


353 


will  be  vastly  more  uniform  with  the  larger  number  of  lamps,  although 
comparative  illumination  tests  with  portable  photometers  under  two 
systems  like  these,  render  the  conclusions  even  more  convincing. 

As  an  example  of  different  points  of  attack  in  two  factory  sections 
with  the  same  dimensions  (floor  area  and  ceiling  height)  Fig.  10  is 
shown.  Here  the  kinds  of  work  are  the  important  factors.  To  the 
left,  the  large  cylindrical  tanks  require  considerable  light  on  the  in- 


Fig.  9. — View  in  which  a  comparison  is  shown  between  the  use  of  one  large  lamp  in  every 
other  bay  and  sixteen  relatively  small  tungsten  units  in  the  same  area. 

side  surfaces  and  thus  make  it  almost  essential  to  use  enough  overhead 
lamps  to  reduce  the  possibilities  of  dense  shadows  on  the  inside  of  the 
tanks.  Of  course,  the  use  of  one  large  flaming  arc  lamp  at  the  center 
of  every  other  bay  as  shown,  results  in  dense  shadows  on  the  inside  of 
practically  all  the  tanks  unless  a  tank  happens  to  be  almost  directly 
beneath  a  lamp.  The  use  of  six  2 50- watt  vacuum  tungsten  lamps 
with  prismatic  glass  reflectors  spaced  12  feet  apart,  proved,  under 
trial,  a  very  much  more  satisfactory  scheme. 
23 


354 


ILLUMINATING   ENGINEERING   PRACTICE 


In  contrast  to  the  left-hand  portion  of  Fig.  10,  the  right-hand  por- 
tion may  be  studied  with  profit.  Here  a  single  large  naming  arc 
lamp  was  formerly  used  for  the  bench  surface  illumination,  but, 
because  of  the  poor  distribution,  drop  cords  and  localized  lamps  were 
needed  to  supplement  the  general  illumination  produced  by  the 
single  arc  lamp.  The  use  here  of  2 50- watt  vacuum  tungsten  lamps 


to  replace  the  arc  lamps  was  based  largely  on  the  desire  to  produce 
an  almost  uniform  horizontal  illumination  intensity  over  the  entire 
bench  areas,  whereas  in  the  case  of  the  tanks,  reduced  shadows  prob- 
ably formed  the  most  important  single  factor.  In  the  bench  section, 
the  use  of  the  2 50- watt  units  eliminated  the  need  for  localized  lamps, 
giving  the  space  a  much  more  pleasing  appearance  and  actually  ren- 
dering more  bench  area  available  for  work. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  355 

The  foregoing  cases  are  typical  of  several  actual  locations  with  the 
corresponding  solutions  of  the  given  problems,  and  it  will  be  noted 
that  in  each  case  the  improved  scheme  was  made  possible  by  the 
availability  of  a  small  or  medium-sized  type  of  lamp,  and  the  economy 
with  which  a  relatively  large  number  of  such  lamps  can  be  used  over- 
head. To'  demonstrate  the  practical  nature  of  such  an  improved 
installation  as  compared  with  the  older  practice  of  using  very 
large  units  sparsely  scattered  over  the  ceiling  area,  it  is  usually  a 
good  plan  first,  to  demonstrate  that  the  actual  yearly  cost  of  such 
an  improved  system  is  a  very  small  percentage30  of  the  actual  annual 
outlay  for  wages  in  the  given  section ;  and  second,  to  make  a  sample 
installation  in  several  bays,  which  contain  active  work. 

This  procedure  will  usually  prove  conclusively  to  the  manager 
or  owner  that  the  investment  represents  not  merely  an  expense 
in  the  ordinarily  accepted  meaning  of  the  term,  but  that  it  is  an 
outlay  which  will  be  accompanied  with  more  or  less  tangible  re- 
turns because  of  a  better  and  larger  product  for  the  same  wages  as 
were  previously  expended  under  the  conditions  of  the  older  inferior 
lighting  conditions. 

DISTRIBUTION  CIRCUITS 

In  motor  driven  factories,  there  is  sometimes  a  tendency  to  supply 
energy  to  motors  and  lamps  from  the  same  circuits.  The  motor 
load,  however,  will  most  likely  be  a  much  larger  proportion  of  the 
total  than  the  lighting  load,  and,  due  to  possible  excessive  variations 
in  the  motor  load,  there  is  a  likelihood  that  the  voltage  fluctuations 
at  the  lamps  will  be  unduly  large. 

Where  tungsten  lamps  are  employed,  large  variations  in  the 
supply  voltage  do  not  materially  affect  the  life  of  the  lamps,31 
but  the  candle-power  and  hence  the  illumination  will  vary  over 
wide  ranges.  With  some  other  types  of  lamps,  excessive  voltage 
fluctuations  may  cause  more  serious  difficulties.  As  a  general  rule, 
therefore,  motor  and  lighting  circuits  should  be  separate  and  the 
circuits  individually  designed  and  the  loads  on  each  kept  down  to 
such  values  as  to  insure  approximate  constancy  in  the  lighting 
circuit  supply  voltage  at  all  times. 

For  tungsten  lighting,  no- volt  mains  are  usually  best,  because  of 
the  higher  efficiency  and  lower  first  cost  of  no- volt  lamps  in  con- 
trast to  those  of  the  2  20- volt  class.  If  2  20- volt  service  exists  and 

30  The  percentage  should  be  worked  out  numerically. 

11  Bulletin  20,  Engineering  Dept.,  National  Lamp  Works,  General  Electric  Co.,  p.  19. 


356  ILLUMINATING    ENGINEERING    PRACTICE 

tungsten  lamps  are  to  be  used,  arrangements  may  be  made  for 
no- volt  operation.  On  alternating-current  circuits,  a  frequency 
of  60  cycles  per  second  is  preferable  to  one  of  25  cycles  per  second, 
but  tungsten  lamps  are  operative  on  25  cycle  circuits  with  a  very 
fair  degree  of  satisfaction. 

Switch  Control. — In  the  switch  control  of  a  lighting  system  com- 
posed of  a  large  number  of  small  lamps,  it  will  usually  be  economical 
to  connect  a  small  group  of  lamps  so  that  it  may  be  controlled  from  a 
single  switch.  However,  where  the  energy  cost  is  low  and  the  instal- 
lation expense  for  switch  circuits  is  very  high,  it  is  well  not  to  go 
to  extremes  in  the  subdivision  of  the  circuits.  As  a  general  rule,  the 
lamps  should  be  grouped  in  rows  parallel  to  the  side  walls  containing 
windows.32 

Emergency  Lighting. — The  Illuminating  Engineering  Society's 
Code  of  Factory  Lighting  in  Article  XI,  calls  for  auxiliary  lighting 
in  all  large  work  spaces,  such  lamps  to  be  in  operation  simultaneously 
with  the  regular  lighting  system,  so  as  to  be  available  in  case  the 
latter  should  become  temporarily  deranged. 

In  Section  XVI  of  the  descriptive  portion  of  this  code  (p.  44) 
this  point  is  emphasized  as  follows: 

"The  auxiliary  system  of  lighting  called  for  in  Article  XI  of  the  code, 
is  a  safety-first  precaution,  which  is  insisted  upon  in  a  large  proportion  oi 
the  1 200  buildings  coming  under  the  control  of  the  Bureau  of  Water 
Supply,  Gas  and  Electricity  in  New  York  City,  particularly  such  buildings 
as  are  occupied  by  large  numbers  of  people.  The  same  precaution  is  now 
observed  by  the  Bell  Telephone  Company's  offices  fairly  generally  through- 
out the  country,  also  by  a  large  number  of  private  manufacturers  and  by 
local  ordinances  compelling  all  types  of  amusement  places  to  take  this 
precaution." 

The  Code  of  Lighting  for  Eactories,  Mills  and  Other  Work  Places, 
adopted  by  the  state  of  Pennsylvania  on  June  i,  1916,  contains 
a  clause,  under  the  title  " Emergency  Lighting"  which  reads  as 
follows : 

"Emergency  lighting  shall  be  provided  in  all  work  space,  aisles,  stair- 
ways, passageways,  and  exits;  such  lights  shall  be  so  arranged  as  to  insure 
their  reliable  operation  when  through  accident  or  other  cause  the  regular 
lighting  is  extinguished." 

Eig.  ii  shows  a  space  in  which  ther^e  are  two  systems  of  lighting 
(a)  a  number  of  direct  units,  and  (b)  several  indirect  fixtures. 

82  These  statements  apply  also,  in  a  general  way,  to  the  supply  mains  of  gas  lighting 
systems. 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC. 


357 


Normally,  the  illumination  is  furnished  by  the  direct  units;  if  these 
go  out  for  any  reason  the  indirect  units  are  turned  on  automatically. 

MAINTENANCE 

The  deterioration  of  tungsten  lamps  and  reflectors  due  to  accumu- 
lations of  dust  on  the  lamp  and  reflector  surfaces  is  shown  graphically 


12 


42 


48 


54 


18  24  30  36 

Elapsed  Time  in  Days 

Fig.  12. — Deterioration  of  the  same  kind  of  lighting  equipment  in  two  different  kinds  of 

locations. 


H 


A- Dome  Enameled  Steel 

S-Bowl  Enameled  Steel 

C-  Dense  Opal  Glass 

D- Prismatic  Glass 

E-  Light  Density  Opal  Glass 


B 


n 


12  16  20  24  28 

Elapsed  Time  in  Weeks 

Fig.  13. — Deterioration  of'  various  kinds  of  lighting  equipment  in  the  same  kind  of 
location.  The  deterioration  in  Figs.  12  and  13  are  due  almost  entirely  to  accumulations  of 
dust  and  dirt  on  the  lamps  and  reflectors. 


in  Figs.  12  and  13.  Fig.  1 2  shows  the  rate  of  deterioration  for  a  given 
type  of  lamp  and  reflector  in  two  different  kinds  of  locations; 
while  Fig.  13  shows  the  deterioration  rates  for  a  number  of  different 


358  ILLUMINATING   ENGINEERING   PRACTICE 

types  of  reflectors  used  with  tungsten  lamps  under  one  fixed  kind  of 
location. 

These  curves  illustrate  the  importance  of  systematic  attention  to 
the  cleaning  of  both  shop  windows  and  lighting  units.  With  the 
latter,  it  is  also  very  important  to  renew  all  burned  out  lamps  promptly 
and  to  " re-carbon"  the  arc  lamps  regularly.  To  insure  regularity 
in  such  work,  it  may  be  desirable  to  detail  someone  from  the  lamp  or 
maintenance  department  to  inspect  each  unit  in  the  lighting  systems 
at  fairly  frequent  intervals,  it  being  his  duty  to  report  all  burned  out 
or  defective  lamps  as  well  as  particular  shop  sections  where  lamps 
and  reflectors  are  in  need  of  cleaning.  The  cost  of  reflector  cleaning 
is  sometimes  included  as  one  of  the  fixed  charges,  instead  of  a 
maintenance  item. 

COST  DATA 

Wiring  and  installation  expense  in  factory  buildings  is  exceedingly 
variable  due  to  the  extreme  variety  of  conditions  met  with,  and 
hence  an  idea  of  the  ranges  in  cost  may  advantageously  be  given 
at  the  outset. 

The  actual  total  expense  for  labor  and  material,  including  lamps, 
reflectors  and  switch  circuits  in  tungsten  systems,  may  range  from 
about  $3.00  per  outlet  (each  composed  of  one  loo-watt  lamp)  for 
wood  moulding  on  a  wood  ceiling  of  10  to  14  ft.  height;  up  to  about 
$7.00  per  outlet  for  the  same  size  of  lamp,  for  iron  conduit  work 
attached  to  iron  trusses  on  a  line  16  ft.  above  the  floor.  Extreme 
cases  of  very  high  ceilings,  or  peculiar  difficulties  in  the  installation 
of  the  circuits,  may  run  this  cost  up  to  very  much  higher  values. 
Obviously,  also,  the  cost  per  outlet  complete,  will  be  very  much 
larger  if  the  first  cost  of  the  type  of  lamp  used  is  very  large. 

In  the  Handbook  on  Shop  Lighting  issued  by  the  Industrial  Com- 
mission of  Wisconsin  (prepared  by  F.  Schwarze)  it  is  stated  that  the 
wire  and  conduit  in  a  shop  lighting  installation  cost  150  per  cent, 
more  than  the  cost  of  the  lamp,  and  that  the  cost  of  the  wire  and 
knobs  for  open  work  is  125  per  cent,  of  the  cost  of  the  lamp.  The 
labor  for  a  conduit  installation  is  approximately  45  per  cent,  of  the 
cost  of  lamp  and  wiring  materials.  The  labor  for  an  open-work 
installation  costs  approximately  50  per  cent,  of  the  cost  of  lamp  and 
wiring  materials.  This  does  not  include,  however,  the  cost  of  the 
mains  and  distribution  centers.  The  following  two  cases  are  given 
by  the  same\uthority: 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  359 

CONDUIT  INSTALLATION 

Tungsten  lamp $o .  70 

Conduit,  wire,  etc i .  75 

Labor 1.12 

Reflector i .  25 


Total $4 . 82 

OPEN-TYPE  INSTALLATION 

Tungsten  lamp $o .  70 

Wiring  materials o .  70 

Labor o.  70 

Reflector 1.25 

Total $3.35 

The  total  operating  cost  of  tungsten  lighting  systems  is  very  well 
discussed  in  bulletin  20,  Engineering  Department,  National  Lamp 
Works  of  General  Electric  Co.,  pp.  42  to  45,  on  which  the  following 
information  is  based: 

In  determining  the  total  operating  cost  of  any  system  of  lighting,  three 
items  must  be  considered:  first,  fixed  charges,  which  include  interest  on 
the  investment,  depreciation  of  permanent  parts,  and  other  expenses 
which  are  independent  of  the  number  of  hours  of  use.  Frequently  this 
item  forms  the  greater  part  of  the  total  operating  expense,  yet  it  is  only  too 
often  omitted  from  cost  tables;  second,  maintenance  charges,  which  include 
renewal  of  parts,  repairs,  labor,  and  all  costs  except  the  cost  of  energy, 
which  depend  upon  the  hours  of  burning;  and  third,  the  cost  of  energy, 
which  depends  upon  the  hours  burning  and  the  rate  per  kilowatt-hour. 

The  life  of  a  lighting  system  depends  not  only  upon  the  wearing  put  of 
parts,  but  also  upon  obsolescence.  There  are  no  installations  in  this 
country  which  have  been  in  use  for  a  period  of  seven  or  eight  years  which 
are  not  already  obsolete.  Although  the  lamps  may  be  in  good  operating 
condition,  economy  demands  that  they  be  replaced  by  more  efficient 
illuminants.  There  is  every  indication  that  the  next  few  years  will  see 
even  greater  progress  in  the  development  of  lamps  and  the  use  of  light. 
The  rate  of  depreciation  on  all  permanent  parts  is  equal  to  at  least  i2L£ 
per  cent.  The  investment  required  in  the  tungsten  system  is  relatively 
very  low. 

Table  IV,  compiled  by  the  same  authority,  is  based  on  a  total  invest- 
ment including  the  cost  of  lamps,  reflectors,  holders  and  sockets  only. 
The  investment  in  permanent  parts  is  therefore  the  total  investment  less 
the  price  of  the  lamps.  No  depreciation  is  charged  against  the  lamps  in- 
asmuch as  they  are  regularly  renewed.  The  labor  item  under  fixed 
charges  provides  for  the  cleaning  of  all  units  once  each  month.  For  the 
smaller  units  with  Holophane  steel  reflectors,  the  cost  of  cleaning  is  taken 


360  ILLUMINATING   ENGINEERING  PRACTICE 

TABLE  IV. — ANALYSIS  OF  OPERATING  COSTS — 100  TO  130  VOLT  MAZDA  UNITS* 


Size  of  lamp,  rated  watts 

40 

60 

IOO 

150 

250 

400 

500 

Cost  of  Lamp,  List 

$0.350 
0.315 
1.  155 
i  .470 

$0.450 
0.405 
i  .292 
i  .697 

$0.800 
0.720 
1.566 
2.286 

$1  .200 
I  .O8O 
1.653 
2.733 

$2  .000 
I  .800 
1.653 
3-453 

$3.600 
3.240 
2.617 
5.857 

$4  .  ooo 
3.600 

2  .617 

6.217 

Cost  of  Lamp,  Std.-Pkg.  Disc  
Cost  of  Reflector,  Std.-Pkg.  Disc  
Cost  of  Unit,  Std.-Pkg.  Disc  

Annual  Fixed  Charges: 
Interest  on  Total  Invest.,  6  per  cent. 
Deprec'n  on  Reflector,  12^2  per  cent. 
Labor,  Monthly  Cleaning  

$0.088 
0-144 
0.240 

$0.102 
0.162 
0.240 

$0.137 
0.196 
0.240 

$0.164 

0.207 
0.360 

$0.207 

0.207 
0.360 

$0.351 
0.327 
0.480 

$0.373 

0.327 
0.480 

Total 

$0.472 

$0.504 

$0.573 

$0.731 

$0.774 

$1.158 

$1  .180 

Maintenance  Cost  per  1000  Hours: 
Lamp   Renewals  at  Std.-Pkg.    Dis- 
count   
Lamp    Renewals   at    $iso-Contract 
Discount  
Lamp  Renewals  at  $i2oo-Contract 
Discount  

$0.315 
0.291 
0.256 

$0.405 

0-374 
0.329 

$0.720 
0.664 
0.584 

$1  .080 
0.996 
0.876 

$1.800 

1.660 
1.460 

$3.240 
2.988 
2.628 

$3.600 
3.320 

2  .920 

Energy   Cost  per   1000  Hours  at   ic. 
per  Kw-hr 

$0.400 

$0.600 

$1  .000 

$1  .500 

$2  .500 

$4  .  ooo 

$5.000 

*  The  prices  on  lamps  and  reflectors  upon  which  the  calculations  of  this  table  are  based 
are  subject  to  change  without  notice;  they  are  used  here  solely  for  convenience  in  engineering 
calculations. 

as  $0.02  per  unit  for  each  cleaning.  Data  obtained  from  installations 
where  accurate  cost  records  are  kept,  show  that  this  figure  is  conservative 
for  labor  at  $0.20  per  hour.  The  cost  of  cleaning  other  reflectors  is  taken 
in  proportion  to  the  amount  of  labor  required.  Some  illuminants  require 
attendance  at  regular  intervals;  the  cleaning  is  done  at  the  same  time  and 
is,  therefore,  included  under  the  maintenance  charge.  For  units  which 
require  no  regular  attendance,  the  cleaning  expense  becomes  a  separate 
charge.  It  will  be  noted  that  the  fixed  charges  form  only  a  small  part  of 
the  total  operating  cost  for  a  lighting  system.  The  folly  of  using  cheap 
reflectors,  which  impair  the  efficiency  of  the  units,  is  evident. 

The  maintenance  charge  is  given  for  a  looo-hour  period  of  burning. 
To  find  the  annual  charge  in  any  case,  it  is  necessary  to  multiply  by  the 
ratio  of  the  total  hours  of  burning  to  1000  hours.  Where  lamps  are  sold 
at  other  than  the  prices  given,  the  proper  correction  should  be  applied. 
The  renewal  of  lamps  is  the  only  maintenance  expense. 

The  energy  cost  is  given  for  a  icoo-hour  period  with  energy  at  $0.0 1  per 
kilowatt-hour.  The  energy  cost  per  year  is  found  by  multiplying  by  the 
cost  per  kilowatt-hour  in  cents  and  by  the  time  of  burning  in  thousands 
of  hours. 

An  example  will  illustrate  the  use  of  Table  IV.  It  is  required  to  find 
the  total  operating  expense  per  unit  per  year  for  lighting  a  mill  with 


CLEWELL:  LIGHTING  OF  FACTORIES,  ETC.  361 

2  50- watt  tungsten  lamps.  The  lamps  are  burned  a  total  of  4000  hours 
and  are  purchased  as  the  discount  obtained  on  a  $150  contract.  The  cost 
of  energy  is  $0.02  per  kilowatt-hour.  From  the  table,  the  following  results 
are  obtained: 

1.  Fixed  charges33 $  o.  774 

2.  Maintenance  4.000  X  $1.660 6 . 640 

3.  Energy  4.000  X  2  X  $2.50 20.000 


Total $27.414 

It  is  important  when  making  a  study  of  such  cost  data  to  keep  in 
mind  the  underlying  advantages  of  good  light  as  outlined  in  the  first 
part  of  this  lecture,  and  to  remember  that  small  differences  in  first 
cost  or  in  the  total  operating  costs  of  two  systems  under  consideration, 
should  be  entirely  overlooked,  if  one  system,  from  the  illuminating 
standpoint,  possesses  any  distinct  advantage  over  the  other. 

Bibliography 

Attention  is  called  to  the  selected  list  of  references  pertaining  to  illumination 
design,  contained  in  the  Fourth  Edition  of  the  Standard  Handbook  for  Electrical 
Engineers,  p.  1162. 

The  sections  on  lamps,  lighting  and  illumination  contained  in  the  following 
handbooks  may  also  be  consulted  with  profit : 

American  Handbook  for  Electrical  Engineers,  John  Wiley  and  Sons,  Inc., 
432  Fourth  Ave.,  N.  Y. 

Handbook  of  Machine  Shop  Electricity  by  C.  E.  Clewell,  McGraw-Hill  Book 
Co.,  Inc.,  239  West  39th  St.,  N.  Y. 

Standard  Handbook  for  Electrical  Engineers,  McGraw-Hill  Book  Co.,  N.  Y. 

Bulletins  and  Data  of  the  National  X-Ray  Reflector  Co. 

Bulletin  71,  issued  by  the  Federal  Health  Service,  by  J.  W.  Schereschewsky 
and  D.  H.  Tuck,  Treasury  Dept.,  Washington. 

Code  of  Lighting  for  Factories,  Mills  and  Other  Work  Places,  issued  by  the 
Illuminating  Engineering  Society,  1915.  Contained  in  the  Transactions  of  the 
Society. 

Factory  Lighting  by  C.  E.  Clewell,  McGraw-Hill  Book  Co. 

First  Report,  Departmental  Committee  on  the  Lighting  of  Factories  and 
Workshops,  Great  Britain,  1915. 

Handbook  on  Incandescent  Lamp  Illumination,  General  Electric  Company, 
Harrison,  N.  J. 

Handbook  on  Shop  Lighting  issued  by  the  Industrial  Commission  of  Wisconsin. 

Industrial  Lighting,  Bulletin  20,  Engineering  Dept.,  National  Lamp  Works 
of  G.  E.  Co. 

Lighting  Code,  Pennsylvania  Dept.  of  Labor  and  Industry,  Harrisburg,  Pa. 

Publications  of  the  National  Electric  Light  Association. 

Transactions  of  the  Illuminating  Engineering  Society. 

11  The  value  is  taken  as  in  the  table.  It  will,  of  course,  be  reduced  by  the  difference  in 
interest  on  the  lamp  at  the  standard-package  price  and  at  the  $150  contract  price.  The 
difference  is  practically  negligible. 


OFFICE,  STORE  AND  WINDOW  LIGHTING 
BY  NORMAN  MACBETH 

The  art  of  applying  lighting  units  to  the  production  of  useful  and 
artistic  illumination  in  offices  and  stores  has  not  kept  pace  with  the 
development  of  new  illuminants.  This  is  partly  due  to  the  remark- 
able rapidity  of  this  development,  and  partly  to  the  fact  that  the 
later  illuminants  could  not  be  effectively  applied  after  the  comple- 
tion of  the  buildings. 

A  great  advance  in  the  use  of  artificial  lighting  has  been  made  in 
the  past  decade.  Too  often  this  result  has  been  accompanied  with 
an  unnecessary  sacrifice  in  the  beauty  of  the  building  through  the 
use  of  inappropriate  fixtures  or  the  improper  distribution  of  the 
light.  In  resisting  this  influence  architects  have  sometimes  neg- 
lected to  provide  useful  illumination  in  keeping  with  present-day  de- 
mands. In  such  cases  the  artistic  work  of  the  architect  has  often 
been  undone  by  the  users  in  the  endeavor  to  meet  their  practical 
needs.  There  are  plenty  of  examples  where  bare  lamps  of  higher 
power  have  been  substituted  for  lower-intensity  frosted  lamps,  to  the 
ruination  of  the  artistic  effect  and  a  sacrifice  of  the  physical  effect. 
The  cure  for  this  is  to  provide  ample  artistic  lighting  in  such  a  way 
that  it  cannot  easily  be  spoiled  by  the  inexperienced.  For  example, 
it  is  often  practicable  to  provide  clear  lamps  concealed  in  diffus- 
ing glassware,  tinted  if  desired  to  secure  a  particular  color  effect. 
Pressed  and  blown  glass  in  keeping  with  the  important  periods  of 
architecture  and  decoration  are  now  available,  while  art  glass  can 
readily  be  made  up  into  any  character  of  design. 

All  modern  and  efficient  illuminants  are  too  brilliant  for  use 
without  some  provision  for  screening  and  diffusion. 

It  is  necessary  to  give  particular  attention  to  the  proper  shielding 
of  filaments  and  mantles  of  lamps  for  the  protection  of  our  eyes. 
This  shielding,  whether  with  globes,  shades,  or  reflectors,  should 
be  done  in  a  pleasing  manner  so  far  as  the  design  and  general  arrange- 
ment of  the  fixtures  are  concerned.  To  be  able  to  see  clearly  and 
easily  is  the  first  step  toward  efficiency  and  the  amount  of  energy 
necessary,  while  frequently  the  only  previous  consideration  when 
speaking  of  "efficiency"  as  related  to  lighting,  is  secondary. 

363 


364  ILLUMINATING   ENGINEERING   PRACTICE 

Calculations  by  the  experienced  lighting  man  are  largely  for  a 
check  on  his  judgment.  This  judgment  is  the  result  of  experience 
on  lighting  installations  and  particularly  from  the  results  secured 
from  investigations  which  he  personally  has  made  of  previous  in- 
stallations. These  investigations  should  always  be  accompanied 
with  illumination  and  brightness  measurements. 

While  a  few  years  there  was  very  little  difference  in  efficiency 
between  the  various  sizes  of  incandescent  gas  and  electric  lamps, 
largely  used  for  office  and  store  lighting,  it  was  a  rather  general 
practice  to  check  up  these  calculations  on  the  basis  of  cubic  feet  of 
gas  or  watts  per  square  foot.  With  the  wide  variation  in  efficiency 
of  various  sizes  of  lamps  at  this  time,  the  more  simple  exact  method 
should  be  adopted  of  basing  the  calculations  on  the  total  light  output 
of  the  lamp  in  lumens  or  of  lumens  per  cubic  foot  of  gas  per  hour, 
or  per  watt. 

It  is  necessary  for  good  illumination  that  there  should  be  a  suf- 
ficiently high  intensity,  with  attention  to  uniformity,  diffusion, 
eye  protection,  appearance,  and  efficiency.  The  importance  of  these 
characteristics  of  good  lighting  will  vary  in  different  installations. 
Efficiency  has  at  times  been  given  too  much  attention  and  promi- 
nence. It  is  used  here  to  refer  in  office,  store  or  window  lighting 
to  that  proportion  of  the  generated  light  effective  on  an  assumed 
plane.  At  times  this  consideration  is  extremely  important,  and  at 
other  times  of  practically  no  importance.  It  is  necessary,  of  course, 
for  the  predetermining  of  results,  to  know  the  probable  efficiency 
of  the  installation,  that  is,  what  per  cent,  of  the  total  light  produced 
by  the  lamps  reaches  the  working  plane.  This  is  generally  termed 
"utilization  efficiency"  and  may  vary  from  70  per  cent,  to  10  per 
cent,  or  less  of  the  total  light  from  the  lamps.  It  is  possible  to 
design  a  lighting  installation  which  from  this  single  standard  would 
have  a  high  value,  but  which,  from  the  standpoint  of  assistance  to 
easy  and  clear  vision,  that  is,  from  the  standpoint  of  good  illumina- 
tion, would  be  an  absolute  failure. 

Within  the  past  few  years  there  has  grown  up  a  strong  appreciation 
of  the  ill  effects  of  bright  sources  on  the  eye  and  of  extremes  of 
contrast  between  the  average  brightest  and  darkest  portions  of  the 
room.  There  is  a  tremendous  difference  nevertheless  between 
equipment  which  without  photometric  tests  appears  to  be  quite 
similar  but  which,  from  the  standpoint  of  distribution  of  light 
and  the  contrast  conditions  set  up,  may  vary  in  efficiency  upward 
to  50  per  cent. 


MACBETH:  LIGHTING  OF  OFFICES  365 

Indirect  lighting  or  semi-indirect  lighting  in  which  no  part  of  the 
fixture  is  brighter  than  the  ceiling  is  generally  more  satisfactory 
than  the  average  system  of  direct  lighting  where  clerical  work  is 
done,  as  tests  have  shown  that  the  efficiency  of  the  eye  is  reduced 
very  rapidly  under  any  system  of  illumination  in  which  light  sources 
of  a  brilliancy  of  those  of  our  commercial  types  are  within  the  ordi- 
nary range  of  the  eye. 

Much  of  the  so-called  semi-indirect  lighting  is  but  slightly  modi- 
fied direct  lighting.  This  kind  of  lighting  with  light  density  glass- 
ware has  been  most  general  and  has  undoubtedly  resulted  in  the 
unjust  condemnation  of  semi-indirect  lighting  as  a  whole.  It  has 
been  shown,1  however,  that  of  the  glassware  on  the  market  used  for 
semi-indirect  lighting  fixtures,  at  least  90  per  cent,  of  it  has  too  high 
a  transmission.  A  worthy  endeavor  is  being  made  to  reduce  this  con- 
trast in  lighting  installations  to  within  the  range  of  100  to  i,  that  is, 
the  brightest  object  within  the  range  of  view  should  not  be  more  than 
100  times  brighter  than  the  average  lower  intensity.  The  average 
semi-indirect  lighting  installation  with  light  density  opal  glass  is 
merely  an  inefficient  system  of  direct  lighting.  In  many  locations 
direct  lighting,  particularly  where  the  ceilings  are  low,  would  better 
meet  the  requirements.  In  all  such  cases,  however,  very  deep  bowl 
glass  reflectors  having  low  transmission  should  be  used.  The  lamp 
filament  should  be  covered  down  to  the  65°  point,  and  it  is  also  de- 
sirable that  the  lower  edge  of  the  reflector  be  flared  out  so  that  this 
part  of  the  reflector  interior  as  ordinarily  seen  be  not  overly  bright. 

There  is  an  important  difference  between  diffusion  of  light  and 
diffusion  of  illumination.2  Light  from  a  single  source,  no  matter 
to  what  extent  that  light  may  be  diffused  by  the  enclosing  media 
would,  from  the  standpoint  of  illumination  on  a  desk,  not  result  in 
diffusion.  Diffusion  of  illumination  is  the  important  factor  and  is 
the  result  you  secure  when  the  light  received  on  the  surface  viewed 
is  from  a  number  of  directions.  This  may  be  secured  through  close 
spacing  of  units  or  through  using  the  ceiling  as  the  light  distributor 
either  with  indirect  or  semi-indirect  fixtures.  Good  diffusion  of 
illumination  is  essential  in  offices,  drafting  rooms  and  similar  places 
where  glazed  paper  or  desk  tops  with  polished  surfaces  are  in  use. 
For  stores,  a  high  degree  of  diffusion  is  not  so  necessary  and  is  gen- 
erally present  to  a  sufficient  degree  with  any  system  of  lighting 
because  of  the  large  number  of  outlets. 

Maintenance  of  lighting  equipment  should  not  be  overlooked  and 
it  is  very  important  that  arrangements  be  made  for  a  proper  cleaning 


366  ILLUMINATING   ENGINEERING   PRACTICE 

of  the  equipment  at  periods  v.arying  from  two  weeks  to  a  month  or 
more,  depending  upon  the  dust  conditions  of  the  location.  Deteri- 
oration is  less  with  direct  lighting  reflector  and  lamp  units  than  with 
the  indirect  or  semi-indirect  units.  There  are  locations  where  it 
would  not  be  possible  to  keep  indirect  and  semi-indirect  units  clean 
without  a  cleaning  period  so  frequent  as  to  be  unnecessarily  expen- 
sive. In  these  cases  direct  lighting  equipment  will  permit  of  longer 
periods  between  cleaning. 

It  is  well  to  remember  in  accepting  tables  of  desired  intensity, 
utilization  factors  or  constants,  and  methods  of  calculation  gener- 
ally furnished  by  the  equipment  manufacturers  that  these  values  are 
invariably  for  new  clean  equipment.  An  average  deterioration 
factor  should  be  used  of  10  per  cent,  to  25  per  cent.,  depending  upon 
whether  the  maintenance  is  likely  to  be  good  or  average.  In  consid- 
ering fixture  design,  it  is  worth  while  to  note  also  the  ease  or  difficulty 
as  the  case  may  be  with  which  the  equipment  can  be  cleaned. 

Fixtures  should  be  substantial  and  the  means  of  removing  glass- 
ware for  cleaning  should  be  simple.  Ordinary  labor  is  generally  used 
for  maintenance  and  holders  with  springs  or  similar  complications 
are  not  easily  taken  care  of  by  the  average  cleaner. 

OFFICE  LIGHTING 

Office  employees  as  a  class  are  subjected  to  more  severe  eye-strain 
than  almost  any  other  class  of  workers. 

In  our  large  cities  during  many  hours  of  the  day  and  in  many 
instances  all  day,  they  work  with  a  mixture  of  natural  and  artificial 
light.  The  intensities  of  the  latter,  in  these  days  of  unwise  economy 
of  energy  for  lighting,  are  rarely  adequate:  There  would  seem  to  be 
little  doubt  that  with  a  mixture  of  daylight  and  artificial  light,  a 
higher  intensity  of  artificial  light  is  required  than  where  artificial 
light  alone  is  used.  Whether  this  is  due  to  a  higher  eye  adaptation 
demand  or  to  color  differences,  has  not  been  decided. 

The  frequent  use  of  an  instrument  for  measuring  illumination 
intensities  cannot  be  too  strongly  recommended.  In  a  recent 
installation  complaint  was  made  that  the  clerks  were  having  diffi- 
culty with  their  eyes,  although  apparently  the  lighting  installation 
had  been  given  every  possible  design  and  maintenance  attention. 
On  inspection  it  was  shown  that  the  spacing  and  kind  of  fixtures  were 
satisfactory,  the  contrast  between  the  brightest  and  darkest  object 
in  the  room  was  well  within  the  proper  range  and  the  installation  had 


MACBETH:  LIGHTING  OF  OFFICES  367 

every  appearance  of  being  right.  Illumination  measurements, 
however,  brought  out  the  point  that  the  average  intensity  was  about 
1.5  foot-candles  which  was  certainly  not  high  enough  for  the  charac- 
ter of  clerical  work  performed.  A  simple  increase  in  the  size  of 
lamps  used  corrected  the  difficulty. 

In  a  recent  investigation3  in  a  block  of  office  buildings  in  New 
York  City,  over  85  per  cent,  of  the  workers  were  under  artificial 
light  or  a  mixture  of  natural  and  artificial  light  all  day,  and  over  90 
per  cent,  of  them  worked  more  than  eight  hours  per  day.  Some  of  the 
clerks  and  stenographers  had  only  0.5  to  1.5  foot-candles  on  their 
work.  Others  again  by  the  use  of  portables,  mostly  placed  improp- 
erly, worked  under  30  to  40  foot-candles,  likewise  suffering  from 
headaches  and  eye  discomfort.  An  entire  floor  of  ledger  keepers 
with  a  system  of  semi-indirect  units,  otherwise  satisfactory,  had 
only  0.5  to  under  2  foot-candles  effective  at  the  desks.  The  mis- 
directed economy  demands  of  the  office  building  superintendents  or 
the  competition  demands  of  the  venders  of  lighting  equipment,  to 
do  with  a  less  energy  expenditure  than  necessary,  was  undoubtedly 
accountable  for  these  installations. 

Economy  is  frequently  referred  to  in  considering  office  lighting. 
The  economy  which  is  most  lasting,  however,  is  that  which  avoids 
the  waste  of  human  energy.4 

"  Such  waste  has  no  compensating  return  but  is  an  irretrievable  and  total 
loss.  The  man  who  works  an  entire  day  to  accomplish  that  which,  under 
obtainable  conditions,  he  could  accomplish  in  half  a  day,  has  wasted  a 
portion  of  that  which  is  above  all  price — life Poor  and  insuf- 
ficient light  is  indefensible  from  every  standpoint The  most 

vital  of  all  economies  is  the  saving  of  human  energy Let  us 

not  overlook  the  fact  that  we  work  by  sight,  that  we  see  by  light." 

In  discussing  the  cost  of  lighting,  Professor  Charles  F.  Scott  stated5 
that  in  one  instance  the  cost  of  good  light  for  an  office  was  but  2  per 
cent,  of  the  wages;  that  "the  difference  in  cost  between  good  light 
and  poor  light  would  be  i  per  cent,  of  the  wages,"  noting  that 

"One  per  cent,  of  an  office  day  was  about  five  minutes,  that  if  clerical 
work  can  be  done  with  greater  ease  and  figures  read  more  accurately,  if 
there  is  greater  rapidity  and  fewer  errors,  if  there  is  less  eye  strain,  less 
headache,  greater  comfort  and  satisfaction,  so  that  more  and  better  work 
is  done  in  eight  hours  than  would  be  done  in  eight  hours  and  five  minutes 
with  a  poor  light,  then  the  extra  cost  is  justified." 

It  was  also  shown  that  the  difference  in  cost  of  equipment  between 


368  ILLUMINATING   ENGINEERING    PRACTICE 

satisfactory  and  unsatisfactory  methods  was  insignificant.  He 
advised  that  in  determining  the  real  value  of  good  illumination  the 
cost  of  light  should  be  determined  in  terms  of  the  total  cost  of  pro- 
duction, either  the  labor  alone,  or  the  labor  plus  the  various  other 
charges  which  enter  into  the  total  cost  of  production.  This  was  to 
be  expressed  either  in  per  cent,  or  in  minutes,  adding  that  common- 
sense  judgment  is  a  better  guide  than  detailed  systems  of  cost  which 
fail  to  consider  the  indirect  and  really  important  elements  that  make 
good  illumination  worth  while. 

In  a  discussion  of  costs  for  office  lighting  at  a  hearing  of  the  Heights 
of  Buildings  Committee  of  the  Board  of  Estimate  and  Apportion- 
ment of  the  City  of  New  York,6  comparing  the  costs  of  good  artificial 
lighting  service  to  the  cost  for  daylight,  generally  considered  to  be 
free,  it  was  stated,  that,  if  the  buildings  in  New  York  were  limited  to 
a  height  of  three  or  four  stories  to  secure  the  maximum  angle  of  sky 
effective  in  the  interior  of  these  buildings,  considering  the  ground 
values  and  office  rentals  per  square  foot  per  annum,  the  values  of  the 
remaining  office  space  would  go  up  enormously;  that,  at  the  present 
rentals,  there  were  very  few  locations,  particularly  in  the  office  build- 
ing district,  where  artificial  lighting  could  not  be  supplied  for  ten 
hours  per  day,  300  days  per  year,  at  an  expense  not  exceeding  i  per 
cent,  or  2  per  cent.,  and  certainly  less  than  5  per  cent,  of  the  present 
rentals  per  square  foot  per  year.  Two  or  three  kilowatt-hours  or 
100  cu.  ft.  of  gas  per  square  foot  per  year  would  be  sufficient  energy 
to  furnish  more  good  illumination  than  75  per  cent,  of  the  offices 
in  New  York  now  have.  Good  artificial  lighting  is  certainly  much 
less  expensive  than  daylight  when  considered  on  this  basis. 

The  load  factor  in  a  set  of  typical  office  buildings  in  Chicago  was 
shown  to  vary  from  1.5  to  4  hours  per  day.7 

Modern  office  lighting  practice  calls  for  general  illumination  of  a 
fairly  high  intensity,  evenly  distributed  throughout  the  entire  room, 
in  contrast  to  the  old  method  of  supplying  a  low  intensity  of  general 
lighting  with  points  of  high  illumination  caused  by  local  or  drop 
lamps  over  each  desk.  Originally  this  system  was  necessary  as  in- 
candescent lamps  were  not  efficient  enough  and  the  rates  for  energy 
were  comparatively  too  high  to  warrant  supplying  the  proper  illumi- 
nation throughout  the  entire 'room,  but  with  our  high  efficiency 
illuminants  and  reduced  rates  the  present  system  is  justified.8 

General  lighting  has  become  practically  standard.  No  well  posted 
designer  thinks  of  providing  desk  lamps  for  typical  office  work. 
Although  in  the  private  office  a  different  problem  often  arises. 


MACBETH:  LIGHTING  OF  OFFICES  369 

Local  lighting  has  in  many  instances  proven  objectionable  as  there 
is  a  great  liability  of  glaring  reflections  from  the  desk  surfaces  and 
glazed  paper.  Marked  contrasts  often  exist  between  the  brightly 
lighted  desk  area  and  the  rest  of  the  room,  both  of  which  factors 
cause  a  reduction  in  the  efficiency  of  the  eye. 

An  office  with  a  multiplicity  of  drop  lamps  is  unsightly,  the  cost 
of  wiring  is  high  and  there  is  a  heavy  expense  when  wiring  is  changed 
as  the  position  of  desks  are  shifted.  The  employees  are  likely  to 
change  the  location  of  lamps  by  tying  the  wire  to  some  stationary 
object,  a  practice  which  is  objectionable  from  a  standpoint  of  safety 
and  forbidden  by  the  wiring  codes.  Time  is  lost  moving  the  light- 
sources  about  and  the  breakage  of  lamps  is  likely  to  be  high. 

With  general  lighting,  overhead  units  are  so  placed  that  the  lamps 
are  well  out  of  the  angle  of  vision  and  are  equipped  with  diffusing 
glassware.  The  arrangement,  of  course,  must  be  such  that  dense 
shadows  are  avoided.  Larger  lamps  are  permissible,  which,  in 
general,  are  more  efficient  than  the  smaller  sizes;  fewer  outlets  are 
required,  reducing  the  cost  of  wiring.  When  one  stops  to  consider 
all  the  factors  that  enter  into  an  effective  lighting  system  he  is  soon 
convinced  that  general  illumination  is  really  far  more  economical 
than  local  lighting. 

The  three  general  types  of  lighting  units  as  ordinarily  recognized 
are  direct,  semi-indirect  and  totally  indirect. 

Direct  lighting  with  efficient  reflectors  is  unquestionably  the  most 
economical  for  with  it  the  color  of  walls  and  ceilings  has  less  effect 
on  the  resultant  illumination.  Direct  lighting,  if  improperly  ar- 
ranged, may  produce  glare  either  from  the  light  sources  themselves 
or  by  reflection  from  the  object  lighted,  or  it  may  not  distribute  the 
light  evenly  and  as  a  result  produce  objectionable  shadows.  It  is 
not  generally  as  decorative  as  the  other  methods.  Nevertheless, 
thousands  of  satisfactory  installations  of  good  direct  office  lighting 
are  to  be  seen,  employing  translucent  glassware  rather  than  opaque 
reflectors  thus  avoiding  the  undesirable  condition  of  a  dark  ceiling 
and  a  gloomy  appearance  of  the  room. 

Totally  indirect  lighting  is  probably  the  most  "  fool-proof "  from 
a  standpoint  of  a  glaring  installation.  The  light  is  usually  evenly 
distributed  and  the  effect  comfortable.  Objections  have  been  raised 
that  there  is  a  total  absence  of  shadow,  making  the  room  appear  flat. 
If  the  system  is  properly  designed,  however,  this  is  not  true. 

Semi-indirect  lighting  is  an  intermediate  practice;  it  may  be  more 
efficient  than  totally  indirect  and  much  better  for  the  eye  than  the 
24 


37°  ILLUMINATING   ENGINEERING    PRACTICE 

average  direct  lighting  system.  Semi-indirect  lighting  is  not  glaring 
if  the  proper  unit  is  chosen;  it  can  be  made  very  decorative,  the  light 
can  be  quite  evenly  distributed  and  such  shadows  as  are  produced 
are  soft  and  do  not  become  annoying.  The  fact  that  the  place  where 
the  light  originates  is  readily  discernible  has  a  psychological  effect 
on  the  average  individual  and  is  said  to  make  people  feel  more  at 
ease  than  under  totally  indirect  lighting. 

A  semi-indirect  unit,  first,  should  be  of  quite  dense  glass;  in  other 
words,  transmit  but  a  small  portion  of  the  light  if  the  best  conditions 
for  the  eye  are  to  be  obtained.  If  light  density  glass  is  used,  the 
bowl  becomes  very  bright  and  the  system  loses  many  of  its  advan- 
tages, dropping  back  to  the  direct  lighting  class  where  a  number  of 
fairly  bright  objects  are  in  the  field  of  vision. 

Second,  the  fixture  or  hanger  used  should  be  of  such  a  length,  and 
the  socket  in  the  proper  relative  position  to  the  bowl,  that  the  light 
is  directed  over  the  ceiling  in  such  a  manner  as  to  evenly  illuminate 
it.  Many  cases  can  be  noted  where  the  lamp  is  placed  too  low  in  the 
dish,  concentrating  the  emitted  light  in  a  fairly  narrow  angle,  re- 
sulting in  a  ring  or  circle  of  very  bright  illumination  while  between 
units  it  may  be  comparatively  dark.  At  other  times  to  get  rid  of 
this  effect  the  lamp  is  raised  so  high  that  from  some  parts  of  the 
room  the  filament  becomes  visible,  introducing  glare.  On  the  intro- 
duction of  the  gas-filled  tungsten  lamp  with  its  rather  concentrated 
filament,  this  feature  became  of  more  importance  than  formerly. 

Third,  in  most  localities,  the  glass  used  should  be  smooth  inside 
and,  preferably,  outside  also,  as  roughed  glass  collects  dirt  very 
readily  and  is  difficult  to  clean.  A  plain  but  effective  equipment  of 
this  kind  is  shown  in  Fig.  i. 

Fourth,  the  means  of  suspension  of  the  bowl  should  be  such  that 
there  is  absolutely  no  danger  of  the  glassware  falling  and  it  is 
desirable  to  have  some  convenient  means  of  cleaning. 

Fifth,  in  the  commercial  office  the  decorations  of  the  glassware,  if 
any,  should  be  very  simple,  for  any  appearance  of  excessive  ornate- 
ness  would  be  out  of  keeping  with  the  character  of  the  room.  Deep 
crevices  in  the  glass,  although  they  may  be  decorative,  are  objec- 
tionable from  the  standpoint  of  dust  accumulation.  Fig.  2  shows  a 
semi-indirect  installation  using  a  somewhat  typical  opalescent 
blown  glass  dish,  17  in.  in  diameter  and  5  in.  deep.  The  interior  of 
the  dish  is  fire  polished,  the  exterior  is  roughed  with  an  etched 
decoration. 

There  is  a  factor  which  does  not  enter  into  the  choice  of  the  unit, 


MACBETH:  LIGHTING  OF  OFFICES  371 

but  which  has  an  important  bearing  on  the  system  as  actually 
installed,  namely,  is  the  color  of  walls  and  ceilings.  With  indirect 
systems  it  is  very  essential  that  the  ceiling  be  light  in  color,  white  or 
slightly  cream,  to  secure  a  maximum  efficiency  of  reflection.  Even 
with  direct  lighting,  as  part  of  the  light  goes  upward,  light  ceilings 
are  desirable.  The  upper  part  of  the  walls,  also,  should  be  light,  as 
considerable  light  often  reaches  this  part  of  the  room.  The  lower 
half  of  the  walls  are  not  so  useful  from  this  standpoint,  and  it  is  often 
desirable  to  decorate  these  in  some  darker  neutral  tint  for  this  is  in 
the  natural  field  of  view  and  a  dark  surbase  provides  space  on  which 
the  eye  can  rest  in  comfort.  Matt  or  dull  finishes  are  always  prefer- 
able to  glossy  surfaces  as  they  avoid  the  possibility  of  annoying 
reflections. 

The  single  office  spaces  usually  have  desks  and  cases  placed  next 
to  walls.  Many  have  tables  in  the  middle.9  The  center  outlet 
system  of  lighting,  either  direct  or  indirect,  is  not  considered  entirely 
successful  where  desks  are  to  be  placed  next  to  the  walls.  In  con- 
ference rooms  where  the  work  is  done  around  a  large  table  in  the 
middle,  either  method  is  satisfactory.  It  has  been  stated  that  in 
semi-indirect  lighting  the  amount  of  transmitted  light  should  be 
about  15  per  cent.  This,  however,  is  a  matter  that  has  entirely  to  do 
with  the  brightness  of  the  surroundings  or  the  low  intensity  of  sur- 
faces within  the  range  of  normal  observation.  There  are  many  cases 
where  there  is  no  apparent  advantage  for  a  single  semi-indirect  or 
totally  indirect  unit  in  the  center  of  an  office.  The  distributed  unit 
system  has  been  in  use  a  great  many  years  and  has  proven  quite 
satisfactory.  It  is  always  subject  to  less  deterioration  from  dust 
and  is  less  effected  by  changes  in  color  of  ceilings  and  walls. 

In  planning  the  outlet  arrangement  for  a  large  office  building,  it 
is  important  to  anticipate  a  sub-division  of  office  space  as  in  office 
buildings  the  partition  arrangements  are  particularly  flexible, 
scarcely  two  tenants  requiring  a  similar  division  of  space.  In  many 
instances  it  is  necessary  to  provide  at  least  one  outlet  for  approxi- 
mately each  100  to  200  sq.  ft. 

Spacing  of  Indirect  and  Semi-Indirect  Units. — Ceiling  height 
largely  determines  the  distance  between  outlets  for  these  fixtures. 
This  distance  should  be  approximately  equal  to  the  ceiling  height  or 
may  extend  to  1.5  times  the  ceiling  height.  Where  close  work  is  to 
be  performed  a  less  distance  should  be  chosen.  The  distance  of 
units  from  the  ceiling  is  more  largely  a  matter  of  appearance  from  an 
architectural  point  of  view.  If  the  unit  is  placed  where  it  looks  as 


372 


ILLUMINATING    ENGINEERING   PRACTICE 


though  it  belonged  in  the  room  the  distribution  of  light  can  be  taken 
care  of  by  selecting  the  proper  equipment  for  the  purpose  and 
adjusting  the  filament  or  mantle  in  relation  to  its  reflector  or  the 
angle  of  cut-off  of  the  unit  itself. 

SPACING  AND  FIXTURE  LENGTHS  FOR  VARIOUS  CEILING  HEIGHTS, 
INDIRECT  AND  SEMI-INDIRECT  FIXTURES 


Height  of  ceiling, 
ft. 

Fixture  length, 
ft. 

Maximum  distance  between 
outlets,  ft. 

8 

I-S 

7-5 

10 

2  .0 

Q.O 

12 

3-o 

12  .O 

14 

3-5 

15-0 

16 

4.0 

18.0 

18 

4-5 

22  .O 

20 

S-o 

25.0 

Two-thirds  of  the  above  spacing  distances   may  be  used  under  structural 
conditions  that  warrant  other  spacing  than  given  above. 


BANK  LIGHTING 

Artistic  consideration  in  the  lighting  of  banks10  is  very  important. 
The  architectural  harmony  should  be  given  as  much  consideration 
as  the  utility.  Present  practice  supplies  a  relatively  low  intensity  of 
well  diffused  general  illumination  produced  by  a  decorative  or  semi- 
decorative  system,  and  a  higher  illumination  by  localized  lighting  at 
the  points  which  logically  demand  this.  These  may  be  divided  as 
follows : 

Patrons'  Desks  in  the  Banking  Space  Proper. — In  the  center  of  the 
room,  a  floor  outlet  should  supply  service. to  a  standard  fitted  with 
two  brackets  and  diffusing  reflectors,  or  one  special  trough  type 
reflector  and  clear  lamps.  At  the  sides,  bracket  type  fixtures  and 
similar  equipments  meet  the  requirements. 

Banking  Cages. — Special  cornice  type,  mirrored  trough  reflectors, 
or  short  brackets  and  opaque  reflectors,  should  be  located  well  out 
of  the  way  and  strong  localized  light  provided. 

Bookkeepers'  Desks. — Local  lamps  with  reflectors  so  designed  and 
located  that  there  is  no  direct  reflection  into  the  eye,  should  provide 
the  desirable  intensity  of  evenly  distributed  light. 

For  local  desk  lighting  it  is  practically  impossible  to  secure  satis- 
factory results  by  placing  a  lamp  symmetrically  on  a  desk  as  shown 


Fig.  i.— Office  30  ft.  by  32  ft.,  ceiling  height  10  ft.  6  in.  Ceiling  is  matt  white  in  color, 
walls  medium  cream.  Height  to  bottom  of  units  8  ft.  Seven  semi-indirect  lighting  fixtures 
are  used  with  dense  opal  glass  reflectors.  One  300-watt  gas-filled  lamp  per  outlet. 


Fig.  2. — Medium-size  private  office.  Ceiling  height  10.5  ft.  Ceiling  finish  white,  walls 
dark  cream.  Six  outlets  are  used  with  three  6o-watt  clear  tungsten  lamps  in  semi-indirect 
dishes  at  each  outlet.  Length  of  fixture  2.5  ft. 

(Facing  page  372.) 


Fig.  3. — Individual  desk  lamp  placed  in  the  center  of  the  desk  furnishing  illumination 
for  two  workers,  one  on  each  side  of  the  desk,  a  frequent  but  most  unsatisfactory  location 
owing  to  the  difficulty  due  to  direct  reflection. 


Fig.  4. — Sales  floor  of  large  wholesale  dry-goods  house  using  a  form  of  semi-indirect  fix- 
ture with  translucent  bowl  and  wide  band,  the  interor  of  which  is  lined  with  ripple  mirrored 
glass.  250-watt  vacuum  tungsten  lamps  were  used,  one  at  each  outlet,  with  a  spacing  of 
ii  ft.  by  15  ft.  On  this  floor  the  ceiling  and  upper  side  walls  are  finished  in  a  flat  white. 


MACBETH:  LIGHTING  OF  OFFICES  373 

in  Fig.  3,  without  setting  up  a  serious  condition  of  direct  reflection 
from  the  paper  surfaces  on  which  the  work  is  done.  This  placing  of 
desk  lamps  symmetrically,  bringing  the  work  to  be  done  into  a 
direct  line  between  the  eye  of  the  operator  and  the  lamp,  is  almost 
universal  with  desk  portables  and  is  perhaps  responsible  for  more 
eye  discomfort  than  any  other  single  condition  in  office  lighting. 

Simply  shifting  the  portable  two  or  three  feet  to  the  left  in  the 
case  of  a  right  hand  writer,  and  to  the  right  in  the  case  of  a  left 
hand  writer,  will  successfully  eliminate  this  direct  reflection  possi- 
bility. It  is  rather  surprising,  the  large  number  of  eye  discomfort 
cases  among  clerical  workers  that  can  be  corrected  in  this  simple 
manner.  It  is  a  general  conclusion  also  that  the  lamps  used  in  desk 
portables  are  invariably  too  large.  There  is  no  practical  necessity 
of  intensities  beyond  10  foot-candles  for  this  work  and  yet  with  the 
average  portable  the  intensities  range  upwards  to  25  and  50  foot- 
candles. 

As  a  simple  test  to  determine  the  satisfactory  position  on  a  desk  at 
which  work  may  be  performed  or  to  note  whether  a  portable  lamp  has 
been  moved  out  of  the  danger  zone,  the  operator  may,  when  seated  in 
the  regular  working  position,  place  a  mirror  at  various  parts  of  the 
working  plane  and  take  observations  as  to  whether  or  not  this  mirror 
shows  the  reflection  of  a  lamp  or  the  image  of  any  bright  surface.  A 
satisfactory  working  condition  may  be  secured  by  either  shifting  the 
working  area,  or  the  lamp  in  the  event  of  it  being  a  portable,  to  such 
a  point  that  all  reflections  seen  in  the  mirror  will  be  of  low  intensity 
surfaces. 

Another  cause  of  eye  discomfort  in  offices,  particularly  with  the 
liberal  expanse  of  window  surf  aces  now  provided  for  in  most  buildings, 
is  due  to  the  large  angle  of  sky  within  the  normal  range  of  vision. 
Recently,  in  an  office  in  one  of  our  large  modern  buildings,  two  steno- 
graphers after  working  under  this  condition  where  they  faced  a  wide 
angle  of  sky  daily  for  a  couple  of  months  suffered  from  headaches 
and  eye-strain.  One  even  found  it  necessary  to  wear  glasses.  By 
turning  their  desks  around  so  that  they  faced  a  moderately  bright 
wall  rather  than  the  bright  sky,  their  eye  difficulties  were  relieved. 

STORE  LIGHTING 

The  general  requirements  for  the  best  illumination  of  stores  and 
especially  department  stores,  may  be  considered  to  be:  First,  the 
goods  displayed  should  be  properly  illuminated.  Second,  there 


374  ILLUMINATING   ENGINEERING   PRACTICE 

should  be  absence  of  glare.  Third,  the  lighting  units  or  fixtures 
should  be  attractive  in  appearance.  Fourth,  the  light  generated  by 
the  lamps  should  be  utilized  efficiently. 

While  all  of  these  general  requirements  are  of  great  importance, 
the  order  in  which  they  are  given  above  may  be  considered  the  order 
of  their  importance,  in  the  average  case. 

The  primary  requirement  is  to  have  the  merchandise  well  illumi- 
nated. In  the  first  place,  the  intensity  of  illumination  should  be 
sufficient.  There  is  a  tendency  toward  using  higher  intensities  of 
illumination  for  artificial  lighting  year  after  year.  Care  should  be 
taken,  therefore,  to  use  an  intensity  sufficiently  high.  There  is  no 
harm  or  discomfort  to  the  eyes  in  doing  this,  if  the  eyes  are  properly 
protected  from  glare.  Second,  the  distribution  of  illumination  should 
be  reasonably  uniform.  In  order  words,  one  part  of  the  store  should 
not  be  appreciably  brighter  than  other  parts. 

The  term  uniformity  is  used  generally  to  express  the  evenness  of 
illumination  over  a  working  plane  and  is  understood  to  refer  to  the 
values  that  would  be  shown  by  illumination  measurements  if  at  every 
point  on  the  plane  the  illumination  intensities  were  similar.  Abso- 
lute uniformity  of  illumination,  however,  is  never  necessary  in  prac- 
tice. The  eye  is  not  adapted  to  detect  variations  even  as  great  as 
35  per  cent,  in  a  room,  provided  the  minimum  intensity  is  above 
one  foot-candle. 

Third,  the  light  should  have  the  proper  color  value.  The  color  of 
the  light  given  by  the  standard  gas  and  electric  lamps,  which  are  now 
universally  used  for  the  larger  stores,  is  an  excellent  color  for  illumi- 
nating a  large  proportion  of  the  merchandise  generally  displayed. 
This  color  is  not  true  white,  however,  and  where  the  goods  should  be 
shown  in  their  true  colors,  the  same  as  in  daylight,  the  excess  of  red 
rays  can  be  filtered  out  by  the  proper  kind  of  glass. 

There  has  been  a  great  deal  of  discussion  among  lighting  men  on 
color  of  light  for  department  stores.  It  is  rather  difficult  to  separate 
the  demand  for  color  in  light,  or  rather  lack  of  color,  for  a  light 
tending  toward  white,  from  that  due  to  the  lamp  and  glassware 
manufacturers '  sales  enthusiasm.  The  fact  remains,  however,  that 
ten  years  ago  a  great  many  department  stores  were  lighted  with 
direct-current  enclosed  arc  lamps,  from  which  a  whiter  light  was 
received  than  that  given  by  the  incandescent  electric  lamps  that 
have  almost  universally  replaced  the  arcs.  In  the  electric  field  the 
tungsten  incandescent  lamp  is  in  general  use  to-day  with  isolated 
instances  of  an  endeavor,  either  with  colored  glassware  or  colored 


MACBETH:  LIGHTING  OF  OFFICES  375 

bulbs,  to  filter  out  some  of  the  excess  red  in  this  lamp  and  so  im- 
prove its  color  value.  Very  few  of  these  attempts,  however,  from 
the  standpoint  of  the  final  installation,  are  nearer  a  white  light,  and 
in  many  instances  are  less  close  than  was  the  direct-current  enclosed 
arc  lamp  which  they  replaced  and  which  produced  light  of  a  better 
white  light  approximation  more  efficiently  than  many  of  these  later 
methods.  There  has  in  the  past,  furthermore,  been  very  little 
demand  in  large  department  stores  for  incandescent  mantle  gas 
lamps  because  of  the  color  of  the  light,  although  from  the  beginning 
these  lamps  have  produced  a  light  resulting  in  less  distortion  of 
colors.  There  is  an  opinion  that  in  the  endeavor  to  see  fabric,  absence 
of  predominating  color  in  light  is  most  important.  This  is  not  true 
unless  greater  attention  is  at  the  same  time  given  to  intensity  of 
light.  Recent  tests  have  shown  that  intensities  from  50  to  100 
foot-candles  help  this  situation  to  a  greater  extent  than  does  light 
having  the  proper  absence  of  excess  color  if  used  as  an  intensity  of 
approximately  3  foot-candles,  or  less.  Direction  of  light,  that  is, 
a  minimum  of  diffused  light,  is  especially  necessary  for  the  examina- 
tion of  fabrics. 

Much  of  the  talked-of  demand  for  whiteness  of  light  has  been 
for  "color  matching."  In  many  instances  the  light  from  our 
ordinary  sources  is  better  for  color  matching,  particularly  if  the 
fabrics  selected  are  ever  going  to  be  seen  under  the  ordinary  lighting 
of  our  homes  or  places  of  business.  There  are  many  colors  that 
will  match  under  white  light  that  are  far  enough  off  under  ordinary 
artificial  light  to  be  unsatisfactory.  Accurate  color  matching  has 
only  been  secured  where  the  match  is  effective  with  all  the  light 
sources  under  which  the  materials  will  later  be  seen  in  combination. 
Fabrics  matched  under  a  source  having  for  instance  a  30  per  cent, 
white-  light  sensation  value  will  not  necessarily  match  under  either  the 
ordinary  artificial  light  of  the  home  or  of  the  daylight  of  the  street. 

The  near  white  light  or  so-called  approximate  white  light  is  use- 
ful to  a  more  limited  extent  in  color  identification  to  prevent  con- 
fusion in  selecting  a  blue  or  a  green  for  black,  a  pale  yellow  or  orange 
or  pink  for  white,  etc. 

Glare  is  produced  when  the  light  units  are  so  bright  that  they 
decrease  the  ability  of  the  eye  to  see  clearly  or  cause  discomfort 
to  the  eyes.  The  ordinary  observer  does  not  know  what  is  the 
cause  of  his  discomfort,  and  may  not  attribute  it  to  the  lighting. 
Customers,  however,  will  not  stay  long  in  store  where  the  light 
is  trying  to  the  eyes,  even  though  they  do  not  realize  why  they  do 


376  ILLUMINATING   ENGINEERING   PRACTICE 

not  feel  like  staying.  For  example,  the  proprietor  of  a  large  billiard 
room,  after  rearranging  his  lighting  system  along  the  lines  of  elimi- 
nating glare,  became  convinced  that  his  customers  were  playing 
billiards  from  one  to  two  hours  longer  at  a  session  than  formerly.  He 
had  not  realized  that  his  old  lighting  system  was  too  glaring  for 
comfort  until  he  saw  the  difference  actually  working  out  in  increased 
revenue  from  his  tables. 

It  is  possible  so  to  light  a  store  that  the  customers  will  feel  com- 
fortable and  not  suffer  through  abuse  of  their  eyes,  and  still  the  goods 
displayed  will  be  brilliantly  illuminated.  The  most  important  thing 
is  to  have  all  light  sources  low  in  brilliancy  at  the  angle  at  which 
they  are  apt  to  be  viewed.  It  is  further  desirable  not  to  have  extreme 
contrasts  of  light  and  shadow.  The  latter  usually  takes  care  of  itself 
on  account  of  the  light-colored  finishes,  now  universally  employed 
for  ceilings,  in  modern  stores.  It  is  sometimes  thought  a  desira- 
ble thing  to  have  the  light  units  appear  brilliant  so  that  the  store 
will  look  attractive  from  the  outside.  The  comfort  of  the  customer 
when  he  comes  inside,  however,  should  not  be  sacrified  to  obtain  a 
brilliant  and  glittering  appearance  from  the  outside.  The  public 
is  becoming  educated  along  these  lines  and  is  not  attracted  by 
glare  as  much  as  formerly.  Furthermore,  if  the  goods  themselves 
are  well  illuminated,  the  store  will  look  attractive.  This  appear- 
ance of  attractively  good  illumination  may  be  enhanced  by  taking 
care  to  display  goods  of  light  color  near  the  entrances  so  that  the 
illumination  will  appear  at  its  best  from  the  outside  or  on  first 
entering. 

It  is  not  necessary  to  go  into  detail  regarding  attractive  appear- 
ance of  the  lighting  units.  Obviously,  these  should  be  pleasing  to 
the  eyes  and  in  harmony  with  the  surroundings.  The  store  owner 
is  more  apt  to  overemphasize  this  point  than  to  underestimate  it 
in  comparison  with  the  other  requirements. 

Efficiency  in  the  utilization  of  light  is  important.  This  point  is 
often  apt  to  be  overlooked  by  the  store  owner.  In  comparing  any 
two  systems  of  illumination,  the  choice  should  not  be  made  alone  upon 
the  question  of  appearance  of  the  unit,  or  even  the  quality  of  illumi- 
nation obtained.  In  a  large  store  the  amount  of  electrical  energy 
used  is  considerable  and  it  is  important  for  the  owner  to  get  the 
best  returns  for  the  money. 

Economy  as  applied  to  the  lighting  of  a  store  must  not  be  mis- 
takenly understood  to  refer  only  to  the  cost  of  lamps  and  fixtures 
or  the  expense  of  operating  them.  This  is  the  debit  side  of  the 


MACBETH:  LIGHTING  OF  OFFICES  377 

account.  On  the  credit  side  are  the  sales  that  result  directly  from 
the  inviting  effective  manner  in  which  the  goods  are  shown. 

Lighting  Systems. — Stores  may  be  properly  lighted  by  direct,  semi- 
indirect  or  indirect  lighting  equipments. 

In  large  stores  direct  lighting  is  usually  the  most  preferred  on 
account  of  its  high  efficiency  in  the  utilization  of  light.  -Direct 
lighting  systems  vary  considerably  in  efficiency,  however,  and  the 
least  efficient  of  these  is  on  about  the  same  plane  as  indirect  and 
semi-indirect  lighting. 

Semi-indirect  lighting  produces  to  a  greater  extent  a  more  thorough 
diffuse  illumination,  which  is  the  quality  of  illumination  obtained 
by  having  light  come  from  a  great  many  directions.  This  is  accom- 
plished by  having  a  large  part  of  the  light  reflected  from  the  ceiling 
so  that  illumination  at  any  one  point  is  produced  by  light  from  many 
directions.  Diffusion  of  illumination  as  thus  produced  is  character- 
ized by  the  absence  of  sharp  and  dense  shadows  and  by  the  minimiz- 
ing of  high  lights  or  glint  reflections.  This  quality  of  diffusion  is 
desirable  in  many  classes  of  store  lighting  but  not  in  all  classes.  It 
is  undesirable,  for  example,  in  the  display  of  jewelry,  silverware, 
glassware,  etc.,  where  direct  lighting  should  always  be  used,  as 
high  lights  or  glint  reflections  are  a  necessary  part  of  the  display.  A 
combination  equipment  may  be  used  to  meet  these  conditions  where, 
as  shown  in  Fig.  5,  indirect  lighting  fixtures  were  installed  for  general 
illumination  with  direct  lighting  units  over  the  counters. 

We  secure  a  shadow  effect  with  predominating  direct  lighting 
which  assists  the  eye  considerably  in  determining  the  structure  of 
fabrics  and  other  goods,  while  under  illumination  that  is  largely 
diffused  these  details  disappear. 

In  using  semi-indirect  lighting  it  is  always  desirable  to  use  a  dense 
glass  so  that  the  maximum  amount  of  light  is  reflected  from  the  ceil- 
ing, and  the  bowl  is  low  in  brilliancy — preferably  not  much  brighter 
than  the  ceiling  itself.  This  not  only  gives  the  maximum  degree  of 
diffuse  illumination,  but  it  also  reduces  the  liability  of  obtaining 
glare  as  pointed  out  above. 

Indirect  lighting  has  the  same  general  characteristics  of  illumi- 
nation as  semi-indirect  lighting.  A  high  degree  of  diffusion  of  illumi- 
nation is  obtained  by  having  the  light  come  from  the  ceiling.  It  is 
usually  considered,  however,  that  semi-indirect  lighting  is  more 
attractive  in  appearance  on  account  of  the  illuminated  bowl.  With 
indirect  lighting  there  is  also  a  contrast  between  the  dark  bowl  and 
the  brilliant  ceiling.  Semi-indirect  lighting  is  usually  preferable  on 


378  ILLUMINATING   ENGINEERING   PRACTICE 

this  account,  although  indirect  lighting  is  often  as  efficient  and  it 
gives  the  same  desirable  diffuse  illumination  as  the  semi-indirect. 

Types  of  Direct  Lighting  Equipment. — Direct  lighting  fixtures  may 
be  fitted  with  open  reflectors,  enclosing  globes,  or  semi-enclosing 
globes. 

Open  reflectors  are  the  most  efficient  in  the  utilization  of  light. 
Prismatic  reflectors,  clear  or  velvet  finish,  and  heavy  density  opal 
glass,  are  highly  desirable  on  account  of  their  efficiency.  These  may 
be  used  in  single  units  or  in  clusters. 

Enclosing  globes  are  often  preferred,  however,  in  order  to  obtain  a 
more  distinctive  appearance.  Any  direct  lighting  unit  which  utilizes 
the  light  efficiently,  results  in  a  great  deal  of  the  light  being  directed 
downward  and  at  angles  not  far  from  the  downward  direction.  This 
means  that  in  standing  under  the  unit  and  looking  up,  the  brilliancy 
will  be  too  great.  It  is  not  possible,  however,  to  protect  the  eyes 
against  such  conditions  with  direct  lighting.  In  the  ordinary  uses  to 
which  a  store  space  is  put,  the  occupants  should  never  have  occasion 
to  look  at  the  light  units  from  directly  underneath.  Opal  enclosing 
globes,  however,  which  do  not  redirect  the  light  in  useful  directions 
but  simply  diffuse  it  over  the  surface  of  the  globe,  have  nothing  to 
recommend  them  for  store  lighting  except  their  appearance.  While 
the  brilliancy  is  not  as  great  as  that  of  a  bare  lamp,  it  is  still  higher 
than  is  desirable.  Furthermore,  the  light  is  not  distributed  effi- 
ciently and  the  wattage  required  is  consequently  greater  than  would 
be  necessary  for  an  installation  of  efficient  units.  On  the  other  hand, 
opal  globes  are  made  up  in  such  a  large  variety  of  attractive  designs 
that  it  is  not  possible  to  condemn  them  entirely  for  store  lighting. 
When  the  globe  is  large  so  that  the  light  is  diffused  over  a  large  sur- 
face and  when  the  store  owner  feels  willing  to  pay  the  additional  cost 
in  order  to  obtain  the  appearance  desired,  it  may  be  justified. 

Semi-enclosing  globes  are  usually  made  in  the  form  of  a  flat  or 
shallow  reflector  with  a  bowl  suspended  directly  underneath.  These* 
are  more  efficient  in  the  utilization  of  light  than  enclosing  opal 
globes.  The  brilliancy  of  the  bowl  is  usually  even  higher  than  the 
opal  enclosing  globes  and  on  that  account  the  units  are  not  as  desir- 
able. Semi-enclosing  globes  of  this  type  should  not  be  confused  with 
semi-indirect  lighting  fixtures.  In  semi-indirect  lighting  a  large  part 
of  the  light  comes  from  the  ceiling  so  that  the  light  received  on  the 
plane  where  illumination  is  desired  comes  from  many  different  direc- 
tions and  the  illumination  is  diffused.  In  the  case  of  the  semi- 
enclosing  globes  with  flat  or  shallow  reflecting  surfaces,  all  the  light 


Fig.  5. — General  view  of  jewelry  store  lighted  with  indirect  lighting  fixtures  for  general 
illumination  and  direct  lighting  units  bracketed  out  from  the  shelving  over  the  counters. 
Store  22  ft.  by  60  ft.,  ceiling  height  16  ft.  Length  of  fixtures  3-5  ft.,  using  in  the  five  outlets 
three  soo-watt  lamps  and  two  750-watt  lamps,  the  latter  on  outlets  No.  2  and  4. 


Fig.  6. — View  of  floor  of  large  department  store  using  enamelled  steel  indirect  lighting 
fixtures  with  short  suspension.  A  better  distribution  of  light  could  be  secured  in  this  kind  of 
location  if  the  fixtures  were  lower  so  that  the  ceiling  would  be  more  uniformly  lighted. 

(Facing  page  378.) 


Fig.  7- — Department  store  floor  using  single-chain  fixtures  with  one-piece  opalescent  glass- 
ball  globes.      This  is  a  typical  department  store  floor. 


Fig.  8. — Semi-indirect  gas  fixtures  as  standardized  for  use  in  a  chain  of  grocery  stores. 


MACBETH:  LIGHTING  OF  OFFICES  379 

comes  from  the  bowl  and  the  reflector,  and  this  is,  of  course,  a  much 
smaller  area  than  the  area  of  the  ceiling.  Semi-enclosing  globes 
should,  therefore,  be  classified  as  direct  lighting  units  and  not  as 
semi-indirect  units. 

The  high-grade  shop  is  usually  small  in  size,  lavishly  furnished, 
located  in  some  fashionable  section  and  handling  only  the  best  grade 
of  goods.  The  proprietor  is  accustomed  to  spending  large  sums  for 
rent,  equipment  and  general  upkeep  and  more  money  can  be  spent 
for  individuality  of  layout.  A  distinctive  lighting  system  is,  there- 
fore, appropriate.  Artistic  appearance  is  the  predominating  factor, 
and  efficiency  a  secondary  consideration.  The  lighting  system 
should  harmonize  with  the  architecture  and  preferably  be  designed 
to  be  strictly  in  accord  with  some  predetermined  plan. 

SMALL  STORE  LIGHTING 

Small  stores  may  be  divided  roughly  into  five  groups,14  the  first  being 
those  requiring  equal  illumination  on  the  side  walls,  shelves,  and  on 
the  counters,  as  bakeries,  drug  stores,  grocery  and  china  stores. 
Stores  of  medium  width,  may  be  lighted  satisfactorily  with  two  rows 
of  lamps.  This  will  result  in  a  high  intensity  of  direct  light  on  the 
counters  and  sufficient  diffused  light  for  the  walls.  In  narrow  stores, 
one  row  of  lamps  down  the  center  of  the  store  will  give  satisfactory 
results. 

The  second  class  of  stores  demand  good  illumination  on  the  coun- 
ters and  a  small  amount  of  light  on  the  side  walls,  such  as  haber- 
dashery, jewelry,  and  stationery  stores,  in  which  locations,  inspec- 
tion of  the  goods  is  on  the  counter  which  must  be  lighted  with  a  fairly 
high  intensity  with  a  requirement  for  lesser  intensities  on  the  side 
walls  or  shelving.  In  jewelry  stores  particularly,  this  treatment  is 
necessary  and  local  lighting  over  the  counters  is  desired  with  clear 
lamps  which  result  in  a  better  appearance  of  engraved  objects  and 
jewels. 

In  the  third  group  are  stores  that  demand  the  highest  intensity  on 
the  wall  surfaces  and  a  low  general  illumination.  Art  and  music 
stores,  paint  and  hardware  stores,  are  in  this  group.  In  the  art 
stores  pictures  are  displayed  on  the  walls,  and  in  the  music  stores  it 
is  necessary  to  have  a  sufficiently  high  intensity  on  the  shelving 
for  the  reading  of  labels  on  the  boxes. 

In  the  fourth  group  of  stores  are  the  clothing,  confectionery,  milli- 
nery, and  shoe  stores.  In  these  locations  general  illumination  is  re- 
quired with  local  lighting  at  convenient  places  in  the  clothing  and 


380  ILLUMINATING   ENGINEERING    PRACTICE 

millinery  stores  that  a  proper  direction  of  light  at  high  intensities  may 
enable  the  customer  actually  to  see  the  fabrics  in  a  manner  not 
possible  under  diffused  illumination  with  the  ordinary  low  intensity 
values. 

In  the  fifth  group  are  the  small  barber  and  manicure  shops  which 
require  localized  lighting. 

Gas  Lighting. — Gas  lighting  is  in  far  better  condition  to-day  than 
ever  before.  There  is  a  better  understanding  of  the  kind  of  lamps 
and  fixtures  required  to  meet  the  general  demand  and  more  attention 
has  been  given  to  the  successful  production  of  these  fixtures.  Gas 
companies  have  shown  awakened  interest  in  good  gas  lighting  and, 
in  many  places,  the  consumer  can  count  upon  a  grade  of  service  not 
possible  a  few  years  ago.  Maintenance  with  gas  lamps,  as  with  all 
of  our  artificial  light  sources,  is  of  particular  importance  and  in  most 
cities  to-day  it  is  possible  to  secure  a  high  grade  of  service  from  the 
local  supply  companies  at  a  nominal  charge  and,  in  some  instances, 
without  charge.  There  is  a  strong  recognition  by  the  gas  companies 
that  their  service  to  the  consumer  means  lighting  service  rendered, 
rather  than  merely  gas  by  the  cubic  foot. 

Good  lighting  is  worth  its  cost  and  the  merchant  to-day  is  more 
willing  than  at  any  time  in  the  past  to  meet  that  cost. 

Semi-indirect  lighting  with  gas  has  proven  to  be  entirely  satisfac- 
tory. The  maintenance  of  these  fixtures  is  simple  and  the  renewal 
cost  low.  A  gas  lighting  installation  similar  to  Fig.  8  is  all  that  could 
be  desired  and  is  certainly  a  better  proposition  than  has  ever  before 
been  offered  in  gas  lighting  to  a  similar  class  of  store. 

The  general  rules  given  for  the  use  of  other  illuminants  may  be 
utilized  in  the  planning  of  gas  lighting  installations  with  such  modi- 
fications only  as  may  be  imposed  by  the  differences  in  the  size  of 
units  available.  For  mechanical  and  other  reasons,  semi-indirect 
lighting  is  particularly  favorable  to  the  use  of  gas.  A  large  number 
of  mantles  may  be  placed  within  a  single  bowl  and  lighted  by  means 
of  a  single  pilot  flame.  It  is  furthermore  unnecessary  to  locate  the 
units  or  to  select  glassware  with  particular  reference  to  the  illumi- 
nation of  individual  areas,  and  with  this  system  it  is  possible  to 
secure  much  higher  intensities  without  the  resulting  glare  that  is  so 
usual  with  the  older  types  of  gas  lamps  used  for  direct  lighting.  This 
kind  of  fixture  has  been  recommended  for  all  classes  of  store  lighting 
with  the  exception  of  jewelry  stores  where  direct  lighting,  as  stated 
above,  will  give  better  results  from  the  standpoint  of  reflection  of 
light  from  silverware,  cut  glass,  jewels,  etc. 


MACBETH:  LIGHTING  OF*  OFFICES  381 

The  method  of  store  lighting  in  considerable  use  before  semi- 
indirect  gas  fixtures  were  available  is  shown  in  Fig.  9.  This  illus- 
tration shows  a  section  of  a  department  store  illuminated  by  means 
of  direct  lighting  cluster  units. 

An  installation  where  "gas  arc"  lamps  are  used  is  shown  in  Fig. 
10.  This  was  at  one  time  the  most  common  method  of  gas  store 
lighting,  largely  because  of  the  lower  cost  of  installation  and  main- 
tenance of  this  lamp  as  compared  to  a  cluster  of  small  lamps.  It  is 
only  fair  to  the  latter,  however,  to  state  that  in  many  situations  where 
costs  have  been  analyzed,  the  advantage  has  been  found  on  the  other 
side,  the  cluster  of  small  units  being  less  expensive  to  maintain  where 
the  conditions  of  gas  supply  were  favorable  and  less  frequent  atten- 
tion was  demanded.16  This  large  direct  lighting  unit  is  still  favored 
in  those  places  where  low  first  cost  is  the  important  consideration. 
Even  this  field  is,  however,  being  taken  care  of  to  an  increasing 
extent  by  the  large  single  inverted  mantle  lamp  shown  in  Fig.  n. 

SHOW-CASE  LIGHTING 

The  show  case  is  a  miniature  show  window10'17.  It  should  stand 
out  in  contrast  to  the  surroundings  and  should  have  at  least  twice 
the  lighting  intensity  of  the  store  proper  effective  on  its  goods. 

Good  illumination  renders  sales  work  easier,  for  close  selection  can 
be  made  without  removing  the  goods  from  the  case.  This  decreased 
handling  is  an  advantage  in  largely  eliminating  shop  wear. 

Show  cases  should  be  lighted  by  lamps  placed  within  the  case  and 
hidden  from  view.  The  unit  used  must  be  quite  small,  as  it  must 
be  placed  at  the  upper  edge  in  the  corner  of  the  case.  It  should 
harmonize  in  appearance  with  the  general  finish  of  the  fittings. 
Lastly,  the  lamps  used  must  be  of  low  wattage  to  avoid  heat. 
Tubular  lamps  with  suitable  trough  or  individual  reflectors  meet  the 
requirements  excellently,  and  with  the  line  source  form,  as  shown  in 
Fig.  12,  a  given  wattage  is  spread  over  quite  an  area. 

With  the  counter  show  case  the  goods  are  viewed  from  above,  and 
the  general  direction  of  light  must  be  downward.  With  the  high 
show  cases,  in  which  lay  figures  are  on  display,  the  light  must  come 
from  an  angle. 

If  the  illumination  in  the  store  proper  is  not  too  great,  150  lumens 
per  running  foot  of  show  case  is  quite  satisfactory,  and  in  cases  where 
a  high  intensity  exists  outside  of  the  show  cases,  this  allowance  may 
be  increased. 


382  ILLUMINATING   ENGINEERING  PRACTICE 

There  are  considerable  differences  in  the  equipment  offered  for 
this  work;  many  of  the  trough  reflectors  that  may  be  secured  for 
use  in  the  front  corner  of  the  case  do  not  screen  the  standard  bulb 
lamps  and  even  in  some  cases,  the  smaller  tubular  lamps  that  have 
been  used,  from  the  eyes  of  the  clerk  behind  the  counter.  This  is 
largely  a  matter  of  cheap  equipment  and  lack  of  attention  to  this 
important  point,  as  good  show  case  lighting  equipment  can  be  read- 
ily secured.  Fig.  13  is  an  illustration  of  an  individual  reflector 
installation  with  which  small  lamps  are  used. 

Lamps  of  low  wattage  are  necessary  to  distribute  properly  the  light 
where  the  cases  are  relatively  small  and  also  to  reduce  the  heating 
effect  to  a  minimum.  This  is  of  particular  importance  in  confection- 
ery stores  where  heat  from  the  larger  lamps  is  sufficient  in  the 
summer  time  to  melt  much  of  the  stock  on  display.  These  cases  are 
generally  of  the  closed  type  and  the  heat  dissipation  is  through  the 
circulation  of  air  in  the  case  and  radiation  from  the  glass  surfaces. 
Where  this  heat  radiation  has  given  considerable  difficulty,  the 
problem  has  been  met  by  using  small  candle-power,  low  voltage, 
miniature  lamps  in  series.  While  this  method  is  not  generally  rec- 
ommended, it  has  met  the  demand  where  apparently  nothing  else 
would  do.  The  total  number  of  lamps  required  for  a  series  should  be 
kept  within  one  case. 

It  is  advisable  when  determining  the  intensity  to  use  to  treat 
all  show  cases  in  one  store  alike  whether  for  light  or  dark  goods  as 
the  character  of  display  may  be  changed. 

SHOW-WINDOW  LIGHTING 

Undoubtedly  there  has  been  considerable  guess-work  and  ill- 
directed  experiment  in  show-window  illumination.18 

Architects  and  builders  have  apparently  given  very  little  consid- 
eration to  this  important  question.  The  space  in  the  window  at 
the  disposal  of  the  window-lighting  specialist  is  frequently  a  matter 
of  a  few  inches. 

We  have  only  to  observe  the  show  windows  in  our  home  city  to 
realize  that  this  very  important  problem  of  illumination  has  been 
given  very  little  consideration.  Windows  high  or  low,  shallow  or 
deep,  are  frequently  given  the  same  treatment.  Windows  containing 
dark  goods  adjoining  displays  of  light  goods  are  given  the  same 
quantity  of  light.  Little  attention  has  been  paid  to  the  amount 
of  reflection  from  materials  or  fabrics,  or  to  the  quality  or  quantity 
of  either  goods  or  light.  Windows  finished  in  light  wood  or  decora- 


Fig.  9. — Small  department  store  using  three-lamp  fixtures  with  single  inverted  mantle  gas 
lamps  with  prismatic  reflectors.     Fixture  length  2  ft. 


Fig.  10. — A  plant  and  seed  store  using  inverted  mantle  "gas  arc"  lamps.  This  type 
of  installation  is  favored  in  those  places  where  low  first  cost  is  the  most  important  considera- 
tion. 

(Facing  page  382.) 


Fig.   ii. — Bakery  store  using  large  single  inverted  mantle  gas  lamps,  store  width  25  ft. 
spacing  between  outlets  10  ft.     Height  of  ceiling  12  ft.,  height  of  lamps  8  ft. 


Fig.  12. — Show  cases  with  small  continuous  trough  reflectors  using  11  in.  tubular  lamps 
with  single  straight  filament  and  contacts  on  each  end  of  the  tube.  This  equipment  for 
case  lighting  occupies  a  minimum  of  space  in  the  corner  of  the  case  and  is  reasonably  in- 
conspicuous. 


Fig.  13. — Show  case  lighted  with  small  individual  reflectors  using  is-watt  round  bulb 
candelabra  base  tungsten  lamps.  These  reflectors  are  usually  installed  on  lo-in.  to  24-in. 
centers.  The  reflector  equipment  is  also  adapted  to  small  lamps  and  medium  screw-base 
receptacles. 


Fig.  14. — Show  window  using  concentrated  single-piece  mirrored  glass  reflectors  with 
loo-watt  vacuum  tungsten  lamps  on  15- in.  centers.  Curtain  used  at  top  of  window  to 
screen  view  of  the  lamps.  Depth,  7  ft.  height,  floor  to  ceiling,  n  ft. 

(Facing  Figs,  n  and  12.) 


Fig.   15. — Appearance  at  night  of  show  window,  a  vertical  section  of  which  is  shown  in  dia- 
gram, Fig.  18. 


Fig.   1 6. — Show  window,  open  in  the  back.     Lamps  in  trough  reflector  screened  from  the 
view  of  those  in  the  interior. 


MACBETH:  LIGHTING  OF  OFFICES  383 

tions  may  have  been  properly  and  sufficiently  illuminated,  but  when 
the  style  of  the  decoration  changes  to  mahogany  or  dark  oak,  the 
illumination  falls  off  so  much  that  the  window  lighting  comes  in 
for  severe  condemnation  on  the  grounds  of  deterioration  in  the 
accessories,  or  gross  carelessness  in  permitting  the  pressure  of  the 
supply  to  drop  off. 

Seldom  has  the  fact  been  made  plain  that  because  of  the  darker 
finishes  a  corresponding  increase  in  intensities  or  change  in  dis- 
tribution of  light  flux  is  necessary. 

In  perhaps  no  other  location  can  the  merchant  secure  a  greater 
return  on  his  investment  than  with  the  comparatively  small  outlay 
for  effective  show-window  lighting.  Window  illumination  is  strictly 
an  advertising  proposition  and,  as  such,  costs  about  10  per  cent, 
of  that  of  any  other  equally  effective  medium. 

The  purposes  of  a  well-lighted  show  window  and  its  tasteful 
display  of  goods  is  to  attract  the  passerby.  A  glaring  light  source 
formerly  in  more  general  use  is,  however,  about  as  good  as  none, 
for  the  goods  cannot  be  seen,  owing  to  the  blinding  effect  upon  the 
observer.  There  should  be  no  exposed  lamps  within  the  field  of 
vision.  In  general,  the  light  must  come  from  in  front  of  the  goods 
to  avoid  bad  shadows,  and  the  background  of  the  window  should 
be  chosen  to  obviate  specular  reflection  from  the  lighting  units. 

The  proper  lighting  of  show  windows  has  become  one  of  great 
importance  with  merchants  and  is  one  that  demands  consideration 
as  a  commercial  proposition.  The  merchant  is  coming  more  to  a 
realization  that  his  display  windows  are  the  medium  through  which 
a  great  deal  of  his  business  comes  and  the  precentage  of  business  that 
he  obtains  through  such  display  either  directly  or  indirectly  depends 
upon  the  manner  in  which  he  has  his  windows  dressed  and  the 
manner  in  which  they  are  lighted  in  comparison  with  that  of  his 
neighbor  merchants;  assuming  that  the  windows  are  equally  well 
dressed. 

Proper  Placing  of  Lamps  for  Show  Windows. — Due  consideration 
must  be  given  in  show-window  lighting  of  the  lighting  conditions 
on  the  street  or  on  the  sidewalk  in  front  of  the  window,  as  it  is 
usually  desired  to  have  the  windows  appear  bright,  it  is  necessary 
to  take  these  outside  conditions  into  consideration.  Any  rules 
given  for  the  amount  of  light  used  in  show  windows  are  therefore 
subject  to  modification  and  more  light  would  have  to  be  used  under 
various  street-lighting  conditions  to  secure  the  desired  effect  of 
brightness. 


384 


ILLUMINATING   ENGINEERING   PRACTICE 


It  is  not  at  all  satisfactory  to  light  show  windows  from  lamps  in 
front,  outside  of  the  window.  Considerable  of  the  light  from  these 
lamps  will  be  reflected  from  the  outer  surface  of  the  plate  glass  and 
tests  have  shown  that  these  outside  lamps  have  a  utilization  effi- 
ciency in  the  interior  of  the  window  of  only  20  per  cent,  of  that  which 
may  be  secured  from  lamps  placed  within  .the  windows.18 

The  illumination  of  store  windows  can,  with  very  few  exceptions, 
be  most  effectively  taken  care  of  with  lamps  arranged  along  the 
front  of  the  window,  Fig.  17.  The  lamps  should 
be  placed  high  and  out  of  the  direct  line  of 
vision.  In  some  cases  it  is  necessary  to  use 
a  painted  band  with  a  sign  transparency  to  hide 
the  lamps;  in  others,  an  ordinary  curtain  or 
shade  will  accomplish  the  purpose,  or,  where 
a  more  simple,  dignified  treatment  is  required, 
a  wooden  or  metal  moulding  of  sufficient  depth 
may  be  fitted  across  the  window  between  the 
lamps  and  the  plate  glass  near  the  top.  The 
lamps  should  be  equipped  with  reflectors,  which 
will  direct  the  light  downward  and  back  into 
the  window;  this  will  insure  the  proper  direction 
of  light  and  natural  shadows.  Shadows  are 
necessary,  but  should  not  be  sharply  defined. 
We  should  have  no  difficulty  in  distinguishing 
detail  in  the  shadows.  This  unsatisfactory  con- 
dition is  quite  noticeable  in  a  window  lighted 
_  with  a  single  high-powered  unit  hung  in  the 
Pig.  17.  —Section  of  center  of  the  window. 

S±.  !nwhaere"  A  window  lighted  from  the  rear  and  below, 
provision  was  made  for  with  the  shadows  upward  and  forward,  would 
flectoTs^™  Hghting  Te~  be  little  more  unsatisfactory  than  the  so-called 
shadowless  window.  All  sense  of  size,  propor- 
tion, distance  and  texture  are  lost  or  are  so  badly  distorted  as  to  repel 
observers  rather  than  to  attract  them.  Windows  have  been  lighted 
successfully  from  the  front  and  below  where  a  few  large  objects  are  dis- 
played on  the  floor  of  the  window  and  where  the  height  of  the  win- 
dow or  structural  conditions  were  such  as  to  render  it  difficult  to  light 
the  window  from  above.  Satisfactory  installations  have  also  been 
made  where,  in  addition  to  the  lighting  from  above,  "foot-light" 
sections  have  been  placed  along  the  front  bottom  of  the  window. 
The  purpose  of  these  sections,  which  should  contribute  intensities 


MACBETH:  LIGHTING  OF  OFFICES  385 

not  more  than  one-third  of  that  effective  from  the  top  of  the  window, 
is  to  illuminate  the  shadows  to  a  lower  intensity  than  the  high  lights 
resulting  from  the  lamps  in  the  upper  part  of  the  window.  This  sys- 
tem is  useful  in  windows  where  the  objects  displayed  have  wide  pro- 
jections under  which  there  would  be  heavy  shadows.  These  "foot- 
light"  sections  are  also  effectively  equipped  with  colored  lamps  or 
color  filters  and  are  used  to  direct  colored  light  into  the  shadows  for 
the  purpose  of  rendering  the  objects  displayed  more  attractive. 

Light  Distribution  Calculation. — In  high,  shallow  windows,  con- 
centrating reflectors  should  be  used;  while  in  deep  windows,  these 
reflectors  would  not  be  satisfactory.  A  very  simple  method  for 
determining  the  distribution  characteristics  of  the  reflector  to  use  is 
to  make  a  scale  drawing  of  a  sectional  elevation  of  the  window, 
showing  the  height  and  depth,  marking  in  the  satisfactory  position 
for  the  lamp  from  a  structural  point  of  view,  and  the  assumed  plane 
of  illumination.  Radial  lines  should  then  be  drawn  from  the  lamp 
center  to  this  plane.  The  length  of  these  lines  can  be  measured  with 
any  scale,  preferably  in  centimeters  or  tenths  of  an  inch.  These 
numbers  squared  will  then  be  a  measure  of  the  proportionate  inten- 
sities required  for  uniform  normal  illumination  over  the  section  of 
plane  assumed.  It  may  be  desirable  to  increase  the  values  toward 
the  front  of  the  window  and  reduce  those  in  the  rear  as  objects  having 
fine  detail,  if  placed  in  the  front  of  the  window,  are  sufficiently  close 
to  the  observer,  and  the  high  intensity  would  be  useful,  whereas,  in 
the  rear  of  the  window,  it  is  more  a  matter  of  discerning  form  and 
outline.  These  values  are  then  plotted  to  a  convenient  scale  which 
will  bring  them  up  as  shown  by  curve  B,  Fig.  18,  and  by  considering 
this  specification  curve  with  the  candle-power  dsitribution  curves  of 
units  that  are  available,  it  is  a  simple  matter  to  select  the  one  which 
will  give  the  best  approximation.  In  this  instance,  the  solid  line, 
curve  C,  was  selected  and  illumination  measurements  afterward 
made  in  the  window  show  the  close  approximation  of  E,  the  measured 
value,  to  D  which  was  calculated,  and  in  the  vertical  planes  G  and  F, 
respectively.  A  considerable  building  up  of  the  values  in  the  front  of 
the  window  can  be  counted  upon  through  the  reflection  of  light  from 
the  inside  surfaces  of  the  plate  glass. 

The  completed  window  is  shown  in  Fig.  15.  Sixty- watt  lamps 
with  focusing  prismatic  reflectors  were  installed  on  13. 5-inch  centers. 
The  lamps  and  reflectors  were  directed  backward  into  the  window 
at  an  angle  of  20  degrees  from  the  vertical.  It  will  be  noted  that 
there  is  clear  glass  in  the  upper  half  of  the  rear  of  this  window.  The 
25 


386 


ILLUMINATING   ENGINEERING   PRACTICE 


purpose  of  this  glass  is  to  admit  daylight  to  the  store.  At  night  this 
glass  is  objectionable  because  of  the  reflection  of  the  lamps  and  re- 
flectors used  in  front  of  the  window.  This  reflection  could  be  elimi- 
nated if  window  shades  were  installed  in  the  back  of  this  window, 
on  the  window  side.  In  many  instances,  shades  have  been  in- 


Fig.  1 8. — Diagram  illustrating  the  method  of  calculation  for  the  predetermination  of  the 
distribution  of  light  in  a  show  window;  the  distribution  curve  of  the  unit  selected  to  meet 
the  specification  and  also  the  calculated  and  resultant  illumination  values  are  given.  A, 
is  the  assumed  line  of  trim;  B,  calculated  photometric  curve  to  produce  uniform  normal 
illumination  on  A;  C,  polar  diagram  distribution  curve  of  the  unit  installed;  D,  calculated 
illumination  values  from  a  unit  having  the  distribution  of  curve  B;  E,  test  values  of  final 
resultant  illumination  on  the  horizontal  plane;  F,  calculated  vertical  illumination  values 
from  unit  corresponding  to  curve  B;  G,  test  values  of  final  resultant  illumination  on  the  ver- 
tical plane. 

stalled  in  locations  similar  to  this  but  they  are  invariably  improperly 
placed  on  the  store  side  of  the  glass. 

It  is  not  assumed  that  it  is  correct  to  base  all  such  calculations 
upon  an  average  window  trim  for  all  windows.  As  a  matter  of  fact 
the  line  of  window  trim  undoubtedly  differs  in  the  majority  of 
windows.  If  a  window  is  merely  flooded  with  light  it  is  not  neces- 


MACBETH:  LIGHTING  OF  OFFICES  387 

sarily  productive  of  the  best  results  but  may  be  an  extravagant  waste 
of  energy  and  money  for  the  merchant.  With  properly  designed 
reflectors  the  goods  in  a  window  may  be  made  to  stand  out  more 
prominently  with  a  lower  power  consumption  than  with  reflectors 
with  a  distribution  not  conforming  to  that  required  to  direct  the 
light  at  such  incident  angles  upon  the  goods  as  to  cause  the  redirected 
rays  to  be  most  effective  upon  the  eye  of  the  observer.  This  is  true 
regardless  of  a  possible  difference  in  efficiencies  of  the  reflectors. 

In  many  stores  and  showrooms  the  show  windows  are  merely  an 
extension  of  the  sales  floor.  The  windows  are  not  backed  up.  It  is 
very  important  in  lighting  a  window  of  this  kind  that  the  lamps  be 
screened  from  the  range  of  vision  of  those  in  the  store.  This  can  be 
done  as  shown  in  Fig  16,  where  a  trough  reflector  was  used,  designed 
in  such  a  manner  that  the  rear  section  of  the  trough  cut  off  all  view 
of  the  lamps  from  the  interior,  the  cut-off  being  effective  right  up  to 
the  back  of  the  window  at  any  position  above  3  feet  above  the  floor. 

Determination  of  Number  of  Outlets. — After  the  candle-power  dis- 
tribution characteristics  of  the  unit  have  been  settled  upon,  it  has 
been  found  sufficient  to  multiply  the  floor  area,  in  square  feet,  by  the 
illumination  desired  in  foot-candles,  then  multiply  this  result  by  a 
value  which  will  range  from  two  to  five  or  more,  depending  upon  the 
efficiency  of  the  light  distribution.  This  result  will  be  the  total 
lumens  required,  which  amount  divided  by  the  total  lumens  per 
lamp,  will  give  the  number  of  lamps.  It  is  important  to  provide  in 
the  placing  of  these  lamps  for  ample  illumination  at  the  ends  or  sides 
of  the  windows.  The  center  will  be  well  taken  care  of  from  the  con- 
tributions from  practically  all  of  the  lamps  in  the  row  unless  exceed- 
ingly concentrating  reflectors  are  used.  In  fact,  it  is  desirable  to  use 
a  wider  spacing  of  units  in  the  center  of  the  window  than  at  the  ends. 
Many  illumination  measurements  made  in  existing  window  installa- 
tions show  that  with  a  uniform  spacing  of  lamps,  the  illumination 
intensities  are  very  much  higher  in  the  center  of  the  window  than 
at  the  ends.  This  is  not  a  result  of  design,  but  of  ignorance  or 
thoughtlessness.  If  the  intensity  at  the  ends  is  satisfactory,  then 
there  is  too  much  in  the  center,  whereas,  if  the  center  is  satisfactory, 
either  a  closer  spacing  of  units  or  larger  lamps  should  be  used  in  the 
ends  of  the  window. 

Because  of  the  amount  of  light  absorbed  by  dark  goods,  windows 
where  this  class  of  goods  is  to  be  displayed  should  have  higher  in- 
tensities than  where  light  goods  only  are  on  view.  In  some  instances 
in  the  past,  this  was  provided  for  by  switching  arrangements  so  that 


388  ILLUMINATING   ENGINEERING   PRACTICE 

when  dark  goods  were  displayed  all  the  lamps  would  be  in  use  and  a 
proportionately  less  number  could  be  turned  on  for  light  goods. 
From  the  standpoint  of  the  merchant,  this  did  not  prove  a  practical 
consideration  as  in  all  cases  investigated  all  of  the  lamps  were  used 
together.  The  conclusion  is  reached,  therefore,  that  if  dark  goods 
are  at  any  time  likely  to  be  displayed,  the  window  lighting  shall  be 
designed  for  dark  surfaces. 

It  is  important  with  gas-filled  incandescent  lamps  that  they  be 
installed  in  such  a  manner  as  to  make  it  difficult,  if  not  impossible,  for 
the  window  trimmer  to  place  goods  close  to  these  lamps  or  attach 
anything  to  them.22  There  is  less  difficulty  with  gas  lamps  because 
window  trimmers  have  a  very  clear  idea  that  with  these  lamps  there 
is  heat;  there  is,  however,  a  sufficiently  great  fire  risk  with  gas-filled 
electric  lamps  to  warrant  this  caution,  and  much  less  of  the  heat 
association  idea. 

Figs.  19  and  20  illustrate  the  value  of  checking  up  the  requirements 
for  light  distribution  in  a  window.  This  was  done  for  the  window  in 
Fig.  19  and  Fig.  20  was  a  direct  duplication  of  the  installation  in  Fig. 
19,  made  with  the  consent  of  the  owners  of  the  first  mentioned  window. 
The  dimensions  are  identical  and  the  same  kind,  size  and  number  of 
lamps  were  used.  Instead  of  the  lamps  being  hung  pendent  with  an 
angle  reflector,  however,  as  in  Fig.  19,  Fig.  20  was  fitted  with  a  very 
similar  prismatic  reflector  which,  instead  of  being  pendent,  was 
tipped  up  at  an  angle  which  resulted  in  directing  most  of  the  light 
into  the  upper  rear  part  of  the  window. 

The  methods  for  installing  gas  lamps  to  illuminate  show-window 
displays  vary  somewhat  with  the  construction  of  the  windows.23 

In  Fig.  23,  are  shown  three  methods,  two  for  the  enclosed  box  type 
window,  and  one  for  the  open  type. 

The  enclosed  type  of  window  can  be  best  lighted  by  units  installed 
in  the  front  of  the  window  through  openings  in  the  ceiling  or  deck. 
The  lamps  should  be  provided  with  reflectors  to  direct  the  light 
downward  and  back  into  the  window.  A  valance  or  screen  should 
be  placed  at  the  top  to  result  in  a  finished  appearance  and  to  hide  the 
reflectors.  There  should  be  openings  in  the  floor  of  the  window  to 
admit  fresh  air.  These  openings  are  to  be  connected  to  air  ducts 
over  which  cheesecloth  has  been  stretched  to  prevent  dust  being 
carried  into  the  windows  and  to  check  the  direct  flow  of  air  which 
might  cause  sudden  draughts.  The  cheesecloth  can  be  stretched  on 
a  wooden  frame  of  such  construction  as  to  permit  of  its  easy  renewal. 
The  glass  in  a  show  window  so  ventilated  will  not  "sweat"  and,  as 


Fig.   19. — Show  window  in  which  the  arrangement  of  lamps  and  reflectors  was  carefully 
calculated.     The  average  intensity  in  this  window  was  30  foot-candles. 


Fig.  20. — Show  window  constructed  in  practically  every  particular  similar  to  Fig.  19. 
These  two  photographs  were  made  on  the  same  kind  of  plate  and  were  given  the  same  time 
of  exposure,  development  and  printing. 

(Facing  page  388.) 


Fig.  21. — Show  window  of  the  boxed-in  type  illuminated  with  five  single  inverted  mantle 
gas  lamps  installed  over  glass  panels  in  the  ceiling  of  the  window  close  to  the  plate  glass. 


Fig.  22. — A  deep  enclosed  window  with  glass  panels  in  the  ceiling  above  which  gas  lamps 
with  concentrating  reflectors  are  used. 


MACBETH:  LIGHTING  OF  OFFICES 


389 


a  consequence,  will  be  free  from  frost  during  cold  weather.  The 
arrangement  of  the  air  ducts,  ventilators,  and  lamps  causes  a  current 
of  air  to  pass  constantly  across  the  back  of  the  glass.  In  any  window 
where  sweating  is  noted,  it  is  merely  necessary  to  arrange  the  lamps, 
fixtures,  and  ventilators  in  such  a  manner  as  to  produce  a  circulation 
of  air  over  the  entire  surface  of  the  glass  and  the  trouble  will 
disappear. 

In  windows  fitted  as  shown  in  Diagram  A,  Fig.  23,  the  openings  in 
the  deck  should  be  large  enough  to  permit  the  lamps  to  be  adjusted 
from  the  interior  of  the  window.  Transoms  above  the  windows 


--y^lIeUl  Oiling   f 

Light:  12  on  Center!; 
ITt^ 


Window  Plmtfor^ ^  VJndow  Platform 

ABC 

Fig.  23. — Illustrating  three  methods  of  installation  for  window  lighting  with  gas  lamps. 

A,  enclosed  type  of  window,  lamps  equipped  with  angle  reflectors  projecting  through  open- 
ings in  the  front  of  the  deck  of  the  window.     B,  the  totally  enclosed  type  of  window,  with 
glass  panels  in  the  front  part  of  the  deck  and  the  gas  lamps  with  opaque  concentrating  re- 
flectors installed  above  this  glass  at  a  sufficient  height  to  enable  the  reflectors,  glassware, 
and  mantles  to  be  removed  without  difficulty.     C,  an  approved  method  for  the  installation 
of  gas  lamps  in  the  open  type  of  window.     The  lamps  are  concealed  in  a  box  built  in  at  the 
top  and  parallel  with  the  plate  glass,  the  bottom  of  this  box  being  fitted  with  glass  panels. 

should  extend  to  within  3  inches  of  the  ceiling  and  should  be 
hinged  at  the  bottom  so  that  when  they  are  open  they  protect  the 
lamps  in  the  window  from  draught  and  prevent  undue  heat  pocketing 
at  the  ceiling. 

For  corner  show  windows,  windows  that  are  shallow,  or  windows 
where  the  display  is  such  that  access  to  the  window  can  be  had  only 
at  irregular  intervals,  the  type  of  construction  illustrated  in  Diagram 

B,  Fig.  23,  is  recommended.     The  glass  panels  in  the  front  of  this 
deck  are  of  ripple  or  cathedral  glass  which  serves  to  break  up  the  image 
of  the  lamp  above  the  glass  and  to  distribute  the  light  effectively 


390  ILLUMINATING   ENGINEERING   PRACTICE 

throughout  the  window  without  the  absorption  losses  of  the  ground 
or  sandblasted  glass  that  has  been  used  in  the  past.  In  this  type  of 
window  the  lamps  should  be  installed  above  the  glass  at  a  sufficient 
height  to  permit  the  mantles  and  glassware  to  be  removed.  The 
reflectors  should  be  opaque  and  of  the  concentrating  type. 

Fig.  21  shows  a  window  of  this  kind  in  which  the  lamps  are 
equipped  with  prismatic  glass  reflectors.  The  opaque  band  with 
translucent  letters  at  the  top  of  the  window  serves  to  cut  off  the  view 
of  the  glass  panels  from  the  observer  on  the  street. 

In  deep  narrow  windows  the  lighting  units  may  be  distributed 
over  the  entire  deck.  A  window  lighted  in  this  manner  is  shown  in 
Fig.  22. 

For  the  open  type  of  show  window,  the  construction  shown  in 
Diagram  C,  Fig.  23,  is  recommended.  The  two  important  factors  in 
a  window  of  this  type  are  :  first,  that  the  location  of  the  lamps  must 
be  such  as  properly  to  illuminate  the  face  of  the  display  presented  to 
the  observer  on  the  outside  and,  second,  those  on  the  inside  of  the 
store  should  be  protected  from  the  glare  which  would  result  with  a  row 
of  open  lamps  used  in  this  position.  The  lamps  can  be  shielded 
from  the  interior  of  the  store  by  this  method  as  shown  on  the  diagram. 
The  rear  section  of  this  box  may  be  of  panelled  wood  or  metal.  The 
bottom  of  the  box  should  be  filled  in  with  panels  of  ripple  glass. 
Opaque  concentrating  reflectors  should  be  used.  As  cathedral  glass 
can  be  secured  in  several  colors,  it  is  advisable  that  the  framework  be 
arranged  so  that  the  glass  can  be  easily  removed  and,  in  this  manner, 
the  color  effects  of  the  lighting  can  be  varied  by  the  use  of  different 
colored  glass. 

There  are  two  practically  standard  methods  used  for  igniting  the 
lamps  in  these  show  windows.  First,  the  jump  spark  system,  the 
necessary  energy  for  which  is  supplied  by  a  battery  of  dry  cells  and, 
second,  ignition  by  pilot  flames.  The  gas  supply  to  the  lamps  can  be 
controlled  by  a  magnet  valve  operated  from  the  dry  cells. 

BIBLIOGRAPHY 

*W.  A.  DURGIN  and  J.  B.  JACKSON.—  "  Semi-direct  Office  Lighting  in  the 
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2  "Lighting  Handbook."     I  vanhoe-  Regent  Works  of  General  Electric  Co., 


3  M.  MCMILLAN.  —  "  Better  Lighting  Supervision  Would  Preserve  Health  in 
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4  E.  L.  ELLIOTT.  —  "Economy."     111.  Eng.,  1912,  page  623. 


MACBETH:  LIGHTING  OF  OFFICES  391 

5  CHAS.   F.   SCOTT.— "Cost  and  Value  of  Light."     Electric  Journal,   1910, 
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6  "Lighting  Survey."    L.  J.,  1913,  page  206. 

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J.  P.  MALIA.— "Office  Indirect  Lighting."     Electrical  World,  1913,  page  335. 

C.  E.  CLEWELL. — "Illumination  Design  Notes  Based  on  the  New  Hill 
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T.  H.  ALDRICR  and  J.  P.  MALIA. — "Indirect  Illumination  of  the  General 
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S.  G.  HIBBEN. — "General  Suggestions  for  the  Proper  Installation  and  Use 
of  Semi-indirect  Lighting  Fixtures."  L.  J.,  1915,  page  145. 

H.  D.  BUTLER  and  J.  A.  HOEVELER. — "Indirect  Illumination  in  a  Large 
General  Office."  L.  J.,  1913,  page  196. 

"Comparison  of  Office  Building  Lighting  Equipments."  Electrical  Age, 
1915,  page  41. 

A%  B.  ODAY  and  R.  E.  HARRINGTON.— "Illumination  Systems  for  Good 
Lighting  of  Offices."  Electrical  World,  1915,  page  814. 

C.  E.  CLEWELL. — "New  Lighting  in  the  Engineering  Building  of  the  Uni- 
versity of  Pennsylvania."  L.  J.,-i9i5,  page  196. 

W.  E.  CHAPMAN.— "Artificial  Lighting  of  Typical  Offices  in  State,  War,  and 
Navy  Department  Building."  Trans.  111.  Eng.  Society,  1915,  page  651. 

M.  SPENCER. — "Scientific  Illumination  of  Working  Surfaces."  111.  Eng., 
1912,  page  132. 

"  Unit  Lighting  System."     Electrical  World,  1911,  page  1512. 

"Drafting  Room  Indirect  Illumination."     Electrical  World,  1912,  page  832. 


392  ILLUMINATING   ENGINEERING   PRACTICE 

W.  S.  KILMER.— "Office  Building  Lighting."  Electrical  World,  1912, 
page  264. 

W.  S.  KILMER.— "The  Lighting  of  an  Office  Building."  111.  Eng.,  1912, 
page  223. 

10  "Handbook  on  Incandescent  Lamp  Illumination."     Edison  Lamp  Works  of 
General  Electric  Co.,  1916. 

W.  N.  GOLDSCHMIDT. — "Indirect  Lighting  in  an  Insurance  Company's 
Office."  111.  Eng.,  1911,  page  140. 

F.  W.  WILLCOX.— "The  Illumination  of  the  Turbo  Drawing  Office,  The 
British-Thomson-Houston  Co.,  Ltd."     111.  Eng.,  1911,  page  319. 

G.  H.  SWANFELD. — "Lighting  the  Largest  Publishing  House  in  America." 
111.  Eng.,  1911,  page  624. 

L.  H.  SULLIVAN. — "Lighting  the  People's  Savings  Bank,  Cedar  Rapids, 
Iowa."  111.  Eng.,  1911,  page  631. 

S.  G.  HIBBEN. — "When  Architect  and  Engineer  Cooperate."  L.  J.,  1913, 
page  35. 

D.  WOODHEAD. — "The  Lighting  of  a  Bank  and  a  Large  General  Office." 
L.  J.,  1913,  page  292. 

"Indirect  Lighting  of  a  Bank  in  Los  Angeles."     Gas  Age,  1914,  page  386. 

F.  J.  McGuiRE  and  F.  R.  NUGENT.— "The  Lighting  of  New  York's  Great 
Municipal  Building."  L.  J.,  1914,  page  125. 

C.  M.  BUNN. — "Concealed  Lighting  Fixtures  in  the  Swedish-American 
Bank,  Chicago."  L.  J.,  1914,  page  30. 

W.  R.  MOULTON.—"  Modern  Lighting  of  a  Bank  by  Reconstruction  of  Old 
Fixtures."  L.  J.,  1916,  page  102. 

C.  L.  LAW. — "Illumination  Test  on  Semi-indirect  and  Cove  Lighting  in  a 
Combined  Office  and  Salesroom."  L.  J.,  1915,  page  12. 

"Cove  Lighting  of  a  Store."     Electrical  World,  1916,  page  378. 

11  W.  S.  KILMER.— "  Semi-indirect  Lighting  Applied  to  Large  Areas."    L.  J., 
1913,  page  40. 

12  W.  N.  GOLDSCHMIDT. — "Jewelry  Store  Lighting  with  Indirect  Fixtures." 
L.  J.,  1915,  page  105. 

E.   J.   DAILEY. — "Clothing   Store  Lighting   with  Type   C   Mazda  Lamps." 
L.  J.,  1915,  page  2. 

W.  R.  MOULTON.— "Lighting  of  Stores  and  Public  Buildings."  Electrical 
Review  and  Western  Electrician,  1916,  page  918. 

13  A.  L.  POWELL. — "Large  Dry  Goods  and  Department  Store  Lighting."     L. 
J.,  1913,  page  142. 

C.  L.  LAW  and  A.  J.  MARSHALL. — "The  Lighting  of  a  Large  Store."  Trans. 
111.  Eng.  Society.,  1911,  page  186. 

T.  E.  RITCHIE.— "  Color  Discrimination  by  Artificial  Light."     Prog.  Age, 

1912,  page  199. 

"Filene  Store,  Boston."     Electrical  World,  1913,  page  579. 
W.    S.    KILMER. — "Semi-indirect  Illumination   in   a   Department   Store." 
L.  J.,  1913,  page  151. 

H.  W.  SHALLING. — "Department  Store  Lighting."     Trans.  111.  Eng.  Society, 

1913,  page  17. 

"Illumination  Features  in  N.  Y.  Department  Store."     Electrical  World, 

1914,  pages  1134,  1145,  1397. 


MACBETH:  LIGHTING  OF  OFFICES  393 

"The  Lighting  of  a  Large  Department  Store."    L.  J.,  1915,  page  245. 

H.  T.  SPAULDING.— "Modern  Lighting  Practice  in  Department  Store." 
Cen.  Sta.,  Dec.,  1915,  page  150. 

"Lighting  Features  of  Department  Store,  Boston."  Electrical  Review  and 
Western  Electrician,  1915,  page  312. 

"An  Innovation  in  Store  Lighting."     111.  Eng.,  1911,  page  354. 

M.  H.  FLEXNER  and  A.  O.  DICKER. — "Illumination  of  a  Furniture  Store." 
L.  J.,  1913,  page  141. 

E.  F.  OLIVER. — "Modernizing  Furniture  Store  Lighting."  L.  J.,  1915, 
page  153. 

14  A.  L.  POWELL.— "The  Lighting  of  Ordinary  Small  Stores."     L.  J.,  1913, 
page  122. 

C.  L.  LAW  and  A.  L.  POWELL.— "  Present  Practice  with  Tungsten  Filament 
Lamps — Small  Store  Lighting."  Electrical  Review  and  Western  Electrician, 

1912,  page  775. 

C.  L.  LAW  and  A.  L.  POWELL. — "  Small  Store  Lighting  with  Tungsten  Fila- 
ment Lamps — Present  Practice  in."  Trans.  111.  Eng.  Society,  1912,  page  437. 

C.  L.  LAW  and  A.  L.  POWELL.— "  Distinctive  Store  Lighting."  Trans.  111. 
Eng.  Society,  1913,  page  515. 

A.  L.  POWELL. — "Store  Lighting."    L.  J.,  1913,  page  90. 

C.  L.  LAW  and  A.  L.  POWELL. — "Distinctive  Store  Illumination."  Isolated 
Plant,  Dec.,  1913,  page  42. 

A.  L.  POWELL. — "Shop  Lighting."    L.  J.,  1914,  page  4. 

A.  L.  POWELL. — "Store  Lighting  with  High  Efficiency  Mazda  Lamps."     L. 
J.,  1914,  page  166. 

15  "Lighting  of  All-Package  Grocery  Stores."     Gas  Age,  1916,  page  523. 
"Store  Lighting."     Am.  Gas.  Lt.  J.,  1910,  page  1139. 

L.  F.  BLYLER. — "Lighting  a  High  Class  Haberdashery  Store."     111.  Eng., 
1911,  page  656. 

J.  N.  COOK. — "Commerical  Lighting."     Prog.  Age,  1911,  page  418. 

B.  K.  CARLING. — "Store  Lighting."     Prog.  Age.,  1911,  page  435. 

E.  H.  MARTIN. — "Lighting  a  Rug  Display."     Prog.  Age,  1911,  page  487. 

R.  M.  THOMSON. — "Holding  Lighting  Business."  Prog.  Age,  1911,  page 
610. 

E.  M.  OSBOURNE. — "Store  Lighting  with  Gas  Arcs."  Prog.  Age,  1911, 
page  987. 

J.  M.  COLES. — "  Gas  Arc  Lamps  in  a  Millinery  Goods  Show  Room."    L.  J., 

1913,  page  125. 

J.  E.  PHILBRICK.—"  Store  Lighting."  Trans.  111.  Eng.  Society.,  1913, 
page  499. 

R.  ff.  PIERCE. — "Lighting  Installation  Planning."  Am.  Gas  Lt.  J.,  1915, 
page  321. 

16  C.  I.  HODGSON.— "The  Use  of  Detailed  Maintenance  Records."    L.  J., 
1915,  page  274. 

W.  S.  KILMER.— "  Special  Illumination  from  a  Tubular  Source  of  Light." 
111.  Eng.,  1911,  page  18. 

J.  A.  VESSY. — "Show  Case  Lighting."     Electrical  World,  1912,  page  1223. 

W.  S.  KILMER. — "Modern  Show-case  Lighting."  Electrical  Review  and 
Western  Electrician,  1913,  page  162. 


394  ILLUMINATING   ENGINEERING   PRACTICE 

"Indirect  Lighting  in  a  Large  Retail  Clothing  Store."  Electrical  Review 
and  Western  Electrician,  1913,  page  670. 

H.  B.  WHEELER. — "Lighting  by  Indirect  System  High  Class  Stores."  Elec- 
trical Engineering,  1913,  page  439. 

W.  R.  MOULTON.— "The  Lighting  of  an  Exclusive  Clothing  Store."  L.  J., 
1915,  page  251. 

17  A.  L.  POWELL.— "Show  Window  and  Show-case  Lighting."     L.  J.,   1913, 
page  173. 

F.  H.  M.  RILEY.— "  Value  of  the  Lighting  Engineer."  Electrical  World, 
1913,  page  407. 

18 Lectures,  Johns-Hopkins.     111.  Eng.  Society,  1911,  page  778. 

R.  BEMAN. — "Reflection  from  Plate  Glass."     111.  Eng.,  1912,  page  209. 

19  H.  B.  WHEELER.— "The  Illumination  of  the  New  Hub  Store,  Chicago." 
L.  J.,  1913,  page  116. 

J.  G.  HENNINGER. — "Show  Window  Lighting."     Trans.  111.  Eng.  Society, 

1912,  page  178. 

20  A.  L.  ABBOTT  and  C.  M.  CONVERSE. — "Show  Window  Installation."     L.  J., 

1913,  page  39. 

H.  B.  WHEELER  and  J.  A.  HOEVELER. — "Illumination  of  Small  Show  Wind- 
ows." Electrical  World,  1914,  page  335. 

H.  B.  WHEELER.— "The  Lighting  of  Show  Windows."  Trans.  111.  Eng. 
Society,  1913,  page  555. 

J.  C.  KING.— "Show  Window  Lighting  at  Stern  Brothers'  New  Store,  New 
York."  L.  J.,  1913,  page  264. 

"Show  Window  and  Display  Lighting."  Electrical  Review  and  Western 
Electrician,  1914,  page  275. 

C.  B.  PATE. — "Department  Store  Show  Window  Lighting."  L.  J.,  1913, 
page  1 70. 

21  E.    R.    TREVERTON. — "Combination   Gas   and   Electric   Office  Lighting." 
L.  J.,  1914,  page  264. 

22 1.  CLYDE.— "Danger  in  Show  Windows."     Electrical  World,  1911,  page  335. 

23  "Report  of  Committee  on  Window  Display."     Proc.  N.  C.  G.  A.,  1914, 
page  350. 

B.  F.  BULLOCK. — "Window Lighting."     Prog.  Age,  1911,  page  575. 

A.  H.  JOHNSTON. — "Methods  of  Window  Lighting."  Prog.  Age,  1911, 
pages  705-6. 

R.  M.  THOMSON. — "Decked  Window  Lighting."     Prog.  Age,  1912,  page  5. 

S.  SNYDER. — "Lighting  a  Window  Display  with  Gas."  Prog.  Age,  1912, 
page  196. 

24  P.  EVES. — "Store  Window  Lighting."     Prog.  Age,  1912,  page  572. 


THE  LIGHTING  OF  THE  HOME 

BY   H.    W.    JORDAN 

Many  discussions  coming  under  the  title  of  illuminating  engineer- 
ing are  so  embellished  and  surrounded  with  technical  terms  and  ex- 
pressions that  the  mind  of  the  average  practical  man  becomes  much 
confused  in  listening  to  or  reading  about  them  and  he  is  often  as 
much  in  the  dark  at  the  end  as  at  the  beginning. 

The  average  central  station  operator  or  salesman's  knowledge  of 
illuminating  terms  is  more  likely  to  be  limited  to  a  general  practical 
understanding  of  fixtures,  lamps,  candle-powers  and  wattages,  rather 
than  lumens,  lamberts  or  ultra-violet  radiation. 

I  have  no  intent  to  speak  lightly  of  the  subject  as  an  exact  science, 
or  of  the  real  value  of  technical  knowledge  applied  to  illuminating 
engineering;  but  my  experience  has  proven  to  me  that  there  is  a  real 
need  of  more  simple  information  on  this  subject  that  could  be  ap- 
plied alike  by  the  central  station  man  and  the  lighting  service  sales- 
man, and  it  is  for  such  men  that  this  lecture  has  been  prepared. 

The  possession  by  the  central  station  man,  the  salesman  or  the 
electrician,  of  a  thorough  knowledge  of  the  principles  of  illumination 
would  be  of  the  greatest  advantage,  but  even  though  he  appreciated 
that  it  would  mean  much  to  him,  the  average  man  will  not  apply 
himself  to  a  technical  study  of  these  principles. 

There  is  encountered  frequently  at  the  present  time  the  problem 
of  the  old  installations  with  fixtures  often  of  barbarous  design.  If 
these  were  short-lived,  one  problem  of  poor  illumination  would  be 
solved;  but  while  the  residence  owner  may  quickly  see  the  disad- 
vantage of  antiquated  plumbing  and  remedy  it,  he  and  his'  anti- 
quated lighting  fixtures  "grow  old  together."  However,  if  the  solic- 
itor, directly  in  contact  with  the  owner  and  the  builder  of  the  small 
residence,  had  a  knowledge  of  even  the  elementary  rules  of  illumina- 
tion, there  would,  I  believe,  be  a  vast  improvement. 

The  effect — whether  conscious  or  unconscious — of  the  harshly  or 
insufficiently  lighted  home,  is  as  subtle  and  as  uncomfortable  in  the 
case  of  a  small  residence  as  in  that  of  the  large.  It  is  true  that  in 
the  small  residence  the  question  of  expense  must  be  more  carefully 

395 


396  ILLUMINATING   ENGINEERING   PRACTICE 

considered  than  in  the  large,  yet  when  the  owner  realizes  that  in- 
correct illumination  often  means  larger  bills  than  proper  illumination, 
he  is,  as  a  rule,  anxious  to  have  the  trouble  corrected.  Poor  lighting 
is  by  no  means  always  attributable  to  a  desire  on  the  part  of  the 
owner  to  economize,  but  is  more  often  due  to  a  misunderstanding 
as  to  what  constitutes  correct  lighting. 

In  the  lighting-service  salesman's  mind,  selling  and  service  should 
be  side  by  side.  The  average  residence  owner  must  be  credited 
with  common  sense,  and  it  is  reasonable  to  suppose  that  if  the  solic- 
itor explained  intelligently  the  essentials  of  correct  illumination,  the 
owner  would  not  knowingly  select  the  incorrect.  When  proper 
lighting  is  better  understood,  it,  rather  than  economy,  will  be  the 
primary  factor. 

I  believe  that  the  illuminating  companies  are  alert  to  the  bene- 
ficial results  of  an  understanding  on  the  part  of  their  solicitors  as  to 
what  constitutes  a  home  correctly  lighted,  and  desire  to  cooperate 
with  architects,  fixture  designers  and  decorators  in  this  respect. 
These  companies  do  realize  the  great  need  to  the  public,  and  there- 
fore, to  themselves,  of  proper  installations  and  illuminants  in  the 
home.  This  can  be  judged  in  a  way  from  their  advocacy  of  the 
most  efficient  lamps,  their  adherence  to  the  policy  of  free  advice 
to  present  and  prospective  customers,  and  to  their  support  of  the 
departments  of  illuminating  engineering. 

No  fixed  rule  can  be  given  for  home  lighting.  There  always  arises 
the  question  of  the  use  to  which  the  rooms  in  the  house  are  to  be 
put  by  the  individual  owners,  their  personal  tastes,  whether  artistic 
effect  is  desired,  or  whether  it  is  entirely  a  question  of  economy.  I 
am  sure  no  matter  what  the  residence  owner's  taste  may  be,  there 
always  exists  the  desire  to  have  the  lighting  artistic  and  efficient  and 
the  cost  reasonably  low. 

PSYCHOLOGICAL  ASPECTS 

The  principal  object  in  home  lighting  is  without  question  the 
psychological.  It  is  our  earnest  desire  to  produce  a  plan  of  illumi- 
nation that  will  be  pleasing  and  agreeable  to  those  who  linger  in 
its  presence.  It  is  a  recognized  fact  that  our  visual  perceptions  and 
sensations  are  agreeable  or  disagreeable,  pleasant  or  unpleasant. 
The  illuminating  engineer  must  give  this  fact  great  consideration. 
A  too  brilliant  light  source  takes  away  that  atmosphere  of  restful- 
ness  nearly  always  desired  and  under  it  one  is  prompted  to  sit  up 


JORDAN:  LIGHTING  OF  THE  HOME  397 

straight  on  the  edge  of  a  chair  rather  than  to  sit  peacefully  at  ease, 
as  one  would  feel  like  doing  in  the  presence  of  a  reasonable  amount 
of  light  of  soft  agreeable  colors. 

After  all,  proper  and  correct  illumination  is  that  which  obtains 
pleasing  and  agreeable  results  and  effects.  Surely,  the  emotional 
factor,  is  very  important  in  the  lighting  of  the  home  on  account  of 
its  direct  influence  upon  the  emotions  of  both  the  conscious  and  the 
subconscious  mind. 

People  have  stated  that  the  true  indirect  system  in  small  interiors 
has  a  most  peculiar  effect  on  the  mind;  some  complain  that  it  gives 
them  the  blues,  others  say  that  it  makes  them  feel  depressed,  and  I 
personally  do  not  favor  it  for  residence  lighting. 

Psychology  enters  into  the  consideration  of  aesthetic  effects  and 
also  the  physical.  It  is  a  familiar  fact  that  artificial  lighting  has 
been  done  heretofore  practically  with  illuminants  giving  much  yel- 
low, the  colors  blue  and  green  being  deficient  until  very  recent  times. 
I  believe  that  the  most  agreeable  effects  are  obtained  by  illuminants 
that  give  a  proper  proportion  of  yellow  and  red.  For  instance, 
light  that  has  a  sufficient  percentage  of  yellow  and  red  produces  a 
very  agreeable  effect  upon  the  complexion,  whereas  one  that  does 
not  have  a  sufficient  proportion  of  yellow  and  red  will  "show  up" 
wrinkles  and  freckles  and  produce  a  disagreeable,  harsh  appearance. 
This  fact  points  directly  to  the  advisability  of  using  shades  to  tone 
the  color. 

Time  will  not  permit  going  far  into  the  psychological  aspects  of 
illuminating  engineering,  my  intention  being  to  mention  it  briefly 
in  so  far  as  it  has  a  direct  bearing  upon  the  lighting  of  the  home. 

PHYSICAL  ASPECTS 

It  is  conceded  that  most  of  the  eye  troubles  of  to-day  are  traceable 
to  the  fact  that  we  are  using  our  eyes  much  more  than  heretofore, 
and  that  much  of  our  reading  is  now  done  in  the  evening.  By  the 
infinite  possibilities  of  lighting  equipment,  the  problems  as  pre- 
sented to  the  layman  are  at  present,  and  have  been  for  some  time 
past,  rendered  comparatively  easy,  the  limitations  placed  upon  him 
being  comparatively  few.  If  he  decides  that  one  system  is  bad,  he 
tries  another,  or  increases  the  intensity  of  light,  and  the  whole  time 
he  may  be  getting  deeper  and  deeper  in  trouble.  Here  is  where 
such  unlimited  freedom  may  and  often  does  form  a  dangerous  gift. 
The  allurement  to  excess  in  the  quantity  of  light,  is  always  present. 


398  ILLUMINATING    ENGINEERING    PRACTICE 

In  the  days  of  our  forefathers,  lighting  problems  were  very  simple; 
the  tallow  candle  or  the  whale  oil  lamp  furnished  all  the  light  con- 
sidered necessary,  and  in  many  cases  the  newspaper  or  books  of 
those  days  were  read  by  the  light  from  the  fireplace.  That  the  per- 
centage of  eye  troubles  was  less  than  at  present  is  probably  due  to 
the  fact  that  one  could  not  read  for  any  great  length  of  time  by  those 
methods,  reading  matter  was  not  as  common  then  as  now,  and  people 
usually  retired  shortly  after  dark.  At  the  present  time  there  is  no 
limit  to  the  kinds  of  magazines  and  papers  possible  to  obtain  at  any 
newsstand,  and  the  polish  is  such  that  most  of  their  pages  could 
almost  be  used  as  a  reflector  in  a  projector  lantern. 

There  is  no  question  that  the  eye  has  become  accustomed  to  light 
received  obliquely  from  above.  This,  I  believe,  is  one  of  the  rea- 
sons the  eye  is  affronted  by  light,  harsh  or  strong,  coming  too 
brightly  from  any  other  direction.  The  need  of  giving  serious 
thought  to  the  lighting  of  the  home  from  a  hygienic  standpoint  is 
at  once  apparent,  because  the  faculty  of  sight  is  of  supreme  im- 
portance. The  aim  should  be  not  only  to  have  the  necessary  light 
to  hold  the  eye  to  its  regular  work,  but  also  give  the  eye  its  normal 
amount  of  vision.  Eyesight  declines  with  passing  years,  and  illu- 
mination in  the  home  must  be  of  such  a  character  as  not  to  increase 
this  disadvantage. 

The  old  rule  that  light  for  reading  should  come  obliquely  over 
the  left  shoulder,  well  hints  that  direct  rays  should  be  kept  out  of 
the  eye.  In  lighting  a  room  for  reading  or  for  work  that  is  pro- 
longed, it  is  always  desirable  to  avoid  too  strong  shadows,  and  glare 
either  direct  or  reflected,  while  not  doing  away  with  shadows 
altogether. 

ARTISTIC  ASPECTS 

The  present-day  lighting  service  requirements  in  the  small 
residences,  in  the  homes  of  the  middle  class,  is  low  cost  and  utility. 
In  the  larger  residences,  the  homes  of  the  rich,  the  selection  of  fix- 
tures may  be  governed  almost  entirely  by  artistic  considerations. 
Here  the  words  science  and  art  may  be  synonymous,  and  there 
arises  the  opportunity  for  the  illuminating  engineer  to  combine 
the  two  in  the  production  of  devices  for  agreeable  and  pleasing 
effects. 

It  is  desirable  that  all  illumination  when  possible,  shall  be  aes- 
thetically correct.  When  one  considers  that  the  quantity  or 
quality  of  light,  or  type  of  fixture  adds  or  detracts  from  the 


JORDAN:  LIGHTING  OF  THE  HOME  399 

arrangements  and  the  decorative  appeal  of  a  room,  one  recognizes 
the  necessity  of  giving  these  much  thought.  The  purpose  for  which 
the  room  is  to  be  used  and  its  character  must  receive  consideration. 
In  the  large  residence,  in  many  instances,  only  an  artist  can  do 
justice  so  far  as  fixtures  are  concerned. 

One  of  the  blessings  of  to-day  is  that  lighting  auxiliaries  are  more 
artistically  designed  than  heretofore,  and  there  is  not  left,  therefore, 
much  excuse  for  inartistic  lighting  equipment. 

In  these  days  of  period  furnishings  great  care  should  be  taken  in 
the  selection  of  fixtures.  There  are  many  cases  when  efficiency 
must  be  sacrificed  in  order  to  permit  the  use  of  a  fixture  absolutely 
in  harmony  with  the  surroundings  and  the  period.  For  example, 
in  a  large  parlor  of  the  Louis  IV  period,  with  its  gold  furniture  with 
light  coverings,  delicate  hangings  at  the  windows,  and  other  decora- 
tions in  keeping  with  the  times,  imagine  how  a  shower,  a  semi- 
indirect,  a  true  indirect,  or  a  Colonial  fixture  would  look!  A  room 
of  this  type  demands  a  chandelier  of  the  time  with  its  cut-glass,  and 
if  the  client  really  has  the  courage  of  his  convictions,  he  may  equip 
it  with  real  candles;  but  if  he  has  not  quite  reached  this  point,  the 
candles  can  be  replaced  by  the  candle  lamps  which  probably  will 
be  cleaner  and  cause  less  trouble. 

Occasionally  a  dining  room  is  furnished  in  early  English  style  with 
the  carved  table  the  old  court  cupboard  and  the  Jacobean  sideboard 
and  chairs,  the  setting  being  provided  by  a  spacious  room  with 
patterned  ceiling  and  oak-paneled  walls.  In  a  room  of  that  char- 
acter the  ordinary  stock  fixture  would  be  out  of  the  question,  and 
one  would  make  use  of  a  more  ornate  chandelier,  multiple  wall 
brackets  and  candelabra  lamps. 

One  of  the  first  things  to  understand  even  from  the  briefest  study 
of  period  furnishings  is  that  all  furniture  and  all  kinds  of  decoration 
that  have  come  down  to  us  weighted  with  historic  tradition,  were 
evolved  as  a  natural  result  of  certain  conditions  of  life;  hence  the 
various  types  that  were  commonly  used  together,  will  always  look 
well  when  brought  together. 

It  is  of  very  great  importance  to  study  the  lighting  problem  from 
every  angle,  and  if  necessary  to  arrive  at  the  conclusions  in  the  selec- 
tion of  fixtures  by  the  process  of  elimination.  In  no  case  should  one 
upon  entering  a  room  immediately  decide  upon  a  certain  type  of 
fixture  simply  because  he  recently  saw  one  at  a  fixture  house  that 
impressed  him. 

Artistic  aspects  and  beauty  in  a  room,  are  a  matter  of  harmonious 


400  ILLUMINATING   ENGINEERING   PRACTICE 

relationships,  and  good  taste  in  illumination  demands  a  correct 
association  of  fixture  and  of  light,  with  the  proper  background 
setting. 

PRACTICAL  APPLICATION 

The  lighting  service  representative  in  search  of  the  residence  class 
of  business  has  probably  the  best  opportunity  to  start  the  client  on 
the  right  track,  because  he  is  usually  the  one  to  come  in  contact 
with  him  first,  and  it  is  needless  to  say  that  he  should  possess  a 
comprehensive  knwledge  of  the  subject  of  home  lighting. 

He  should,  of  course,  be  familiar  with  the  different  sizes  of  lamps, 
candle-powers,  wattages,  cost  of  operation,  and  should  possess 
other  practical  information.  It  is  obvious  that  should  he  be  possessed 
of  technical  knowledge,  his  value  to  his  company  might  be  in- 
creased; but  the  salesman's  scientific  knowledge  is  usually  limited 
and  perhaps  fortunately  so,  for  a  technically  trained  man  is  seldom 
a  clever  salesman. 

Not  infrequently  when  a  customer  equips  his  house  initially  with 
the  most  efficient  types  of  lamps,  replaces  the  burnt-out  ones  with 
old  carbon  lamps,  the  change  comes  so  gradually  that  it  is  scarcely 
noticed  until  the  bills  show  nearly  double  as  much  energy  used  as 
previously,  and  the  result  is  a  complaint. 

Another  factor  which  enters  surprisingly  into  the  economical  use 
of  lighting  is  proper  and  convenient  switching.  For  instance,  if 
the  entrance  hall  lamp  is  not  controlled  from  the  second  floor  as 
well  as  from  the  first,  it  may  be  left  in  service  much  longer  than  is 
necessary  because  some  one  on  the  way  up  stairs  may  have  forgotten 
to  turn  it  off,  and  is  too  lazy  to  retrace  his  steps. 

Another  method  of  wasting  energy  is  that  of  the  careless  or 
neglectful  person  who  goes  down  cellar  to  "fix  the  furnace" 
and  allows  the  cellar  lamp  to  remain  in  service  all  night.  To 
obviate  this  condition  a  pilot  lamp  could  be  installed  over  the  cellar 
door  where  it  would  prove  as  useful  as  one  for  a  flatiron  or  a  range. 

Another  cause  of  high  bills  is  misplaced  outlets.  I  have  seen  out- 
lets so  badly  located  that  it  was  necessary  to  produce  approximately 
10  foot-candles  in  one  end  of  a  room  in  order  to  obtain  the  necessary 
2  foot-candles  at  the  other.  Obviously,  the  lamps  should  be  so  placed 
as  to  produce  light  where  it  will  be  most  used,  thus  not  only  adding 
to  the  pleasing  effect  of  the  illumination,  but  reducing  the  cost  of 
lighting.  Such  matters  are  entirely  under  the  control  of  the  builder 
and  contractor.  When  we  realize  how  limited  the  appropriations 


JORDAN:  LIGHTING  OF  THE  HOME  401 

are  for  wiring  some  of  the  smaller  houses,  we  wonder  how  they 
provide  as  much  as  they  do.  This  remark  applies  principally  to  the 
" ready  built"  houses,  where  financial  gain  is  the  only  thing  thought 
of.  It  is  unfortunate  that  this  evil  exists,  but  at  present  there 
seems  to  be  no  remedy. 

Globes  or  shades,  the  indispensable  adjuncts  to  the  lighting 
fixture,  are  made  in  all  shapes  and  sizes  and  of  all  colors  of  the 
rainbow.  Some  are  so  thin  that  they  are  of  scarcely  any  help  in 
concealing  the  lamp  filament,  and  others  are  so  dense  that  barely 
any  perceptible  amount  of  light  can  be  obtained  through  them; 
both  of  these  extremes  are  to  be  avoided.  The  kind  of  glass  selected 
should  be  given  considerable  thought,  as  glass  absorbs,  transmits 
and  reflects. 

The  safest  way  of  protecting  one's  self  against  bad  fixtures  is  to 
reduce  the  fixture  appropriation  to  a  point  that  will  insure  simplicity. 
The  worst  fixtures  to  be  seen  are  the  gaudy  ones  of  medium  price, 
where  an  effort  has  been  made  to  obtain  a  highly  decorative  effect 
without  the  skill  in  design  and  finish  in  execution  really  necessary 
for  good  results. 

The  difference  in  cost  of  installation  and  fixtures  between  good  or 
bad  never  is  so  wide  that  the  builder  would  not  select  the  good  if  he 
realized  the  evils  of  the  bad.  Houses  sell  more  readily  when  they 
contain  practical  and  artistic  electrical  equipment. 

COOPERATION 

A  realization  of  the  importance  of  illuminating  engineering  in  the 
vast  fields  which  are  opening  to  us  have  demonstrated  the  desirabil- 
ity for  the  cooperation  of  the  illuminating  engineer  with  the  architect 
and  the  decorator.  A  new  profession,  without  doubt,  is  in  the  process 
of  development.  My  architectural  friends  inform  me  that  they  are 
depending  on  illuminating  engineers  more  and  more  every  day  for 
knowledge  and  aid.  It  is  a  fact  that  illuminants  and  new  devices  with 
their  intricate  details  are  being  developed  so  rapidly  that  even  those 
who  make  special  studies  of  them  can  hardly  keep  pace.  Granting 
this  statement  to  be  true,  I  fail  to  see  how  the  architect  or  decorator 
can  afford  to  spend  as  much  time  on  illuminating  problems  as 
necessary,  without  doing  so  at  the  expense  of  his  profession.  It  is, 
therefore,  advisable  for  the  architect  and  the  lighting  expert  to  work 
together  to  obtain  the  results  for  which  both  are  striving. 

The  engineer  should  be  consulted  where  architectural  changes  are 
26 


4O2  ILLUMINATING    ENGINEERING    PRACTICE 

contemplated,  or,  where  special  lighting  is  wanted  to  emphasize 
architectural  effects,  and  the  architect  has  an  equal  right  to  be  ad- 
vised of  anything  that  concerns  the  house  he  has  designed.  We 
have  fewer  occasions  to  consult  with  the  decorator,  but  the  same 
conditions  apply. 

LIVING  ROOMS 

In  providing  the  lighting  for  the  living  room,  consideration  must 
be  given  to  the  fact  that  of  all  the  rooms  this  one  is  most  used 
by  the  average  family;  as  this  room  is  utilized  for  many  purposes, 
a  somewhat  elastic  lighting  scheme  should  be  arranged.  In  addition 
to  being  the  library  of  the  home,  it  is  often  used  for  social  affairs, 
such  as  card  playing,  and  dancing,  and  at  other  times  one  or  more 
members  of  the  family  and  their  friends  simply  desire  to  lounge  about 
and  converse.  In  most  cases,  more  time  is  spent  in  reading  than 
at  anything  else,  and  it  will  at  once  be  seen  that  good  lighting  is  a 
very  necessary  source  of  comfort  and  one  to  which  the  utmost  con- 
sideration should  be  given.  There  are  a  number  of  ways  of  providing 
light  suitable  for  reading. 

One  way  would  be  to  illuminate  the  room  so  brightly  that  one 
could  see  to  read  in  any  part  of  it,  but  this  method  would  prove 
very  costly  and  consequently  out  of  the  question  in  the  majority 
of  rooms,  and  certainly  would  not  be  considered  artistic. 

Often  selection  is  made  of  a  portable  lamp  fitted  with  an  opaque 
reflector  that  will  throw  the  light  on  the  reading  matter,  but  this 
type  of  lamp,  while  admirable  for  reading,  is  of  so  little  service  in  the 
general  lighting  of  a  room  that  it  cannot,  or  should  not,  be  considered 
seriously  in  the  scheme  of  general  illumination, 

Some  people  attempt  to  obtain  light  for  reading  from  the  chande- 
lier above  by  directing  the  rays  downward,  or  by  attaching  a  short 
extension  cord  to  the  fixture  and  equipping  the  lamp  with  a  pris- 
matic or  even  an  opaque  shade.  This  scheme  is  satisfactory  for 
the  reader,  providing  the  pages  are  turned  at  such  an  angle  that  he 
does  not  receive  the  glare  from  the  paper,  but  it  is  a  makeshift 
arrangement,  unsightly,  and  should  not  be  encouraged. 

Often  in  homes  where  electricity  is  employed  for  lighting  use  is 
made  of  a  kerosene  lamp,  commonly  called  a  " student's  lamp"  for 
reading,  not  really  as  a  matter  of  economy,  but  to  do  away  with  the 
supposed  eye-tiring,  uncomfortable  glare  from  the  incandescent 
lamp,  which  bad  reputation  comes  from  the  use  of  a  too  brilliant 
lamp  unsuitably  placed.  One  can  quite  readily  duplicate  the 


JORDAN:  LIGHTING  OF  THE  HOME  403 

effect  of  the  kerosene  lamp  with  electric  or  gas  lamps  and  with  great 
added  convenience. 

One  reason  that  the  table  lamp  is  commonly  preferred  for  reading 
is  that  it  does  away  with  the  glare  that  is  likely  to  come  from  a 
chandelier  or  a  bracket.  Glare  could  be  avoided  by  assuming  a 
proper  position  for  reading,  by  the  proper  turning  of  the  pages  of  a 
book  to  avoid  it,  or  by  the  use  of  suitable  shading.  Care  should  be 
taken  to  select  a  table  lamp  that  gives  the  proper  light  all  around  the 
table  and  upon  the  reading  matter,  rather  than  on  the  top  of  the 
table,  where  one  can  consider  that  a  large  amount  of  the  light  is 
wasted. 

The  style  or  type  of  lamp  does  not  matter  much,  as  long  as  the 
shades  are  not  dark  and  are  wide  enough  to  allow  the  light  to  cover 
the  page  of  a  newspaper,  held  by  a  person  sitting  near. 

The  selection  of  the  shade  for  the  reading  lamp  is  one  that  natur- 
ally lies  within  the  control  of  the  purchaser.  Unfortunately,  he 
or  she  only  too  often  considers  the  question  of  ornamentation 
before  the  practicability  of  the  lamp  and  the  use  to  which  it  is 
to  be  put.  Individual  taste,  after  all,  is  the  root  of  much  of  the 
present-day  evil  in  illumination.  Naturally,  in  the  selection  of  a 
reading  lamp  for  the  home,  there  is  to  be  considered  the  number  of 
persons  likely  to  use  the  lamp  at  the  same  time. 

If  the  living  room  is  small,  say  14  to  15  feet,  and  is  furnished  with 
a  table  in  the  center,  the  table  is  the  logical  place  for  the  lamp, 
and  there  will  be  ample  room  for  several  persons  to  sit  comfortably 
around  it. 

Where  economy  in  maintenance  is  the  object,  the  single  table 
lamp  for  a  small  room  can  be  recommended,  as  the  same  lamp  can 
be  used  both  for  reading  and  for  the  general  lighting  of  the  room, 
provided  it  is  equipped  with  a  globe  or  shade  that  while  concentrat- 
ing a  considerable  portion  of  the  light  within  the  reading  area,  will 
also  allow  enough  light  to  radiate  in  all  directions  to  give  fairly  good 
illumination  in  other  parts  of  the  room.  When  use  is  made  of  three 
or  four  proper-sized  lamps,  this  arrangement  is  admirable  for  read- 
ing, and  one  is  not  likely  to  be  troubled  by  glare  from  a  page  of  white 
paper  because  the  light  comes  principally  from  one  side.  To  supply 
electrical  energy  to  the  table  lamp  an  outlet  in  the  floor,  under  the 
table,  is  to  be  preferred  and  recommended.  Of  course,  here  enters 
the  question  of  expense,  too  often  uppermost  in  the  small  residence 
owner's  mind,  but  the  slight  extra  expense  may  be  explained  to  him 
from  the  artistic  side  and  the  view-point  of  comfort.  He  may  think 


404  ILLUMINATING   ENGINEERING  PRACTICE 

that  a  cord  could  be  extended  from  the  fixture  that  is  likely  to  have 
been  placed  above  to  take  care  of  the  table  lamp,  but  a  cord  dangling 
above  the  table  is  not  a  source  of  eye-gratification  and  always  seems 
to  have  a  knack  of  hanging  in  the  way. 

Wall  brackets  would  hardly  be  required  in  a  living-room  so  small, 
from  either  a  practical  or  an  artistic  stand-point,  but  if  there  hap- 
pened to  be  a  mantel,  and  of  a  style  which  positively  demanded  some- 
thing to  satisfy  the  artistic  taste,  two  tiny  portable  lamps  equipped 
with  small  candle  light  sources  or  brackets  could  be  placed,  one  on 
either  side  of  the  shelf.  In  decoration  they  should  harmonize  with 
the  surroundings,  but  they  have  little  value  from  a  lighting  stand- 
point. 

If  the  living  room  is  a  large  one,  there  will  probably  be  two  or 
three  tables  scattered  about,  and  use  can  be  made  of  a  suitable 
lamp  on  each  table.  These  lamps  can  be  used  for  reading,  or  simply 
for  ornamentation.  A  room  thus  equipped  with  lamps  ornamented 
with  colored  silk  or  art  glass  shades  produces  a  very  artistic  effect 
when  in  harmony  with  the  furnishings  of  the  room  and  adds  greatly 
to  its  charm. 

The  next  thing  to  consider  is  the  general  illumination  of  the  above 
two  sizes  of  rooms.  In  the  case  of  the  smaller,  either  direct  or 
semi-indirect  lighting  would  be  proper.  If  semi-indirect  is  decided 
upon,  and  the  height  of  the  ceiling  is  about  9  ft.  the  top  of  the  bowl 
should  be  from  30  to  36  in.  from  the  ceiling.  Assuming  that  the  bowl 
is  6  in.  deep,  there  would  be  left  6  ft.  9  in.  head  room.  Fortunately 
one  has  a  large  assortment  of  artistic  and  efficient  stock  bowls  to 
select  from,  some  in  colors  and  some  white.  In  selecting  colors  in 
a  bowl  care  should  be  taken  to  see  that  they  do  not  clash  with  the 
furnishings  of  the  room.  If  there  arises  any  doubt  on  this  point, 
the  pure  white  will  surely  give  satisfaction.  It  is  an  easy  matter 
to  install  colored  lamps  of  any  size  when  a  particular  color  scheme 
is  desired. 

In  case  direct  lighting  is  the  choice,  use  should  be  made  of  the 
multiple  style  of  fixture.  There  are  so  many  styles  of  this  kind  of 
fixture  that  it  would  be  unreasonable  to  specify  any  particular  one. 
To  see  that  the  lamps  have  frosted  bowls  and  are  properly  shaded 
with  soft  colored  shades,  if  desired,  to  harmonize  with  surroundings, 
is  the  most  important  point. 

This  general  lighting  unit  should  always  be  controlled  by  a  wall 
switch  conveniently  located  at  the  entrance  to  the  room.  Economic- 
ally, it  would  be  advantageous  to  have  the  units  for  general  illumi- 


JORDAN:  LIGHTING  OF  THE  HOME  405 

nation  wired  in  two  or  more  circuits  so  that  a  small  amount  of  illumi- 
nation can  be  used  when  full  intensity  is  not  needed. 

In  designing  the  general  illumination  for  a  large  oblong  room,  a 
more  difficult  problem  is  encountered,  and  it  is  here  that  the  coopera- 
tion of  the  illuminating  engineer  with  the  architect  brings  about  the 
best  results.  If  the  ceiling  is  plain,  that  is  to  say,  has  no  beams  and 
no  other  fancy  decorations,  two  ceiling  outlets  could  be  provided, 
one  in  the  center  of  each  half  of  the  room,  the  type  of  fixtures  to  be 
semi-indirect  or  direct,  as  desired.  Ceiling  units  should  be  so 
selected  and  installed  that  they  do  not  break  up  the  continuity  of 
the  ceiling.  If  the  furnishings  are  to  be  distinctive,  say,  for  instance, 
Colonial,  the  semi-indirect  type  of  fixture  would  be  entirely  out  of 
place,  and  use  should  be  made  of  special  fixtures  of  a  Colonial  char- 
acter. Fixtures  of  this  type  have  been  on  the  market  for  some  time 
and  are  attractive  by  day  and  efficient  by  night.  If  the  furnishings 
are  not  of  any  special  design  or  period,  then  the  semi-indirect  unit 
could  be  employed. 

In  case  of  a  very  low  or  beamed  ceiling,  the  fixtures  could  be 
omitted,  and  a  number  of  multiple  wall  brackets  supplied,  for  when 
properly  designed  they  are  very  ornamental  whether  lighted  or  not. 
A  room  thus  equipped  with  decorative  wall  brackets  and  table  or 
floor  lamps  is  very  attractive. 

In  a  room  having  a  fireplace  and  mantel,  there  should  be  an  outlet 
on  either  side,  their  location  on  or  beside  the  chimney  depending 
upon  the  type  of  fireplace.  If  the  brick  work  extends  to  the  ceiling 
and  is  not  too  elaborate,  a  bracket  over  each  end  of  the  shelf  would 
be  suitable.  If  ornamental  brick-laying  was  attempted,  the  brackets 
'  could  be  installed  on  the  wall  at  the  sides.  If  the  mantel  is  of  wood 
and  somewhat  delicate  in  design,  a  more  delicate  type  of  wall  bracket 
should  be  selected. 

DINING  ROOM 

Probably  next  in  importance  to  the  living  room  is  the  dining  room, 
which  in  some  small  residences  is  used  as  a  living  room.  Here,  after 
the  evening  meal,  gathers  the  family  around  the  table,  some  reading 
and  some  otherwise  engaged.  In  a  room  of  this  character,  one  would 
not  hesitate  to  recommend  the  art  glass  dome  provided  the  table  is 
the  center  of  attraction  during  mealtime  and  the  light  in  other  parts 
of  the  room  can  be  to  a  degree  subdued.  The  dome,  if  so  suspended 
as  not  to  obstruct  the  view  of  persons  looking  across  the  table,  makes 


406  ILLUMINATING   ENGINEERING  PRACTICE 

a  very  effective  and  practical  dining-room  unit  and  is  also  admirable 
for  reading. 

In  selecting  the  dome  one  should  be  careful  not  to  obtain  too 
gaudy  colors,  or  one  ornamented  with  a  fringe  hanging  from  the 
edge  the  effect  of  which  is  very  disturbing  by  casting  its  scraggling 
alternating  dark  and  light  lines  upon  the  faces  and  clothing  or  on 
books  and  papers. 

The  dome  could  be  equipped  with  two,  three  or  four  sockets  and 
round-bulb,  all-frosted  lamps  which  should  be  well  shaded  from  the 
eyes  of  people  sitting  in  a  normal  position  about  the  table. 

A  house  of  the  class  here  considered  would  not  be  provided  with 
a  superabundance  of  baseboard  receptacles,  and  hence  one  or  more 
of  the  sockets  in  the  dome  could  conveniently  be  used  for  any  one 
of  the  several  cooking  appliances.  The  dome  circuits  should  be 
controlled  by  a  wall  switch,  and  the  individual  lamps  by  pull  chain 
sockets. 

In  the  dining  room  of  the  higher  priced  home,  the  lighting  expert 
has  a  better  opportunity  to  exercise  his  art.  Here  the  semi-indirect 
bowl  will  usually  meet  with  satisfaction.  A  ly-in.  bowl,  containing 
three  or  four  sockets,  with  25-watt  or  40- watt  lamps,  gives  consider- 
able leeway,  the  size  depending  upon  the  color  of  the  walls  and 
^the  amount  of  illumination  desired.  'Economically,  it  would  be 
desirable  to  install  a  wall  switch  so  arranged  that  one  or  all  lamps 
could  be  turned  on  or  off. 

In  the  dining  room,  as  in  the  living  room,  period  furnishings  and 
color  schemes  should  be  given  thorough  consideration.  Semi-indirect 
bowls  are  made  in  various  gently  tinted  colors,  any  one  of  which 
would  tone  down  the  light  of  the  tungsten  lamp  to  the  soft,  warm 
tones  so  much  appreciated  in  the  general  lighting  of  a  dining  room. 
Bright  or  startling  colors  should  be  avoided,  except  on  rare  occasions. 
For  instance,  at  Christmas  time  one  might  wish  to  decorate  the  din- 
ing room  temporarily  in  red  and  green;  at  Hallowe'en  time  with 
black  and  yellow;  St.  Valentine's  Day  with  pink,  and  so  on.  With 
the  semi-indirect  bowl  or  ceiling  fixture  one  can  easily  produce  almost 
any  desired  color  scheme  in  lighting  by  the  employment  of  various 
colored  silks  or  papers. 

Another  attractive  style  of  fixture  is  one  with  three  or  four  lamps 
pointing  up,  and  equipped  with  colored  silk  shades,  cylindrical  in 
shape,  but  smaller  at  the  top  than  at  the  bottom,  the  colors  selected 
depending  on  personal  taste  and  the  surroundings.  This  style  of 
fixture  should  be  equipped  with  round-bulb  all-frosted  lamps. 


JORDAN:  LIGHTING  OF  THE  HOME  407 

Ordinarily  the  delicate  tints  of  rose,  cream,  yellow  or  amber,  will  be 
found  to  harmonize  with  the  other  decorations. 

With  these  last  two  types  of  fixtures  it  would  be  found  desirable 
to  install  a  floor  receptacle  at  about  i  ft.  to  the  right  of  and  i  ft.  out 
from  a  point  beneath  the  center  of  the  table,  the  idea  being  to  extend 
the  lamp  cord  over  the  edge  of  the  table  on  the  right  hand  of  the 
person  liable  to  do  the  serving,  and  to  dodge  the  central  pillar  of  the 
table  if  it  has  one.  In  addition  to  the  floor  outlet  it  might  be  found 
convenient  to  install  a  baseboard  outlet  near  the  serving  table,  in 
order  that  utensils  could  be  used  there  when  desired. 

If  the  room  contains  a  mantel  and  fireplace  their  charm  would  be 
enhanced  by  a  pair  of  candle  lamps.  Thus,  to  equip  one's  home 
harmoniously,  is  to  give  a  new  charm  and  a  new  intimacy,  for  the 
secret  of  the  attractive  home  lies  in  the  graceful  blending  of  lighting 
principles  with  the  accessories. 

It  is  with  some  hesitation  that  I  approach  the  problem  of  illumina- 
tion of  dining-rooms  of  the  palatial  type.  Here  it  is  that  we  again 
come  in  contact  directly  with  our  esteemed  friends  the  architect  and 
the  decorator.  I  am  pleased  to  say  that  the  meetings  with  these 
people  in  such  places  are  more  pleasant  now  than  in  times  gone  by. 

In  stately,  dignified  dining-rooms  large  chandeliers  may  be  used 
with  most  beautiful  effect;  but  they  should  be  supplemented  with 
multiple  wall  brackets  and  candelabra  lamps.  As  most  of  these 
large  rooms  have  period  furnishings,  we  more  and  more  realize  the 
value  of  greater  knowledge  in  period  styles. 

In  this  class  of  dining-room  as  in  many  others,  frequently  with  the 
exception  of  a  small  lamp  near  the  serving  table,  the  other  fixtures 
are  seldom  used,  and  tallow-candles  furnish  all  the  illumination 
desired.  This  effect  is  certainly  very  satisfying  to  the  esthetic 
taste. 

LIBRARY 

Few  of  the  smaller  residences  have  what  would  be  called  a  library, 
and  in  houses  of  the  next  grade  the  library  and  living-room  are  usually 
joined  in  one.  Occasionally  a  room  is  used  exclusively  as  a  library, 
and  by  bad  tradition  it  is  usually  in  rather  dark  finish.  Additional 
absorption  of  light  is  encountered  when  the  walls  are  lined  with 
bookcases  filled  with  books.  Here  sufficient  general  illumination 
should  be  provided  by  ceiling  fixtures  to  enable  the  titles  of  books 
to  be  clearly  read.  Wall  brackets  could  hardly  be  recommended  in 
a  room  of  this  character,  since  with  these  it  is  more  difficult  to  light 


408  ILLUMINATING   ENGINEERING   PRACTICE 

properly  the  bookcases  which  lie  nearly  in  the  same  plane  with  the 
brackets;  and  again,  in  locating  the  outlet  for  the  bracket  almost 
any  place  upon  the  wall  is  liable  to  interfere  with  the  bookcase  space. 
In  this  room  proper  reading  lamps  are  of  primary  importance,  and 
the  same  suggestions  for  reading  lamps  as  have  been  made  for  read- 
ing lamps  for  the  living-room  previously  described,  would  apply, 
not  forgetting  that  a  generous  supply  of  floor  or  baseboard  plugs  is 
especially  useful. 

MUSIC-ROOM 

The  average  home  does  not  have  a  room,  as  a  rule,  devoted  ex- 
clusively to  music,  but  occasionally  such  a  room  does  exist.  Bril- 
liant lighting  is  not  necessary  here  except  when  the  room  is  used  for 
other  purposes.  However,  near  the  piano,  which  instrument 
usually  gives  the  room  its  name,  considerable  localized  light  should 
be  provided.  This  is  best  accomplished  by  means  of  a  portable  floor 
lamp  equipped  similarly  to  the  reading  lamp  as  previously  described, 
supplied  with  energy  from  floor  or  base-board  plug.  As  certain 
occasions  may  require  the  presence  of  several  musicians  or  enter- 
tainers, a  soft  general  illumination  is  often  necessary.  In  all  cases 
the  lamps  should  be  well  shaded  from  the  eyes  of  the  guests. 
Wall  brackets  are  objectionable  from  the  fact  that  when  lighted, 
they  are  eternally  shining  into  people's  eyes,  much  to  their  distress 
and  discomfort. 

The  music-room  is  probably  the  only  one  in  a  residence  where 
cove  lighting  could  be  considered.  Before  decision  is  made,  how- 
ever, due  thought  must  be  given  not  only  to  the  client's  pocket- 
book,  but  also  to  the  possibilities  of  the  lamps  and  trough  being 
kept  clean — which  they  generally  are  not. 

In  case  use  is  rriade  of  the  semi-indirect  or  chandelier  source  for 
general  illumination,  it  must  be  carried  high  enough  so  as  not  to 
distract  the  attention,  or  in  any  way  interfere  with  the  field  of  view, 
and  the  lamps  must  be  thoroughly  shaded. 

DENS 

From  the  character  of  "den"  rooms,  it  would  seem  desirable  to 
provide  considerable  general  illumination,  due  to  the  fact  that  the 
walls  are  usually  decorated  to  the  ceiling  with  pennants,  crossed 
swords,  trophies,  Indian  relics,  skulls  and  other  articles  of  a  rather 


JORDAN:  LIGHTING  OF  THE  HOME  409 

gloomy  nature.  It  would  be  a  pity  for  a  guest  to  miss  seeing  any 
of  these  most  interesting  curios. 

The  finish  and  furnishings  of  these  rooms  being  usually  dark, 
more  energy  should  be  provided  than  in  ordinary  rooms  of  the  same 
size.  In  case  of  a  small  room,  a  fairly  deep  semi-indirect  bowl 
equipped  with  a  loo-watt  lamp,  or,  if  the  room  is  large,  a  shallow 
bowl  containing  three  40-watt  lamps,  would  make  a  well  lighted 
room. 

Should  localized  light  be  required  for  a  desk  or  table,  I  doubt  if 
the  type  of  lamp  that  would  likely  be  selected  to  harmonize  with 
surroundings  would  be  suitable  for  either  writing  or  reading,  its 
only  value  being  for  decorative  purposes,  and  it  would  be  serviceable 
only  when  a  little  light  is  desired.  The  lamp  cord  should  be  con- 
nected to  a  baseboard  receptacle  and  have  chain  pulls.  A  wall 
switch  near  the  door  should  control  the  ceiling  fixture. 

SUN  PARLORS  OR  CONSERVATORIES 

These  rooms  being  usually  filled  with  plants  and  flowers,  some  of 
which  grow  to  the  ceiling,  soft,  general  illumination  is  required,  and 
the  semi-indirect  bowl  will  produce  a  beautiful  effect.  In  addition 
to  the  general  illumination  a  table  lamp  will  prove  very  serviceable. 

KITCHENS  AND  PANTRIES 

These  are  the  working  portions  of  the  house  and  should  receive 
careful  consideration.  In  the  small  kitchen  a  60- watt  lamp  equipped 
with  a  shallow  prismatic  reflector,  located  well  up  in  the  center  of 
the  room,  will  give  the  light  required  in  all  parts.  In  the  small 
pantry  one  2$-watt  or  40- watt  lamp,  similarly  equipped  over  the 
working  table,  would  provide  ample  light. 

In  the  large  kitchens,  lamps  should  be  installed  at  one  or  more 
points,  as  the  arrangement  of  the  working  space  requires,  such  as 
the  stove,  the  sink,  and  very  likely  a  working  table.  In  case  of  a 
hooded  range,  one  or  two  lamps  should  be  placed  under  the  hood. 

A  ceiling  lamp  should  be  provided  for  general  illumination,  under 
control  of  a  wall  switch  conveniently  located;  in  some  cases  it  may 
be  found  convenient  to  control  this  lamp  also  from  the  second  floor. 

All  kitchens  should  be  equipped  with  receptacles  for  flatirons  and 
other  appliances.  Butlers'  pantries,  where  glasses  and  dishes  are 
washed  and  stored,  usually  require  two  ceiling  fixtures,  one  over  the 


410  ILLUMINATING    ENGINEERING   PRACTICE 

sink   and   another   to   illuminate   the   shelves.     Twenty-five   watt 
lamps,  with  enclosing  shades,  furnish  the  proper  equipment. 

HALLS 

In  most  of  the  new  residences  the  halls  are  almost  as  much  a 
living  part  of  the  house  as  are  some  of  the  other  rooms.  In  nine  out 
of  ten  of  the  houses  a  person  in  the  living-  or  dining-room,  can  see 
into  the  hall.  With  this  arrangement  the  lamps  should  be  carefully 
shaded  so  that  the  glare  may  not  be  offensive  to  persons  in  the  other 
rooms.  This  remark  applies  in  the  case  of  direct  lighting  by  lantern 
or  small  fixture. 

A  semi-indirect  unit  makes  a  pleasing  hall  light  source.  The  bowl 
selected  should  be  of  the  deep  type  and  not  very  large.  If  the  hall 
is  long,  and  the  back  part  is  left  in  comparative  darkness,  it  may  be 
necessary  to  supplement  the  principal  fixture  with  wall  brackets. 
In  case  of  an  exceptionally  low  ceiling,  wall  brackets  are  to  be  pre- 
ferred, the  lighting  being  balanced  by  means  of  one  or  two  on  each 
side  wall,  depending  upon  the  length  of  the  space.  The  fixture,  or 
one  of  the  wall  brackets,  should  be  controlled  by  a  switch  near  the 
front  door  and  also  from  the  second  floor. 

The  second  floor  halls  generally  do  not  require  as  much  light  as 
do  the  lower  halls.  If  the  ceilings  are  low,  say  8  or  9  feet,  a  hemi- 
sphere or  squat  ball  makes  a  good  unit.  If  there  is  only  one  lamp, 
it  should  be  placed  near  the  top  of  the  stairs.  If  the  hall  is  large, 
another  should  be  installed  in  the  other  part.  Both  of  these  should 
be  controlled  by  wall  switches  and  the  one  most  used  should 
be  controlled  also  from  below. 

Wall  brackets  if  used  at  all  in  halls  or  passageways  should  be 
installed  rather  high,  because  there  is  danger  of  persons  running 
into  them  in  the  dark. 

Back  halls  and  passageways  require  very  little  light.  The  loca- 
tion of  fixture  should  be  such  that  the  lamps  will  thoroughly  light 
the  stairways. 

In  the  halls  of  the  more  imposing  character,  fixtures  of  special 
design  will  be  necessary.  These  should  give  a  well  diffused  light 
and  fairly  soft  shadows.  Should  the  upper  part  of  the  room  be 
decorated  with  special  architectural  features,  due  consideration 
should  be  given  this  fact,  and  arrangements  should  be  made  to  give 
them  proper  significance.  Not  infrequently  there  are  paintings,  and 
these  also  may  require  special  treatment,  by  lamps  placed  to  illumi- 
nate them  directly. 


JORDAN:  LIGHTING  OF  THE  HOME  411 

BEDROOMS 

In  the  bedrooms  the  handiwork  of  the  feminine  sex  will  be  every- 
where in  evidence.  Probably  in  no  other  part  of  the  house  will  she 
be  as  much  concerned  as  here,  she  has  decided  upon  the  location  of 
the  dressing  table  and  planned  to  have  the  various  rooms  decorated 
in  appropriate  colors.  It  is  necessary  for  the  engineer  to  know  what 
these  colors  are  to  be  in  order  that  the  shades  selected  may  be  of  a 
tint  to  match.  I  mention  these  frivolous  matters  first,  not  because 
of  their  importance,  but  because  in  the  lighting  of  the  home  similar 
matters  are  first  brought  to  the  attention  of  the  illuminating  engineer 
by  the  lady  of  the  house,  and  if  he  fails  to  gain  her  favor  he  may  as 
well  stop  before  beginning. 

It  is  a  recognized  fact  that  the  bedroom  suffers  more  from  mis- 
placed fixtures  than  from  insufficient  light,  for  the  amount  of  light 
required  is  not  large  in  rooms  of  medium  finish. 

Now,  assuming  that  the  architect  has  provided  space  enough 
between  the  windows  for  a  dressing  table,  and  then  a  little  more,  one 
can  recommend  a  pair  of  swing-arm  wall  brackets,  one  on  either 
side  of  the  mirror,  as  the  most  satisfactory  lighting  equipment.  If 
the  cheval  mirror  or  a  mirrored  door  is  among  the  articles  of  furni- 
ture, it  could  be  equipped  in  a  similar  manner.  When  it  is  definitely 
known  where  the  other  articles  of  furniture  are  to  be  placed,  one  can 
easily  provide  lighting  for  the  remainder  of  the  room. 

Wall  brackets,  properly  shaded,  are  to  be  preferred  for  bedroom 
lighting.  For  those  who  follow  the  practice  of  reading  in  bed,  an 
additional  small  reading  lamp  can  be  installed  for  use  beside  the  bed. 
This  lamp  should  be  controlled  by  a  pull  chain  or  pendant  switch, 
easily  reached  from  the  bed. 

There  is  no  serious  objection  to  the  use  of  a  ceiling  lamp  in  a 
bedroom,  but  it  should  be  resorted  to  only  in  cases  where  extreme 
economy  must  be  observed. 

At  least  one  lamp  in  the  room  should  be  controlled  from  a  switch 
beside  the  door. 

BATHROOMS 

There  is  little  to  be  said  about  bathrooms.  The  best  method  is  to 
install  two  wall  brackets  about  on  a  level  with  a  person's  face,  one  on 
each  side  of  the  mirror.  These  will  not  only  give  ample  general 
illumination  in  the  largest  of  bathrooms,  but  are  most  suitable  for 
shaving. 


412  ILLUMINATING   ENGINEERING    PRACTICE 

In  very  small  rooms  one  center  ceiling  lamp  does  very  well  when 
placed  high.  A  switch  should  be  placed  near  the  door. 

In  this  room  should  be  placed  a  receptacle  for  a  water  heater. 

CELLARS  AND  LAUNDRYS 

Cellars  usually  require  only  a  very  moderate  amount  of  lighting. 
Use  could  be  made  of  one  25-watt  tungsten  lamp  near  the  foot  of  the 
stairs  and  another  in  front  of  the  heater.  If  the  cellar  is  sub-divided, 
other  lamps  placed  in  the  dark  portions  will  be  helpful.  The  lamp 
near  the  foot  of  the  stairs  and  the  other  one  near  the  heater  should 
be  controlled  by  a  switch  at  the  top  of  the  stairs. 

There  is  seldom  much  work  to  be  performed  in  the  laundry  after 
dark,  but  in  city  houses,  the  laundries  are  often  so  located  that  no 
daylight  enters  at  all.  Here  there  should  be  one  lamp  over  the 
tubs,  one  over  the  ironing  table,  and  one  for  general  illumination. 
The  lamp  over  the  table  and  tubs  should  be  equipped  with  steel 
porcelain  enamel  reflectors,  and  controlled  by  key  sockets.  The 
center  lamp  should  be  controlled  by  a  switch  located  near  the  door. 
Receptacles  should  be  installed  in  the  laundry  for  washing  jnachines 
and  flat  irons. 

PORCHES 

The  porch  does  not  require  any  high  intensity  of  illumination, 
for  we  would  refrain  from  making  it  look  like  a  store  front.  The 
purpose  for  which  a  porch  lamp  is  used  is  welcoming  the  arriving 
and  speeding  and  the  departing  guests,  illuminating  the  steps  and 
enabling  the  people  in  the  house  to  scrutinize  a  caller. 

If  the  front  porch  lamp  has  any  value  for  the  last-mentioned 
reason,  then  why  not  a  back  porch  lamp?  Its  advantage  would  at 
once  be  appreciated  by  the  average  woman  in  the  home,  when  the 
back  door  bell  rings,  perhaps  late  in  the  evening,  or  when  she  is 
alone  in  the  house.  A  25-watt  lamp  in  an  enclosing  ball  near  the 
ceiling  of  the  porch  and  controlled  by  a  switch  inside  the  door,  is 
all  that  is  required. 

GENERAL  REMARKS 

I  am  not  in  favor  of  elaborate  massive  fixtures  anywhere  in  the 
ordinary  house.  The  more  simple  and  unobtrusive  ones  are  much 
to  be  preferred.  In  selecting  fixtures  for  the  whole  house,  the  nearer 
they  match  each  other  in  design  the  better.  The  metal  parts  should 


JORDAN:  LIGHTING  OF  THE  HOME  413 

particularly  be  of  the  same  design  and  finish,  although  the  glassware 
or  shades  can  be  different  in  shape  and  color. 

In  other  words,  in  the  smaller  residences  all  the  fixtures  should 
be  as  nearly  the  same  style  as  possible,  or  the  house  will  look  like 
a  second-hand  fixture  establishment.  In  large  houses,  the  type  of 
decoration  is  the  controlling  element. 

In  conclusion,  I  want  to  call  attention  to  the  fact  that  central 
station  managers  are  beginning  to  realize  the  importance  of  in- 
creasing the  residence  load.  The  great  variety  of  house  wiring 
campaigns  now  under  way  certainly  emphasize  this  fact  and  great 
effort  is  being  made  to  equip  old  houses  for  electric  service. 

In  the  territory  of  the  Edison  Electric  Illuminating  Company  of 
Boston  alone,  there  were  many  thousands  of  unwired  houses  before 
the  house  wiring  campaign  was  started,  and  at  the  end  of  three 
years  about  4000  of  these  houses  had  been  wired  on  the  "Easy 
•Payment  Plan"  with  an  approximate  annual  income  of  $100,000 — 
an  average  of  $25  per  year,  per  customer.  This  means  an  addition 
to  the  lighting  load  of  about  4000  kw.  which  load,  coming  as  it  does 
on  a  late  peak,  is  very  desirable,  a  fact  that  is  being  appreciated 
more  and  more. 


THE  LIGHTING  OF  STREETS— PART  I 

BY   PRESTON   S.  MILLAR 

This  lecture  is  to  be  considered  as  complementary  to  that  of 
Mr.  Lacombe  which  is  to  follow.  The  two  are  intended  to  em- 
brace the  entire  subject  of  street  lighting.  The  division  between 
the  two  is  arbitrary  and  of  course  cannot  be  absolute. 

HISTORICAL 

.  Within  the  limitations  of  time  imposed  it  is  not  desirable  to  dwell 
upon  the  historical  aspects  of  the  subject.  The  evolution  of  street 
lighting  from  the  torch  bearing  stage  through  the  period  of  candle 
and  oil  lamps,  maintained  individually  by  citizens,*  up  to  the  begin- 
ning of  community  or  municipal  maintenance,  has  little  practical 
bearing. 

The  modern  history  of  street  lighting  begins  with  the  invention  of 
illuminating  gas  and  assumes  its  second  interesting  phase  with  the 
development  of  the  direct-current  series  carbon  arc  lamp.  Follow- 
ing these  two  stages  of  progress,  the  historical  aspects  of  street 
lighting  naturally  ramify  into  the  history  of  street  illuminants,  of 
street  lamp  mountings,  of  street  lamp  equipments  and  accessories 
and  of  the  development  of  ideas  of  street  lighting  principles.  These 
several  phases  of  the  subject  are  treated  under  their  respective  head- 
ings in  either  this  lecture  or  in  Mr.  Lacombe's  lecture. 

PURPOSES 

The  earliest  street  lighting  was  adopted  as  a  measure  of  protection 
for  the  wayfarer  against  the  criminally  inclined.  At  first  this  pur- 
pose was  served  partially  by  the  carrying  of  torches. 

*  Some  milestones  in  the  historical  progress  of  street  lighting  are  as  follows: 

1558 — Paris  the  first  modern  city  to  attempt  civic  street  lighting.     Inhabitants  ordered 

to  hang  lighted  candle  lanterns  in  front  of  houses. 
1766 — Pitch  or  resin  bowls  substituted  for  candle  lanterns. 
1809 — First  street  lighting  by  illuminating  gas  in  London. 
1821 — Illuminating  gas  used  in  Baltimore  for  street  lighting. 
1884 — Arc  lamps  used  in  Philadelphia  for  street  lighting. 
1896 — Gas  mantle  lamps  first  used  in  this  country  for  street  lighting. 

415 


41 6  ILLUMINATING   ENGINEERING   PRACTICE 

Then  the  lamps  were  placed  permanently  along  the  way,  affording 
better  protection  and  also  marking  the  route.  From  this  stage  street 
lighting  was  evolved  into  a  means  of  safeguarding  traffic  against 
collision  and  of  revealing  obstructions  or  holes  in  the  roadway. 
More  recently  it  has  been  developed  into  an  ornate  system  which  not 
only  accomplishes  these  purposes  but  also  promotes  commerce  and 
embellishes  the  highway  both  by  its  own  artistic  qualities  and  by  the 
lighting  effects  which  it  produces. 

Modern  street  lighting  serves  the  combined  purposes  which  have 
been  named,  these  being  fundamental  perhaps  in  the  order  in  which 
they  are  mentioned.  The  protective  element,  however,  is  sometimes 
taken  for  granted,  and  most,  if  not  all,  attention  is  directed  to  the 
last  purposes  named.  All  street  lighting  ought  to  serve  these  several 
purposes  and  the  importance  of  each  purpose  in  a  particular  installa- 
tion must  depend  upon  the  local  conditions,  principal  factors  among 
which  are  traffic  density,  real  estate  development  and  criminal 
hazard.  In  portions  of  cities  in  which  evil  resorts  exist,  the  police 
protection  factor  is  of  first  importance.  In  interurban  highways 
the  marking  of  the  way  and  provision  against  collision  are  essential. 
The  dignified,  high-class  avenue  must  be  so  lighted  as  to  reveal  its 
character  by  exhibiting  to  advantage  the  architectural  features  of 
the  buildings.  For  police  purposes  the  lighting  should  be  designed 
primarily  to  reveal  large  objects  on  the  street  and  sidewalk. 

To  serve  the  purposes  of  the  automobilist  the  same  requirement 
exists,  and  in  addition  raised  places  or  depressions  in  the  roadway 
should  be  revealed,  and  the  curb  or  limitations  of  the  driveway  should 
be  perceptible.  His  is  the  most  difficult  requirement  to  meet  be- 
cause of  the  high  rate  of  speed  at  which  he  travels.  Safety  requires 
that  he  must  be  able  to  detect  the  presence  of  objects  in  a  single  glance. 

The  pedestrian  requires  somewhat  the  same  lighting  design,  though 
he  is  especially  concerned  in  having  inequalities  in  the  sidewalk 
revealed.  In  the  important  streets  he  should  be  able  to  distinguish 
faces  of  passers-by. 

From  the  point  of  view  of  aesthetics  the  fundamentals  of  design  in 
all  visible  portions  of  the  lighting  system  should  be  observed.  This 
applies  especially  to  posts,  fixtures  and  glassware.  The  incongruous 
should  be  absent;  nor  should  the  general  effect  of  the  street  be 
neglected.  The  fixtures  should  be  of  pleasing  design  in  themselves 
and  suitable  to  their  surroundings.  They  should  be  installed  at  such 
intervals  and  in  such  locations  as  to  enhance  the  attractiveness  of 
the  street  by  day  and  by  night. 


MILLAR:  LIGHTING  OF  STREETS  417 

Minor  requirements  of  street  lighting  present  themselves.  For 
example,  one  may  wish  to  observe  the  time  indication  of  his  watch, 
or  to  read  an  address  on  a  card,  or  to  find  an  article  which  he  has 
dropped,  or  to  read  a  number  on  a  house  front.  All  such  require- 
ments are  distinctly  minor  and  should  not  have  an  important  place 
in  a  discussion  of  this  kind.  Many  of  them  can  be  met  by  moving 
near  to  a  street  lamp;  others  occur  so  infrequently  as  not  to  demand 
much  consideration.  In  general,  lighting  which  serves  the  major 
purposes  outlined  above  serves  also  these  minor  purposes  as  well  as 
they  can  be  served  with  a  given  expenditure  for  lighting. 

All  of  the  foregoing  may  be  summed  up  in  a  statement  of  the 
purposes  to  be  served  by  street  illumination  put  forward  by  the 
Street  Lighting  Committee  of  the  National  Electric  Light  Associa- 
tion in  1914  as  follows:1 

1.  Discernment  of  large  objects  in  the  street  and  upon  the  sidewalk. 

2.  Discernment  of  surface  irregularities  on  the  street  and  on  the  sidewalk. 

3.  Good  general  appearance  of  the  lighted  street. 

In  studying  the  street  lighting  requirements  for  a  particular 
locality,  much  time  and  thought  profitably  may  be  devoted  to  defin- 
ing for  that  locality  the  principal  purposes  of  the  street  lighting. 
With  these  purposes  clearly  in  mind,  best  efforts  are  more  likely  to 
be  made  to  design  the  installation  in  conformity  with  the  require- 
ments. Most  serious  mistakes  in  the  design  of  street  lighting  have 
occurred  because  those  responsible  have  failed  properly  to  analyze 
the  purposes  to  be  served  by  the  lighting  and  have  proceeded  in 
design  without  sufficient  thought  or  according  to  preconceived  ideas 
of  the  requirements  of  good  street  lighting.  It  is  almost  as  unreason- 
able to  design  street  lighting  without  reference  to  the  local  require- 
ments as  it  is  to  design  interior  illumination  without  reference  to 
the  character  and  decorations  of  the  room. 

EXTENT  AND  SCOPE 

Street  lighting  is  regarded  as  one  of  the  indispensable  functions 
of  municipal  government.  As  a  rule  in  this  country  it  is  rendered 
by  a  private  corporation  under  contract  with  the  municipality. 
Cities,  towns,  and  even  the  smallest  villages  are  lighted  very  gen- 
erally. It  has  been  stated  that  the  lighted  streets  of  New  York 
City  alone,  if  placed  end  to  end,  would  form  a  single  lighted  highway 
extending  from  New  York  to  Reno,  Nevada. 
27 


418  ILLUMINATING   ENGINEERING  PRACTICE 

It  is  the  usual  practice  to  adjust  the  intensity  of  street  lighting 
to  the  real  estate  and  traffic  conditions  of  streets,  lighting  the  im- 
portant streets  more  brilliantly  than  the  secondary  streets.  Ap- 
propriations for  street  lighting  purposes  are  often  somewhat  inade- 
quate with  the  result  that  all  classes  of  streets  are  not  as  well  lighted 
as  the  engineers  in  charge  would  like  to  have  them. 

In  recent  years  the  great  growth  of  automobile  traffic  has  com- 
plicated the  street  lighting  situation  in  a  number  of  ways — for  one 
thing  the  use  on  the  automobile  of  too  brilliant  lamps  at  night  has 
introduced  a  serious  problem.  Where  street  lighting  is  so  inadequate 
as  to  make  their  use  important,  automobile  lamps  for  lighting  the 
roadway  safeguard  the  automobilist  but  introduce  a  large  measure 
of  annoyance  and  some  danger  for  other  travelers.  Where  street 
lighting  is  reasonably  adequate  within  municipal  limits,  it  is  en- 
tirely feasible  to  abolish  the  use  of  head-lamps,  as  is  demonstrated 
by  the  experience  of  some  years  in  New  York  City.  If  instead  of 
seeking  to  eliminate  the  difficulty  solely  by  limiting  the  light  from 
head-lamps  municipal  authorities  would  see  to  it  that  the  street 
lighting  is  adequate,  the  whole  problem  could  be  dismissed  by  pro- 
hibiting the  use  of  head-lamps  within  the  city  limits. 

The  advances  of  the  last  ten  years  in  efficiency  of  light  production 
have  led  to  considerable  improvement  in  street  lighting.  Not  all 
of  these  advances  have  been  put  into  increased  lighting;  part  have 
been  realized  by  municipalities  in  reduced  lighting  costs.  Thus 
the  great  opportunities  offered  by  the  new  illuminants  for  better 
street  lighting  have  not  been  grasped  in  their  entirety,  though  very 
large  improvement  has  resulted. 

In  the  last  two  or  three  years  marked  impetus  has  been  given  to 
the  lighting  of  village  roads  and  of  rural  highways.2  Counties, 
towns  and  villages  are  awaking  to  the  value  of  such  highway  light- 
ing. Street  lighting  growth  js  reactive.  If  a  street  is  lighted  be- 
cause there  is  traffic  requirement  for  lighting,  the  result  is  increased 
traffic,  which  in  turn  brings  the  demand  for  better  street  lighting. 

The  tendency  of  municipalities  to  appropriate  inadequate  sums 
for  street  lighting  has  offered  an  opportunity  which  the  American 
business  man  has  not  been  slow  to  grasp,  with  the  result  that  there 
has  entered  into  American  practice  the  merchants '  display  lighting 
system,  the  so-called  " white  way"  lighting,  whereby  a  given  street 
through  the  activity  of  a  merchants'  association  is  much  more 
brilliantly  lighted  than  adjacent  streets.  This  is  considered  good 
advertising  in  that  it  attracts  people  to  the  locality.  Installations 


MILLAR:  LIGHTING  OF  STREETS  419 

of  this  sort  are  generally  of  distinctive  design;  many  of  the  earlier 
ones  were  of  the  incandescent  lamp  cluster  type;3  some  consisted  of 
arches  across  the  street;  others  of  festoons  of  lamps  mounted  over  the 
curbs.  The  more  recent  installations  have  consisted  of  a  post  and  a 
single  ornate  lighting  unit  enclosing  either  an  arc4  or  an  incandescent 
lamp. 

When  well  organized  and  operated  under  the  strict  supervision 
of  a  strong  merchants '  association  or  of  the  municipality,  this  spo- 
radic street  lighting  has  been  successful  in  setting  a  new  and  higher 
intensity  for  street  lighting,  tending  to  advance  the  standard  every- 
where. When  not  controlled,  it  has  sometimes  led  to  isolated  and 
ill-considered  instances  of  merchants '  lighting  which  has  resulted  in 
a  hodge-podge,  often  three  or  four  different  lighting  units  being 
installed  on  a  single  block,  some  lighted  at  night  and  others 
unlighted. 

In  a  few  instances  municipalities  have  installed  special  high 
intensity  street  lighting  systems  at  a  greater  expense  than  would 
ordinarily  have  been  incurred  for  the  street  selected,  the  city  having 
in  mind  something  of  the  same  considerations  as  have  actuated  mer- 
chants '  associations,  namely  the  advertising  of  the  locality  in  order 
-to  bring  traffic  to  it. 

Prevailing  intensities  in  street  lighting  practice  in  a  measure  are 
dependent  upon  the  extent  and  intensity  of  private  lighting  with 
which  they  are  brought  into  frequent  comparison.  In  a  city  where 
the  private  lighting  is  highly  developed,  the  general  level  of  street 
lighting  intensities  is  likely  to  be  higher  than  in  other  cities.  In 
passing  it  may  be  noted  that  the  converse  is  also  true,  and  that  the 
introduction  of  higher  intensities  in  street  lighting  tends  to  increase 
the  intensities  of  private  lighting  with  which  it  is  contrasted. 

The  general  practice  is  to  light  street  lamps  shortly  before  dark 
and  to  allow  them  to  continue  in  service  until  after  daybreak.  The 
4000-hour  year  in  this  latitude  is  standard.  No  self-respecting 
city  continues  the  archaic  moonlight  schedule  whereby  street  lamps 
are  not  lighted  when  the  almanac  states  that  the  moon  should  be 
shining.  A  more  reasonable  modification  of  lighting  practice, 
applicable  to  merchants '  lighting  rather  than  to  civic  lighting,  is  the 
reduction  of  the  amount  of  street  lighting  after  the  midnight  hour, 
as  for  example,  the  extinguishing  of  a  certain  percentage  of  the 
lamps. 

Like  many  other  municipal  enterprises,  street  lighting  serves 
the  entire  populace  and  is  a  matter  for  community  interest.  More 


420  ILLUMINATING   ENGINEERING   PRACTICE 

than  most  such  municipal  activities,  its  status  is  apparent  to  the 
citizen  and  to  the  visitor.  A  city  is  likely  to  be  judged  by  its  muni- 
cipal undertakings,  and  the  most  casual  observer  takes  cognizance 
of  the  condition  of  the  street  lighting. 

1  'A  city  is  judged  by  impressions.  It  may  have  the  finest  climate 
in  the  world;  it"  may  be  fortunately  situated  near  rivers  and  rail- 
ways; it  may  have  every  natural  advantage  that  a  business  man  may 
desire.  Yet  if  it  be  unattractive,  dirty  and  gloomy,  its  development 
will  be  slow.  When  it  does  develop,  the  first  impetus  will  be  given 
by  changing  its  appearance  for  the  better;  and  in  that  change  street 
lighting  will  play  an  important  part."5 

Something  of  this  view  is  manifesting  itself  in  many  cities  of  the 
country,  and  the  newer  installations  are  reflecting  in  their  enhanced 
attractiveness  and  effectiveness  the  municipal  pride  which  underlies 
design. 

The  cost  of  civic  street  lighting  per  capita  generally  ranges 
from  60  cents  to  $1.20.  It  amounts  to  perhaps,  from  2  to  3 
per  cent,  of  the  total  municipal  expenditure.  Its  value  must  be 
taken  to  include  not  alone  the  safety  features  which  are  its  primary 
purpose,  but  as  well  a  part  of  the  city  growth  and  the  promotion 
of  the  industry  and  the  welfare  of  its  citizens.  It  may  be  claimed 
also  that  the  large  expenditures  for  highway  construction  are 
rendered  of  greater  utility  by  street  lighting,  and  that  to  a  degree 
the  street  lighting  must  be  credited  with  the  promotion  of  the  enor- 
mous traffic  which  these  highways  bear.  To  quote  a  recent 
expression: 

"It  is  earnestly  believed  that  when  the  economic  value  of  a  system  of 
street  lighting  is  compared  with  other  public  works,  such  as  schools,  bridges, 
police  force,  fire  department,  etc.,  the  price  required  to  be  paid  for  the 
protection  of  the  life  and  limb  of  the  entire  citizenry  of  the  inhabitants; 
the  protection  of  our  wives  and  daughters  from  crime  and  annoyance;  the 
protection  of  our  property  from  burglary  by  supplementing  the  police 
force  by  adequately  lighted  streets;  the  various  conveniences  to  the 
public,  secured  by  sufficient  illumination  on  the  streets,  and  the  adver- 
tising and  aesthetic  values  of  adequate  street  lighting  to  the  city  at  large, 
and  to  the  individuals — the  cost  of  an  adequate  street-lighting  system 
will  be  found  to  be  insignificant  compared  to  the  value  received,  and  the 
expenditure  well  worth  while."6 

ILLUMINANTS 

Recent  History. — The  earliest  development  of  importance  to  this 
discussion  was  that  of  the  open  carbon  arc  lamp  and  series  direct 


MILLAR:  LIGHTING  OF  STREETS  421 

• 

current  lighting  systems.  The  early  records  of  street  lighting  in 
this  country  show  a  number  of  competing  companies  manufacturing 
equipment  for  such  systems.  They  became  recognized  standard 
street  lighting  systems  and  continued  as  such  until  the  development 
of  the  enclosed  carbon  arc  lamp  in  i894.7  The  open  carbon  arc 
lamp  in  this  country,  generally  speaking,  did  not  attain  to  the 
development  which  it  received  abroad.  It  is  understood  that  the 
carbon  electrodes  produced  in  America  were  inferior  to  those  later 
employed  in  Europe.  The  superiority  of  the  electrodes  available 
and  the  lower  prevailing  cost  of  labor  in  Europe  led  to  the  continua- 
tion of  the  open  carbon  arc  lighting  system  for  years  after  its  decline 
began  in  this  country. 

The  enclosed  carbon  arc  lamp  by  reason  of  its  relatively  long 
electrode  life  and  low  maintenance  cost  received  in  this  country  a 
measure  of  development  which  it  could  not  experience  abroad.  It 
possessed  other  advantages  over  the  open  carbon  arc  lamp  which 
aided  its  success,  including  a  better  light  distribution  characteristic 
and  a  greater  steadiness  of  light.  In  the  course  of  the  decade  suc- 
ceeding its  invention,  the  enclosed  carbon  arc  lamp  became  the 
standard  street  lighting  lamp  in  this  country. 

As  subsidiary  to  the  arc  lamp,  small  illuminants  were  used  in  the 
lighting  of  streets  of  minor  importance.  These  were  the  gas  mantle 
lamp,  the  gasoline  lamp  and  the  series  carbon  incandescent  lamp. 

The  period  of  modern  street  lighting  illuminants  was  ushered  in 
by  the  development  of  the  metallic  electrode  lamp  in  1904.*  This 
lamp,  known  as  the  magnetite  lamp,  and  the  metallic  flame  arc  lamp 
and  also  as  the  luminous  arc  lamp,  attained  great  eminence  in  street 
lighting  practice  in  this  country  during  the  decade  following  its 
invention. 

Within  this  same  decade  the  flaming  arc  lamp  was  the  subject 
of  much  experimental  and  development  work  on  the  part  both  of 
arc  lamp  manufacturers  and  of  electrode  makers.  The  same  diffi- 
culty of  high  maintenance  costs  in  this  country  placed  it  beyond 
practicability  to  utilize  the  short-life  flaming  arc  lamps  of  European 
development.  As  in  the  case  of  the  carbon  arc  lamp  the  difficulty 
was  reduced  by  enclosing  the  arc  and  securing  a  much  longer  elec- 
trode life.  For  a  time  the  enclosed  flame  arc  lamp  with  long  life 
electrodes  bade  fair  to  become  a  real  factor  in  the  street  lighting 
situation  in  this  country.  Several  important  extensive  installations 
were  made  with  more  or  less  satisfactory  results.  The  development 
of  the  gas-filled  tungsten  lamp  known  as  the  "  Mazda  C"  lamp,  in 


422  ILLUMINATING   ENGINEERING   PRACTICE 

IQI4,9  however,  introduced  competition  which  the  flame  arc  lamps 
of  present  types  have  not  been  able  to  meet  except  in  special  cases. 

The  Mazda  C  lamp  superseded  a  number  of  illuminants  in  the 
street  lighting  field.  It  hastened  the  displacement  of  open  and  en- 
closed carbon  arc  lamps  which  were  still  in  the  field.  Its  availability 
in  small  sizes  resulted  in  its  substitution  very  generally  for  the  gas 
and  gasoline  mantle  lamps  which  had  very  largely  claimed  the 
secondary  streets  for  their  own,  and  of  course  it  displaced  the  ineffi- 
cient carbon  series  lamps  wherever  they  were  in  service. 

The  development  of  street  lighting  practice  is  so  largely  involved 
with  the  development  of  street  illuminants  that  this  brief  account 
of  the  development  of  the  latter  serves  to  recall  the  history  of  street 
lighting  as  a  whole.  The  development  of  other  phases  of  street 
lighting  practice  has  perhaps  lacked  the  definite  steps  of  advance 
which  are  apparent  in  the  record  of  the  illuminants  employed,  but 
the  progress  has  been  none  the  less  real  on  that  account. 

Modern  Lamps. — As  electric  and  gas  illuminants  are  the  subjects 
of  other  lectures,  their  qualities  need  not  be  discussed  in  detail  in 
this  connection.  Street  lighting  lamps  in  this  country  are  as 
follows: 

Gas  filled  tungsten  lamps  (Mazda  C). 
Arc  lamps  (principally  magnetite  lamps). 
Low  pressure  gas  and  gasoline  mantle  lamps. 

The  incandescent  lamps  are  usually  of  the  series  type,  though  in 
some  cities,  notably  New  York  City,  multiple  lamps  are  employed. 
American  selection^  of  illuminants  differs  somewhat  from  that  in 
European  countries  due  to  the  higher  labor  costs  and  greater  dis- 
tances prevailing  here.  Thus  it  may  prove  economical  for  us  to 
sacrifice  something  of  efficiency  in  order  to  secure  lower  maintenance 
cost,  while  in  Europe  the  maintenance  cost  is  not  so  large  a  factor 
in  the  total.  This  in  part  accounts  for  the  fact  that  flaming  arc 
lamps  and  pressed  gas  lamps  have  not  come  into  use  largely  in  this 
country  as  abroad.  On  the  contrary,  the  magnetite  lamp  has  been 
used  largely  here  and  hardly  at  all  abroad. 

Through  the  courtesy  of  the  Lamp  Committee  of  the  Association 
of  Edison  Illuminating  Companies,  the  accompanying  table  of  data 
on  electric  street  lamps  is  available. 

In  this  connection  Fig.  i  will  be  of  interest.  This  shows  the 
lumens  produced,  the  watts,  and  by  reference  to  the  diagonal  lines, 
the  efficiency  of  the  principal  electric  street  lamps.  The  lumens 
per  watt  output  of  street  lamps  is  by  no  means  a  final  measure  of 


MILLAR:  LIGHTING  OF  STREETS 


423 


TABLE  I. — RELATIVE  LIGHT  PRODUCING  EFFICIENCIES  OF  MAZDA  C,  FLAMING 
ARC  AND  MAGNETITE  LAMPS 

Mazda  C  lamps 


Description 

Bare  lamp 

Equipped  for  service  as- 
suming 25  per  cent, 
absorption  in  accessory 

Total 
lumens 

Average 
watts 

Lumens 
per  watt 

Total 
lumens 

Average 
watts 

Lumens 
per  watt 

6  6-amp     ''  6o-cp." 

600 
1,000 
2,500 
4,000 
6,000 
6,000 

10,000 

46.9 
71.9 
155-0 
245-0 
367-0 
310.0 
518.0 

12.8 

13-97 
16.12 
16.32 
16.32 
19.3 
19.3 

450 
750 
1,875 
3,000 
4,500 
4.500 
7,500 

46.9 
71-9 
155-0 
245-0 
367.0 
310.0 
518.0 

9-6 
10.4 
12.  5 

12  .2 
12.3 
14-5 

14  5 

10.5 
II.  5 
12.7 
13-5 

6.6-amp.,  "roo-cp." 

6  6-amp.    "2SO-cp."   . 

6.6-amp.,  "40O-cp."   

6.6-amp.,  ''6oo-cp."  

2O-amp.,  "6oo-cp."  (compensator)., 
ao-amp.,  "  xooo-cp."  (compensator). 

no-volt,  "200-watts"  
no-volt   "40O-watts" 

2,795 
6,130 
12,740 
17.960 

200.0 

400.0 
750.0 
1,000.0 

13.97 

15  33 
16.99 
17.96 

2,098 
4,600 
9,550 
13.480 

200.0 
400.0 
750.0 
1,000.0 

no-volt   "75O-watts" 

no-volt   "looo-watts" 

Magnetite  lamps 


Description 

Bare  lamp 

Equipped  for  service 

Am- 
peres 

Electrode 

Globe 

Total 
lumens 

Average 
watts 

Lumens 
per  watt 

Total 
lumens 

Average 
watts 

Lumens 
per  watt 

4 
4 
5 
5 
6.6 

Standard 

Clear 
Clear 
Clear 

2.991 
4.649 
5.768 
7,655 
8.708 

310 
323 
390 
371 
SI  I 

9-65 
14.4 
14.8 

20.6 
17.0 

High  efficiency.  .  .  . 
Standard 

High  efficiency.  .  .  . 
Standard  .  .  . 

Clear 
Clear 

7.5     I  (Yellow) 


A.C.  enclosed  flame  arc  lamps 
Clear 


8.557 


480        17.8 


the  relative  economy  of  the  several  types,  but  it  is  one  important 
factor  entering  into  the  problem. 

Fig.  2.  shows  change  in  illuminating  power  and  output  or  effi- 
ciency with  change  in  electrical  values  for  the  several  types  of 
modern  street  lamps. 

All  lamps  used  for  street  lighting  are  variable  as  to  candle-power. 
There  are  inherent  initial  variations.  Other  variations  are  due 
to  operating  irregularities,  including  those  of  supply  and  of  main- 
tenance. Arc  and  gas  lamps  are  subject  to  additional  variations 
involved  in  adjustment,  and  irregularities  in  the  light-giving  ele- 
ments. Most  of  these  variations,  however,  do  not  detract  materially 


424 


ILLUMINATING   ENGINEERING   PRACTICE 


from  the  utility  of  the  lamps  in  service.  It  is  important  to  note 
their  existence  to  avoid  misapprehensions  affecting  engineering 
practice  or  specifications  rather  than  to  include  them  as  factors 
affecting  the  value  of  the  lighting  service  to  the  public. 


10000 


9000 


400 


500 


600 


Watts 
Fig.   i. — Typical  initial  values  of  electric  illuminants. 

ACCESSORIES 

Since  lighting  accessories  are  covered  thoroughly  in  another 
lecture,  it  is  the  purpose  here  to  touch  upon  them  but  briefly. 
In  street  lighting  the  lamp  accessory  serves  to  protect  the  lamp, 
to  redirect  the  light,  to  reduce  the  brightness,  and  to  improve 


MILLAR:  LIGHTING  OF  STREETS 


425 


the  appearance  of  the  unit.  Sometimes  all  of  these  purposes  are 
accomplished;  again,  only  one  or  two  of  them  may  be  served.  While 
many  varieties  of  accessories  are  available  on  the  market,  yet  it  is 
fair  to  say  that  the  development  along  this  line  is  far  from  complete. 
The  characteristics  which  are  regarded  as  desirable  in  all  lamp 
accessories  intended  to  serve  these  purposes  include:  Simplicity, 
sturdiness,  cleanliness,  low  cost,  and  low  light  absorption.  Ap- 
parently the  advantages  of  securing  the  best  balance  among  these 
desirable  qualities  is  not  generally  appreciated,  nor  are  accessories 
always  selected  upon  the  basis  of  data  concerning  their  various 
qualities. 

In   the   selection   of   accessories,   after   the   generally   desirable 
qualities  have  been  sought,  the  question  of  a  desirable  light-direct- 


SSSSS|SSf|22:S     SSJSSSSS3SS2SS22     g  gS§S|SS|S2  22 
Percent  Volts  or  Amperes        Percent  Volts  or  Amperes       Percent  Volts  or  Amperes 
Fig.  2. — Variation  of  input  and  output  with  change  in  voltage. 

ing  characteristic  remains.  This  should  not  be  considered  to  the 
exclusion  of  other  qualities;  but,  other  things  being  equal,  a  generally 
desirable  form  for  various  classes  of  streets  may  be  indicated.  Thus 
in  the  lighting  of  residence  streets  and  of  highways,  little  or  no 
light  is  desired  in  the  upper  hemisphere,  and  a  form  of  distribution 
curve  similar  to  that  provided  by  the  pendant  magnetite  lamp  or 
by  the  tungsten-filament  or  Mazda  lamp  with  reflector  or  refractor 
may  be  acceptable.  On  the  other  hand,  in  the  illumination  of  prin- 
cipal avenues  and  business  streets  of  a  city,  some  light  above  the 
horizontal  is  essential  to  the  good  appearance  of  the  street.  Here 
some  form  of  diffusing  globe  becomes  desirable. 

The  absorption  of  light  in  fixtures  and  accessories  usually  is  an 
important  factor  in  reducing  efficiency.  Much  may  be  accomplished 
through  skilful  design  in  minimizing  this  loss.  Containers  which 


426 


ILLUMINATING   ENGINEERING   PRACTICE 


completely  enclose  the  light  source  are  of  course  likely  to  occasion  a 
greater  absorption  loss  than  are  reflectors  which  receive  only  a 
portion  of  the  total  flux  without  subjecting  the  remainder  of  the 
flux  to  absorption.  In  considering  loss  due  to  absorption  it  is  well 
to  be  more  tolerant  of  absorption  which  results  in  great  reduction  of 
brightness  and  which  effects  desirable  re-direction  of  the  light  flux. 
Absorption  which  results  in  neither  desirable  re-direction  nor  sufficient 
reduction  in  brightness  is  difficult  to  excuse.  Absorption  involved  in 
either  re-direction  or  brightness  reduction  without  accomplishing  the 
other  desirable  purpose,  may  or  may  not  be  overlooked,  depending 
upon  local  conditions.  The  best  design  of  accessories  should  ac- 
complish both  objects  to  a  reasonably  satisfactory  degree. 

Some  statistics  of  absorption  of  light  in  accessories  for  street 
lighting  are  presented  in  Table  II.  This  indicates  for  reflectors 
which  intercept  only  a  part  of  the  light,  absorptions  ranging  from  1 2 
to  32  per  cent.  For  housings  with  enclosing  globes  the  absorptions 
range  from  18  to  39  per  cent. 

TABLE  II. — ABSORPTION  or  LIGHT  IN  ACCESSORIES 
(Mazda  Lamps) 


Lamp 

Accessory 

Loss  of  light, 
per  cent. 

Remarks 

SOO-watt 

Reflectors. 
Steel    enameled    reflector  —  shallow 
curve.     .    . 

29 

250-  watt 

Steel    enameled    reflector  —  shallow 
curve  .... 

32 

25-250  watt 

Radial  wave  reflector 

12—21 

Varying  with  size  of  re- 

400-cp. 

Enclosing  Units  

Concentric  reflector  and  refractor  .  .  . 
2o-in.  concentric  reflector  and  dif- 
fusing globe 

23-40 
36 

34 

flector  and  location  of 
lamp  filament. 
Depending  upon   inten- 
sity of  globe  and  size  of 
housing. 

4OO-cp. 

i8-in.  radial  wave  reflector  and  dif- 
fusing globe 

32 

250-Cp. 

Radial  wave  reflector  and  diffusing 
globe  ,  

39 

Refractor  units 

31—39 

Ornamental   upright    fixtures  —  dif- 
fusing globes         

18-27 

Enclosing   Accessories   alone    (without 
fixtures). 
Diffusing  globes  with  slight  blue  tint. 
White  diffusing  glass  

Approx.  45 
8-29 

C.  R.  I.  glass  

5-  8 

Clear  glass  

Approx.  4 

MILLAR:  LIGHTING  OF  STREETS  427 

In  the  design  of  street  lighting  accessories  in  general  considerable 
progress  has  been  made  during  the  past  few  years.  This  has  consisted 
principally  in  small  improvements  which  have  entered  into  general 
practice.  The  more  general  use  of  diffusing  globes  and  of  the  better 
class  of  reflectors  is  a  matter  of  common  knowledge  and  represents 
the  principal  advance  in  accessories. 

In  recent  years  there  have  been  several  attempts  at  radical  re- 
vision of  accessory  design  to  secure  a  particular  light  distribution. 
Among  these  may  be  mentioned  an  asymmetrical  prismatic  device 
having  reflecting  prisms  on  one  side  and  the  usual  combination  of 
diffusing  ribbings  and  redirecting  prisms  on  the  other  side.  This 
was  designed  to  direct  upon  the  street  a  portion  of  the  flux  which 
otherwise  would  fall  upon  building  fronts  or  fields  along  the  street. 
Another  device  is  the  two-way  (or  four-way)  half  parabola  reflector. 
This  is  designed  to  direct  upon  the  street  most  of  the  light  which 
otherwise  would  be  delivered  above  the  horizontal  and  much  of  the 
light  which  without  it  would  fall  upon  buildings  or  fields  along  the 
street.  A  third  design  represents  an  endeavor  to  avoid  glare. 
It  consists  of  a  prismatic  hood  with  an  opal  envelope.  Its  candle- 
power  distribution  is  symmetrical  and  for  successful  application  it 
requires  to  be  installed  either  at  relatively  short  intervals  or  at 
relatively  great  height. 

The  most  recent  development  along  this  line  is  a  special  prismatic 
refractor,10  which  differs  from  the  usual  prismatic  arrangement  in 
that  the  diffusing  ribbings  and  the  directing  prisms  are  turned 
inward,  the  outer  exposed  surfaces  consisting  of  smooth  glass.  A 
typical  candle-power  distribution  curve  is  shown  elsewhere  in  Fig.  4. 
The  refractor  is  entering  into  practice  more  largely  than  have  any  of 
the  other  light-directing  devices  just  described.  A  recent  modifica- 
tion of  the  refractor  is  known  as  the  band  refractor,  differing  from 
the  ordinary  refractor  in  that  the  lower  part  of  the  bowl  is  missing. 
The  downward  light  from  the  source  is  therefore  allowed  to  escape 
without  redirection  or  absorption.  Candle-power  distribution 
curve  is  shown  elsewhere  in  Fig.  6. 

Diffusing  globes  of  alabaster,  opal  and  cased  glass  are  available, 
offering  a  variety  of  sizes  and  shapes,  as  well  as  a  wide  range  of 
diffusion  and  absorption.  Also  the  use  of  segmented  diffusing 
globes  is  growing.  It  is  therefore  possible  to  choose  for  any  lamp 
fixture  and  post  a  globe  which  will  possess  precisely  the  qualities 
needed  to  produce  the  desired  light  effects,  and  to  comply  with  the 
artistic  requirements  of  the  installation. 


428  ILLUMINATING   ENGINEERING   PRACTICE 

CALCULATIONS  OF  STREET  ILLUMINATION 

The  computation  of  total  or  zonal  light  flux  from  candle-power 
distribution  curves  need  not  be  considered  in  this  connection,  as 
it  is  treated  in  another  lecture.  While  in  the  calculation  of  illumina- 
tion on  the  street  the  same  principles  are  involved  as  in  other 
illumination  computations,  yet  the  applications  are  somewhat 
different  because  the  surface  illuminated  is  a  long,  narrow  strip.* 
While  reflecting  surfaces  such  as  buildings  play  an  important  part 
in  street  illumination,  yet  the  light  which  they  return  to  the  street 
surface  is  usually  not  large  and  it  may  be  ignored  in  these  computa- 
tions. This  simplifies  the  problem  materially. 

In  order  to  carry  out  some  simple  studies  of  light  delivered  upon 
the  street  surface,  four  characteristics  of  vertical  candle-power 
distribution  are  selected  as  follows:  These  are  typical  of: 

Magnetite  lamp  with  band  refractor. 

Magnetite  lamp,  pendant  type,  clear  globe  and  small  internal  reflector. 

Mazda  C  lamp  in  a  bowl  refractor. 

Mazda  C  lamp  in  a  particular  fixture  with  diffusing  globe. 

These  four  distribution  curves  adjusted  to  the  same  total  flux 
appear  in  Figs.  3,  4,  5  and  6.  They  represent  the  range  of  practical 
distribution  characteristics  encountered  in  street-lighting  practice 
at  the  present  time.  It  is  desired  to  emphasize  the  shape  of  dis- 
tribution curve.  It  is  the  purpose  to  deal  with  the  characteristics 
of  distribution  rather  than  to  undertake  a  comparison  of  illuminants. 

In  order  to  afford  a  better  idea  of  the  candle-power  distribu- 
tion represented  by  the  curves,  there  are  presented  solids  of  revolu- 
tion of  the  candle-power  distribution  curves  for  three  of  the  charac- 
teristics selected.  These  solids  (Fig.  7)  therefore  represent  the  mean 
distribution  of  candle-power.  It  is  assumed  that  the  lamps  are 
mounted  22  feet  over  one-curb  of  a  level  and  straight  street,  of  which 
the  roadway  is  50  feet  wide  and  each  sidewalk  is  15  feet  wide.  The 
candle-power  effective  upon  the  street  between  building  lines  is 
represented  by  the  white  portion  of  the  distribution  models. 

The  computation  of  flux  delivered  upon  the  street  consists  in 
ascertaining  what  portion  of  the  total  light  is  represented  by  the 
white  part  of  the  solid.  It  is  apparent  that  the  procedure  to  be 
followed  consists  in  determining  the  zonal  flux  values  for  the 
illuminant  and  in  ascertaining  the  portion  of  the  flux  in  each  zone 

*  This  is  complicated  when  a  lamp  is  mounted  at  street  intersections. 


* 


o 


Fig.  7. — Solids  respresenting  distribution  of  candle-power  corresponding  respectively  with 

Figs.  4,  5,  and  6. 


0.5 


0.4 


o.c 


0.2 


0.1 


LIGHT  FLUX  EFFECTIVE  UPON   STREET 
CONSTANTS  FOR  MEAN  ZONAL  CANDLE- 
POWER;  SOURCE  MOUNTED  AT  ONE  SIDE 


f 


G5 


55 


75 


55 


45 


35 


25 


15 


0  10  20  30  40  50 

Ratio  of  Street  Width  to  Mounting  Height 

Fig.  8. — Chart  for  calculation  of  light  flux  delivered  upon  street  surface  directly  from  lamps. 

(Facing  page  428.) 


MILLAR:  LIGHTING  OF  STREETS 


429 


which  falls  within  the  street  boundaries.  The  solid  of  revolution  of 
the  candle-power  distribution  curve  of  the  light  source  may  be 
divided  into  zones  assumed  as  equivalent  to  zones  of  spheres. 
That  portion  of  each  such  zone  which  lies  between  a  plane  per- 
pendicular to  the  street  and  parallel  to  the  building  line  and  a 
plane  passing  through  the  opposite  boundary  of  the  illuminated 
surfaces,  supplies  a  basis  for  the  computation  of  zonal  constants  by 
means  of  which  the  proportion  of  flux  delivered  upon  a  street  in 

Fig.  3.  Fig.  5. 


105° 


IS 


H 


43°  60°  0°      15°        80°  45° 

Fig.  4.  Fig.  6. 

Figs.  3,  4,  5  and  6. — Vertical  candle-power  distribution  characteristics. 

each  zone  may  be  determined.     These  constants  depend  upon  the 
ratio  of  street  width  to  mounting  height. 

From  such  constants  the  chart  in  Fig.  8  has  been  prepared.* 
To  illustrate  the  application  of  this  chart,  let  us  take  the  assumed 
standard  conditions  and  the  curve  of  the  complete  unit  in  Fig.  4. 
The  following  table  shows  computations  of  the  per  cent,  of  the  total 
flux  in  each  zone  delivered  upon  the  street,  employing  the  mean 
zonal  candle-power.  The  flux  upon  the  sidewalk  on  one  side  of  the 

*  Studies  of  light  flux  delivered  upon  the  street  were  presented  before  the  Illumina- 
ting Engineering  Society  in  ioio.n  Improved  methods  of  calculating  these  values  have 
been  developed  by  Mr.  E.  Peterson  who  has  prepared  the  chart  in  Fig.  8,  and  to  whom 
the  lecturer  is  indebted  for  permission  to  use  it. 


430 


ILLUMINATING    ENGINEERING    PRACTICE 


lamp  and  that  upon  the  driveway  plus  the  sidewalk  on  the  other 
side  have  to  be  added  together  to  obtain  the  total  light  flux  dis- 
tributed upon  the  street. 

FLUX  ON  STREET 

Candle-power  Distribution  as  in  Fig.  4.     Mount  22  ft.,  Street  Width  50  ft., 
Sidewalks  15  ft.     Source  over  One  Curb. 


Driveway  +  one  sidewalk 

One  Sidewalk 

Mid-zone  Angle 

Mean  zonal 
candle- 

R  -  |f  -  *.P5 

R  =  —  =  0.68 

power 

K 

Lumens 

K 

Lumens 

5° 

105 

0.047 

4-9 

0-047 

4-9 

15 

H5 

o.  141 

16.2 

o.  141 

16.2 

25 

130 

0.232 

30.1 

0.232 

30.1 

35 

170 

0.314 

53-4 

0.263 

44-7 

45 

210 

0.387 

81.3 

0.178 

37-4 

55 

270 

0.449 

121.  2 

0.141 

38-1 

65 

480 

0.496 

238.0 

0.105 

50-4 

75 

930 

0.328 

305-0 

0.063 

58-6 

85 

62O 

0.092 

57-o 

O.O2O 

12.4 

907.1 

292.8 

Flux  effective  on  street 1 200  lumens. 

Total  flux  produced  by  unit 2991  lumens. 

Proportion  delivered  upon  street 40  per  cent. 

This  method  of  calculating  light  flux  delivered  upon  the  street  is 
employed  elsewhere  in  this  lecture  in  studies  of  effect  upon  delivered 
flux  of  changes  in  location  of  street  lamps. 

The  characteristic  of  distribution  of  horizontal  and  vertical  illumi- 
nation intensities  for  each  of  the  four  illuminants  which  have  been 
chosen  is  indicated  in  Figs.  9  and  10.  These  show  intensities  com- 
puted along  the  street  in  line  with  a  single  lamp.  The  curves  of 
horizontal  illumination  show  maximum  values  near  the  lamp  to  be 
greatest  in  the  case  of  the  diffusing  globe  equipment. 


DISCERNMENT  UNDER  STREET  ILLUMINATION 

Visual  Characteristics. — The  theoretical  aspects  of  vision  under 
street-lighting  conditions  are  covered  in  another  lecture  of  this  course. 
It  may  be  well,  however,  to  indicate  briefly  in  this  connection  the 
salient  features  which  have  a  direct  bearing  upon  our  problem.  The 


MILLAR:  LIGHTING  OF  STREETS 


HORIZONTAL  ILLUMINATION   INTENSITIES 

FROM  ONE  LAMP 
MOUNTING  HEIGHT  22  FEET 


— Mazda  Diffusing  Globe 

Mazda- Do wl  Refractor 

Magnetite  Arc-Clear  Globe 

Magnetite  Arc-Band  Refractor 


60         80         100       120       140        160       180        200       220       240 


Fig.  9. — Variation  of  illumination  with  distance. 


0.7 

0.6 
« 
|0.5 

a 

8  °-4 
fa 

0.3 
0.2 
0.1 
0 

VERTICAL  ILLUMINATION   INTENSITY  FROM 
ONE  LAMP-MOUNTING  HEIGHT  22  FEET 

0  07 

\ 

Mazda-Diffusing  Globe 
Maz.da-Dowl  Refractor 
Magnetite  Arc-Clear  Globe 
Magnetite  Arc  -Band  Refractor 

0  06 

\ 

0  05 

\ 

\ 

1 

0  04 

§ 

Sy 

5 

+> 

g 

-0.03 
0  02 

\ 

\ 

N 

^^ 

h 

\ 

\ 

\ 

x 

N%NV 

^^^^ 

^ 

.-"""*>• 

A  01 

x* 

^ 

-^^ 

\ 

^v 

^ 

"^ 
^^. 

-^. 

?^x 

^ 

\ 

*^"»«*, 

"  —  • 

—  —  — 

""•J" 

V 

^~~! 

jsiss 

55= 

^•ss* 

gSJJE 

!  im 

m^m 

0        20        40        60        80        100      120      140      160      180      200       220      240 

Feet 
Fig.   10. — Variation  of  illumination  with  distance. 


432  ILLUMINATING   ENGINEERING   PRACTICE 

von  Kries  rod  and  cone  theory  of  vision  now  generally  accepted 
allots  to  the  retinal  rods  the  process  of  vision  at  very  low  intensity. 
The  rods  are  thinly  interspersed  among  the  cones  in  the  central  or 
foveal  portion  of  the  retina  of  the  eye  and  are  more  thickly  clustered 
about  the  peripheral  portions.  In  twilight  or  scotopic  vision  the 
foveal  or  central  portion  of  the  retina  is  less  sensitive  than  the  con- 
tiguous surrounding  portions.  In  such  vision  sensibility  in  general 
is  a  function  of  the  dark  adaptation  of  the  retina.  Complete  dark 
adaptation  is  rarely  realized,  but  various  degrees  of  dark  adaptation 
characterize  vision  under  street-lighting  conditions.  Dark  adapta- 
tion is  built  up  slowly  for  some  minutes  after  the  removal  of  all 
bright  lights  and  more  rapidly  during  a  period  of  one-half  hour  or 
more  until  with  continued  darkness,  it  attains  the  approximate  level 
of  retinal  sensibility  which  the  conditions  will  permit.  Dark  adap- 
tation is  easily  destroyed  by  the  intrusion  of  a  bright  and  powerful 
light  source  within  the  field  of  vision. 

Under  conditions  of  dark  adaptation,  sensibility  near  the  cen- 
tral portion  of  the  retina  is  increased  when  the  area  of  stimulus  is 
increased.  When  the  intensity  of  the  light  stimulus  becomes  feeble, 
the  eye  in  general  becomes  relatively  more  sensitive  to  light  of  the 
shorter  wave  length.  This  is  known  as  the  Purkinje  effect  and  is 
usually  associated  and  complicated  with  the  degree  of  dark  adapta- 
tion of  the  eye. 

From  the  foregoing  statements  we  may  conclude  that  in  street 
lighting,  especially  under  conditions  of  feeble  intensity,  best  vision 
requires  (i)  absence  from  the  field  of  view  of  powerful  and  bright 
light  sources,  (2)  illumination  of  as  large  areas  as  practicable  and 
(3)  light  in  which  the  shorter  wave  lengths  are  prominent. 

Street-lighting  theories  should  be  comprehensive.  While  it  is 
often  desirable  to  separate  a  given  variable  and  to  study  it  to  the 
exclusion  of  other  variables  in  order  to  ascertain  its  characteristics, 
yet  no  one  variable  should  be  considered  with  respect  to  its  effect 
upon  the  problem  as  a  whole  without  taking  into  account  the  effect 
of  all  other  variables.  Thus,  while  the  foregoing  general  require- 
ments for  good  vision  under  street  lighting  conditions  appear  to  be 
fundamentally  sound,  yet  it  may  be  quite  possible  that  other  con- 
siderations may  in  particular  instances  make  it  desirable  to  trans- 
gress these  rules  in  order  to  obtain  a  better  final  result. 

Indoor  vs.  Outdoor  Lighting. — An  important  distinction  between 
street  lighting  and  interior  lighting  is  this — poor  lighting  of  interiors 
results  in  ocular  discomfort  due  to  difficulty  in  seeing  things  well 


MILLAR:  LIGHTING  or  STREETS  433 

enough;  poor  lighting  of  streets  results  in  failure  to  see  things.  In 
the  lighting  of  interiors  the  problem  is  rarely  that  of  detecting  the 
presence  of  objects,  while  in  the  lighting  of  streets  this,  to  a  large 
degree,  is  the  principal  object.  Under  interior  illumination  we 
rarely  have  recourse  to  rod  vision.  Cone  vision  prevails  almost 
exclusively.  In  the  streets  at  night  rod  vision  predominates  and 
the  situation  is  complicated  by  a  frequent  shifting  from  rod  to  cone 
vision.  Under  the  usual  interior  illumination  we  discriminate 
fine  relief;  we  distinguish  colors.  Under  the  usual  street  lighting  we 
discover  objects  and  consciously  or  unconsciously  classify  them  with 
respect  to  type,  but  generally  speaking  we  are  not  able  to,  and  per- 
haps do  not  desire  to,  discriminate  details  or  distinguish  colors. 

When  the  lighting  is  intensified,  as  in  the  important  streets  of 
the  city,  other  secondary  purposes  are  served.  Here  one  may  be 
able  to  identify  the  color  of  an  automobile  or  to  recognize  a  passer-by. 
Visual  conditions  and  revealing  processes  are  more  nearly  similar  to 
those  which  obtain  in  indoor  lighting.  That  is,  one  discriminates 
detail,  color,  etc.  This  more  expensive  street  lighting  of  necessity 
must  be  restricted  to  the  principal  streets  of  a  city.  It  is  limited 
to  a  relatively  small  area.  As  shown  elsewhere  street-lighting  prob- 
lems are  less  difficult  under  such  conditions. 

In  the  great  majority  of  cases  streets  are  lighted  to  a  low  intensity. 
Reliance  is  placed  almost  entirely  in  large  shade  contrasts  and  in 
contour  rather  "than  upon  discrimination  of  detail.  It  is  impor- 
tant to  remember  this  distinction.  If  the  details  of  a  surface  are  to 
be  described,  theoretically  at  least  the  most  important  consideration 
would  be  the  securing  of  uniform  vertical  illumination.  Such  dis- 
crimination, generally  speaking,  is  beyond  the  scope  of  the  average 
street-lighting  system.  In  practice,  perception  consists  first  in  detec- 
tion of  the  presence  of  an  object,  and  second  in  recognition  of  the 
object.  If  the  object  is  of  considerable  size,  it  is  detected  because  it 
is  lighter  than  its  background  and  surroundings,  or  darker,  or  be- 
cause it  casts  a  shadow  which  can  be  seen.  One  can  detect  the 
presence  of  an  automobile,  but  may  be  unable  to  distinguish  the 
make.  The  size  and  contour  classify  the  object,  but  the  color  is 
not  revealed.  The  size  and  contour  may  make  it  evident  that 
another  object  is  a  person.  The  lighting  is  sufficient  to  permit  per- 
ception of  his  movements,  but  it  does  not  reveal  the  color  of  his  dress. 

Silhouetting. — In  warm  weather,  light-colored  fabrics  are  common 
in  the  apparel  of  women  and  children  if  not  of. men.  With  this 
exception,  nearly  all  objects  which  it  is  important  to  perceive  when 


434  ILLUMINATING    ENGINEERING    PRACTICE 

traversing  a  street,  whether  they  be  objects  on  the  street  or  irregu- 
larities in  the  street  surface,  tend  to  be  either  the  same  shade  or  of 
a  darker  shade  than  the  street  surface.  The  majority  of  objects 
and  pavement  irregularities  are  thus  not  light  in  color,  and  on  this 
account  it  is  the  most  usual  consideration  that  the  contrasts  per- 
ceived are  those  of  dark  objects  against  lighter  backgrounds.12  It 
should  be  noted  that  dark  objects  are  the  most  difficult  to  perceive 
and  that  their  perception  involves  the  most  important  part  of  the 
street-lighting  problem. 

When  the  objects  are  large,  in  the  majority  of  cases  they  are  seen 
as  silhouettes.  Fig.  n  is  a  comparison  of  a  man  seen  in  the  street  at 
night;  first  by  direct  light  that  illuminates  to  a  degree  which  makes 
it  possible  to  see  him  even  if  he  were  not  contrasted  against  the 
background,  and  second,  when  seen  as  a  silhouette  in  another  por- 
tion of  the  street  where  the  illumination  is  too  feeble  to  reveal  the 
details.  Even  in  the  first  case  it  will  be  observed  that  he  appears  as 
a  silhouette,  though  less  strongly  contrasted  against  the  background 
than  in  the  second  case. 

Fig.  12  is  a  comparison  of  an  observation  target  viewed  in  turn 
from  opposite  directions.  The  target  is  painted  substantially  the 
same  color  as  the  street.  Viewed  from  the  side  it  is  well  illuminated 
from  a  near-by  lamp.  It  is  very  difficult  to  see  because  its  bright- 
ness is  substantially  the  same  as  that  of  its  background.  Viewed 
from  the  opposite  direction  it  is  dimly  illuminated  from  a  distant 
lamp,  but  is  readily  seen  as  a  silhouette  because  its  background  is 
much  brighter  than  its  observed  surface. 

While  this  discussion  refers  more  especially  to  the  great  majority 
of  street  lighting  which  is  not  of  a  high  order  of  intensity,  yet  it 
may  not  be  out  of  place  to  note  at  this  point  that  no  street  lighting  is 
so  intense  as  to  eliminate  silhouetting  as  a  fundamentally  important 
method  of  discernment.  Fig.  13  is  a  picture  of  a  silhouette  in  a 
brilliantly  lighted  street. 

The  difference  between  brilliantly  lighted  and  dimly  lighted 
streets  in  regard  to  silhouetting  is  that  on  the  brilliantly  lighted 
streets  one  is  not  dependent  solely  upon  the  silhouette  effect  for 
discernment,  whereas  in  many  parts  of  a  dimly  lighted  street  he 
must  rely  exclusively  upon  this  method.  Even  in  brilliant  sun- 
light most  large  objects  are  seen  on  the  street  as  silhouettes  for  the 
reason  that  at  a  distance  one  cannot  discriminate  fine  details  of 
relief  and  color,  and  that  upon  a  near  view  one  ordinarily  is  not 


Fig.   ii. — Man  seen  by  direct  illumination  and  by  silhouetting. 


Fig.   12. — Observational  target  seen  by  silhouetting  and  by  direct  illumination. 

(Facing  page  434.) 


MILLAR:  LIGHTING  OF  STREETS  435 

sufficiently  interested  to  do  so,  especially  when  moving  rapidly,  as 
in  an  automobile. 

When  the  illumination  is  markedly  variable,  objects  between 
lamps  are  seen  as  silhouettes  offering  still  greater  contrast  to  their 
background  because  a  part  at  least  of  such  background  is  more 
brilliantly  lighted  than  under  uniform  illumination.  When  objects 
are  near  to  and  just  beyond  a  lamp  on  a  non-uniformly  lighted  street, 
they  are  more  likely  to  be  seen  as  under  interior  illumination  through 
the  discrimination  of  surface  details  because,  other  things  being 
equal,  the  illumination  at  such  points  is  more  brilliant  than  is  the 
case  of  a  uniformly  lighted  street.  Thus  a  pedestrian  seen  through- 
out the  length  of  a  uniformly  lighted  street  as  a  silhouette  offering 
mild  contrast  against  the  background  would  appear  on  a  non- 
uniformly  lighted  street  first  as  seen  under  dim  interior  illumination 
and  then  as  a  strongly  contrasting  silhouette. 

Direction  of  Light. — Consider  an  abrupt  depression  in  the  road- 
way (see  Fig.  14).  This  is  discerned  readily  if  the  rim  or  the  exposed 
floor  of  the  depression  is  markedly  lighter  or  darker  than  the  sur- 
rounding roadway.  If  the  light  falling  upon  the  depression  is 
derived  from  a  distant  lamp  opposite  the  observer,  the  observed  rim 
is  likely  to  be  left  in  darkness  and  to  appear  much  darker  than  the 
roadway,  thus  revealing  the  presence  of  the  depression.  If  the  light 
falls  at  an  acute  angle  from  a  lamp  behind  the  observer,  the  observed 
rim  is  likely  to  appear  brighter  than  the  surrounding  roadway,  thus 
revealing  the  presence  of  the  depression.  In  either  case  the  floor  of 
the  depression  may  be  darker  than  the  surrounding  roadway,  be- 
cause it  lies  in  the  shadow  and  if  seen  it  also  will  reveal  the  presence 
of  the  depression.  On  the  other  hand,  if  the  light  is  received  from 
a  near-by  lamp  slightly  nearer  the  observer  than  is  the  depression, 
the  rim  and  floor  of  the  depression  may  be  illuminated  to  about  the 
same  brightness  as  the  surrounding  roadway  and  there  may  be  little 
or  no  contrast  to  reveal  the  presence  of  the  depression.  Thus  with 
one-tenth  the  illumination  a  depression  midway  between  lamps  may 
be  more  readily  discerned  and  avoided  than  a  depression  near  the 
lamp  illuminated  to  ten  times  the  intensity.  This  is  an  illustration 
of  the  importance  of  contrast  as  affecting  street-lighting  visibility 
and  incidentally  of  the  fact  that  lighting  effectiveness  is  by  no 
means  dependent  exclusively  upon  illumination  intensity. 

DESIGN 

Street-lighting  design  is  subject  to  certain  fundamental  conditions 
as  follows: 


436  ILLUMINATING   ENGINEERING   PRACTICE 

Class  of  city. 

Municipal  appropriations. 

Class  of  street. 

Buildings  along  the  street. 

Trees. 

Roadway  and  curvature. 

Lamp  characteristics. 

Human  characteristics. 

Let  us  consider  briefly  these  several  unalterable  or  nearly  un- 
alterable conditions. 

Class  of  City. — -Cities  differ  greatly  in  respect  to  their  industries 
and  activities.  In  some  cities,  such  as  New  York  and  San  Fran- 
cisco, night  life  is  highly  developed.  In  others,  which  the  writer 
does  not  have  the  temerity  to  illustrate  by  examples,  the  streets  are 
not  used  largely  by  night.  This  difference  in  characteristic  may 
reasonably  be  expected  to  have  a  bearing  upon  the  intensity  standards 
of  street  lighting.  Differences  in  real  estate  values  and  in  traffic 
density  also  distinguish  cities,  and  these  too  have  a  direct  bearing 
upon  municipal  appropriations  for  street  lighting. 

Municipal  Appropriations. — Money  expended  for  street  lighting 
is  fixed  usually  as  a  compromise  between  the  wishes  of  the  engineers 
having  the  lighting  in  charge  and  the  desires  of  those  who  pass 
upon  the  appropriations.  Ordinarily  the  problem  is  to  secure  the 
most  effective  street  lighting  with  the  money  which  is  available. 

Class  of  Street. — In  every  city,  streets  vary  in  importance  through- 
out a  wide  range.  It  is  customary  to  adjust  intensity  standards  of 
street  illumination  with  reference  to  relative  real  estate  valuations, 
traffic  density  and  safety  requirements  for  each  locality. 

Roadway. — The  nature  of  the  pavement,  street  gradient  and 
curvature  are  important  conditions  which  have  to  be  taken  into 
account.  All  affect  directly  the  problem  of  lamp  locations  and  the 
distribution  of  roadway  brightness  with  respect  to  incident  light. 

Trees. — The  presence  or  absence  of  trees  likewise  enters  into  the 
design  in  an  obvious  manner.  If  lamps  are  mounted  well  out  over 
the  roadway,  the  sidewalks  are  likely  to  be  lighted  poorly.  If 
lamps  are  mounted  low  over  the  curbs,  the  roadway  suffers. 

Lamp  Characteristics. — These  are  treated  under  "Illuminants." 

Human  Characteristics. — These  are  discussed  in  another  lecture 
at  length,  and  visual  characteristics  are  referred  to  briefly  elsewhere 
in  this  lecture. 

Within  the  limitations  of  the  above  conditions  which  cannot  be 


MILLAR:  LIGHTING  OF  STREETS  437 

changed  or  which  can  be  changed  only  with  great  difficulty,  design 
ought  to  be  based  upon  a  thorough  understanding  of  what  the  street 
lighting  is  intended  to  accomplish.  Before  addressing  himself  to 
design  the  engineer  in  charge  should  determine  beyond  all  question 
what  he  expects  of  the  street  illumination.  Does  he  want  uniform 
illumination,  and  if  so  what  component  or  quality  of  the  illumina- 
tion does  he  desire  to  have  uniform?  Does  he  want  to  avoid  glare, 
and  should  he  design  the  installation  with  this  in  view  as  a  para- 
mount object,  or,  as  appears  to  the  writer  to  be  the  more  logical  and 
reasonable  procedure,  should  he  seek  to  accomplish  the  purposes 
set  forth  as  the  three  major  objects  of  street  lighting  described  in 
the  early  part  of  this  lecture?  Not  until  this  point  is  settled  should 
he  proceed  to  consideration  of  means  of  accomplishing  the  desired 
objects  with  the  lighting  which  is  to  be  designed. 

With  the  purposes  to  be  served  denned  after  careful  consideration, 
actual  design  of  the  lighting  installation  may  be  undertaken.  At 
this  point  there  are  certain  principles  of  good  street  lighting  which 
as  generalities  find  proper  place  in  a  lecture  of  this  kind.  These 
embrace  generally  accepted  rules  which  may  be  regarded  as  estab- 
lished, and  other  propositions,  ranging  from  notions  to  rather  gen- 
erally accepted  tenets  which  are  subscribed  to  by  various  engineers, 
but  which  have  not  yet  received  general  acceptance.  Some  of 
these  latter  are  at  present  moot  questions  and  it  is  the  lecturer's 
purpose  in  such  cases  to  indicate  the  fact  in  order  to  enable  his 
audience  to  attribute  proper  weight  to  each  such  proposition. 

There  are  certain  obvious  important  features  of  street  lighting 
which  are  essential  to  effectiveness,  including  good  maintenance  of 
posts,  lamp  fixtures  and  lamps,  reliability  and  continuity  of  service, 
etc.  For  the  purposes  of  this  lecture  such  features  characteristic  of 
a  first-class  street-lighting  service  may  be  assumed.  With  these 
disposed  of,  effectiveness  of  street  illumination  may  be  said  to  depend 
upon  the  following  factors: 

Intensity  of  light  upon  the  street — average  and  variability. 
Brightness  of  street  surface. 
Visual  angle  between  lamps  and  street  surface. 
Extremes   of   contrast  between  lamps   and   street   surface. 
Extent  to  which  the  visual  field  is  illuminated. 

Extent  to  which  the  visual  field  immediately  adjacent  to  light  source 
is  illuminated. 

Contrasts  produced  on  the  street  surface. 
Contrasts  produced  on  objects  on  the  street. 


438 


ILLUMINATING   ENGINEERING   PRACTICE 


Appearance  of  installation  by  day  and  by  night. 
Appearance  of  street  and  buildings  by  night. 

Average  Intensity  of  Light  upon  the  Street. — The  intensity  of  light 
upon  the  street  is  one  important  factor  affecting  the  value  of  the 
street  lighting.  The  total  flux  of  light  or  the  average  intensity 
throughout  the  length  of  the  street  should  however  be  considered  in 
connection  with  the  variability  of  intensity  and  with  the  proportion 
and  utility  of  the  light  delivered  upon  buildings.  The  weight  to 
be  attributed  to  different  degrees  of  variability  as  increasing  or 
decreasing  the  utility  of  the  illumination  cannot  be  stated  definitely. 

Considering  first  average  intensities  the  following  table  is  offered 
to  show  modern  practice  in  this  country. 

TABLE  III. — TYPICAL  STREET  LIGHTING  INTENSITIES 


Class  of  street 

Average  horizontal 
illumination  intensity 

Desirable  characteristic 

Important    avenues    and 

0.5  to  i.o  foot-candle 

Ample  light  on  building 

heavy  traffic  streets. 

(lumen  per  square  foot). 

fronts. 

Secondary  business  streets. 

o.i  to  0.2  foot-candle 

Ample  light  on  buildings. 

(lumen  per  square  foot). 

City  residence  streets  .... 

0.05  to  o.io  foot-candle 

Subdued  light  on  building 

(lumen  per  square  foot)  . 

fronts. 

Suburban  highways  

0.02  to  0.04  foot-candle 

Maximum  light  on  road- 

(lumen per  square  foot)  . 

way. 

Suburban  residence  streets. 

0.005  to  0.02  foot-candle 

Very    subdued    light    on 

(lumen  per  square  foot)  . 

building  fronts. 

The  above  intensities  should  be  considered  in  connection  with  the 
fact  that  municipal  appropriations  very  generally  are  inadequate. 
They  therefore  do  not  necessarily  represent  modern  ideas  of  best 
practice;  they  represent  rather  the  status  attained  in  practice,  the 
occasional  exception  conforming  to  the  criterion  of  desirability  en- 
tertained by  street-lighting  engineers.  As  a  standard  for  guidance 
therefore  it  is  probable  that  the  higher  value  shown  may  be  accepted 
with  greatest  safety. 

Per  Cent.  Flux  Delivered  Directly  upon  the  Street. — With  a  method 
available  for  the  ready  and  convenient  computation  of  light  flux 
delivered  upon  the  street  surface,  we  may  carry  out  some  simple  but 
instructive  studies  of  the  influence  of  changes  in  location  and  equip- 
ment of  street  illuminants  upon  delivered  flux.  For  the  purpose 
we  shall  adopt  certain  standard  conditions  which  will  obtain  unless 


MILLAR:  LIGHTING  OF  STREETS 


439 


otherwise  stated.  These  include  a  level  and  straight  street  free 
from  trees  and  other  obstructions,  the  width  from  curb  to  curb 
being  50  feet  with  sidewalks  15  feet  wide,  lamps  being  mounted  at 
a  height  of  22  feet. 

Fig.  15  shows  for  each  of  the  four  candle-power  distribution 
characteristics  which  have  been  referred  to,  the  change  in  per  cent, 
flux  delivered  directly  upon  our  arbitrarily  selected  street  due  to 
alteration  in  the  location  of  the  illuminant  transverse  of  the  street. 
Thus  when  the  lamps  are  mounted  over  the  middle  of  the  street, 
the  maximum  flux  is  of  course  delivered  upon  the  street  surface. 
When  these  lamps  are  moved  over  to  the  curb  25  feet  away  from  the 


EFFECT  OF  LAMP  LOCATION  UPON 
FLUX  DELIVERED  UPON  STREET 


10  20 

Lamp  Distance  from  Point  over 

Middle  of  Street 
Fig.   15. — Variation  of  percentage  of  flux  delivered  on  the  street  with  position  of  lamps. 

center  of  the  street,  the  same  standard  mounting  height  of  22  feet 
being  preserved,  the  reduction  in  flux  delivered  directly  upon  the 
street  ranges  from  12  to  16  per  cent.  This  is  of  course  based  upon 
the  assumption  that  the  street  is  free  from  interference  due  to  the 
presence  of  trees  and  other  causes. 

From  Fig.  16  we  may  derive  some  interesting  relations  between 
lamp  mounting  height  and  per  cent,  flux  delivered  directly  upon  the 
street.  In  order  to  make  the  diagram  useful  for  other  purposes, 
the  scale  of  abscissa  shows  ratio  of  width  to  height.  The  four  curves 
are  applicable  to  our  four  selected  candle-power  distribution  charac- 
teristics. The  variation  in  per  cent,  light  flux  delivered  upon  the 
street  due  to  a  change  in  the  height  of  the  lamps  or  in  the  width  of 


440 


ILLUMINATING   ENGINEERING   PRACTICE 


the  street  when  the  lamps  are  mounted  over  the  curbs  is  indicated. 
When  the  values  are  taken  from  the  chart  it  will  be  seen  that,  for  the 
candle-power  distribution  characteristics  represented,  when  the 
lamp  is  raised  from  a  height  which  is  half  the  width  of  the  street  to 
one  which  is  three-quarters  the  width  of  the  street,  the  reduction  in 


36 


20 


16 


12 


I 


-  4.0  Amp.  Standard-Magnetite  Arc  -Reflector 
--  4.0  Amp.  Standard  -Magnetite  Arc-Refractor 
---  Mazda  C  Series  Refractor  Unit 
---  Mazda  C  Multiple  Diffusing  Unit 


012  345 

Ratio  Street  Width.  One  Side  of  Lamp  to  Mounting  Height 

Fig.   1 6. — Variation  in  light  flux  delivered  upon  streets  of  various  widths  with  change  in 
lamp  mounting  height. 

flux  delivered  upon  the  street  ranges  from  12  to  21  per  cent.  In 
order  to  consider  a  particular  case,  it  may  be  assumed  that  the  lamps 
are  mounted  over  one  curb  of  our  standard  street.  The  per  cent, 
light  flux  delivered  upon  the  street  with  various  mounting  heights 
will  then  be  as  indicated  in  derived  Fig.  17. 
Light  on  Building  Fronts. — The  total  or  average  light  flux  delivered 


MILLAR:  LIGHTING  OF  STREETS 


441 


upon  the  street  surface  of  course  does  not  represent  a  complete  state- 
ment of  lighting  value,  though  it  is  a  very  important  part  of  such 
complete  statement.  Light  delivered  upon  building  fronts  ranges 
from  perhaps  a  negligible  value  in  residential  streets,  where  it  is 
often  undesirable,  to  a  high  value  in  important  avenues  where  it  is 
indispensable.  Figs.  18  and  19  are  examples  of  building  front  illu- 
mination representing  extremes  of  good  and  bad  practice. 

Variability  of  Illumination  along  Street. — The  complete  qualities 
of  the  illumination  cannot  be  known  unless  there  is  a  statement 


66 
62 
58 

«,   54 

w  50 

% 
c   & 

!« 

88 
34 
30 
26 

FLUX  DELIVERED  UPON  80  FT. 
STREET  BY  LAMP  MOUNTED 
22  FT.  OVER  MIDDLE 
OF  STREET 

\ 

• 

\> 

V 

\\ 

,\ 

Magnetite  Arc.Clear  Globe 
Magnetite  Arc-Dand  Befractor 
——Mazda-Bowl  Refractor 

—  —  Mazda  -Diffusing  Globe 

-v> 

s\  : 

S^ 

^<\ 

tS; 

^^V 

TS 

i? 

\ 

iL_ 

f§ 

^-* 

\ 

32 

»^ 

r    "V 

s; 

>K 

10 


15 


26 

Mounting  Height 
Fig.  17. 


indicating  the  variability  of  the  distribution  of  light  on  the  street 
surface  itself. 

It  has  long  been  a  tenet  of  street  lighting  that  invariable  or  uni- 
form illumination  is  the  great  desideratum.13  This  may  be  quali- 
fied perhaps  by  saying  that  those  who  so  believe  are  satisfied  to 
obtain  illumination  which  is  nearly  uniform,  varying  from  maximum 
to  minimum  by  no  more  than  say  5  to  i.  Recently  it  has  been 
further  qualified  by  the  concession  that  when  the  total  light  produced 
falls  below  a  certain  minimum  which  corresponds  roughly  with 
residence  street  lighting,  the  uniformity  criterion  may  have  to  be 
abandoned  as  impracticable  for  the  reason  that  it  may  be  possible 


442  ILLUMINATING    ENGINEERING   PRACTICE 

to  light  such  streets  better  with  a  degree  of  variable  illumination 
than  with  uniform  illumination. 

While  many  writers  insist  upon  the  general  quality  of  uniformity, 
most  of  them  are  a  little  indefinite  in  stating  what  component  of 
the  illumination  should  be  uniform.  All  desire  uniformity  of  incident 
light,  but  opinion  seems  to  divide  some  favoring  uniform  horizontal 
illumination  and  others  uniform  vertical  illumination. 

It  would  appear  that  the  desire  for  uniformity  arises  from  the  view 
that  by  this  characteristic  the  best  lighting  effects  will  be  obtained 
throughout  the  length  of  the  street.  If  the  horizontal  illumination 
is  desired  to  be  uniform,  it  is  in  order  that  the  street  surface  may 
be  seen  to  best  advantage.  If  the  vertical  illumination  is  desired 
to  be  uniform,  it  is  to  the  end  that  faces  of  passers-by  may  be  dis- 
tinguished. It  may  appear  reasonable  to  consider  that  in  order  to 
maintain  visibility  conditions  uniformly  along  the  street,  the 
intensity  of  incident  light  must  be  uniform.  The  chain  is  only  as 
strong  as  its  weakest  link.  The  minimum  illumination  intensity  is 
said  to  be  the  weakest  link  in  the  chain  of  street  illumination.  There- 
fore, the  minimum  must  be  increased  until  it  attains  the  average.  So 
persuasive  is  this  view  that  in  England  there  appears  to  be  rather 
widespread  belief  that  a  correct  basis  of  rating  street  illumination 
must  be  closely  associated  with  a  minimum  intensity. 

It  is  perhaps  inherent  in  the  problem  that  differences  of  opinion 
should  exist  as  to  the  relative  importance  of  uniformity  in  horizontal 
and  vertical  components  of  the  illumination.  The  two  serve  quite 
different  purposes;  each  is  important.  Unfortunately  the  require- 
ments for  the  two  are  not  identical.  Uniform  vertical  illumination 
imposes  a  requirement  for  a  somewhat  higher  angle  of  maximum 
light  distribution  than  does  uniform  horizontal  illumination.  Ad- 
vocates of  uniformity  in  general  manifest  a  tendency  to  accept  either 
of  the  two  distribution  characteristics  which  it  is  possible  to  obtain, 
being  satisfied  if  they  can  realize  either  the  one  or  the  other  of  these 
uniformity  conditions. 

Uniformity  of  illumination  is  naturally  approximated  when  a 
street  is  lighted  brilliantly  by  closely  spaced  lamps.  It  does  not 
have  to  be  striven  for  and  can  hardly  be  avoided.  In  such  streets 
there  is  ample  intensity  for  all  purposes  and  the  desirability  or 
undesirability  of  strict  uniformity  of  lighting  need  not  be  discussed. 
It  is  only  in  streets  illuminated  to  lesser  intensities  where  uniformity 
can  be  attained  only  by  using  a  larger  number  of  small  lamps  or  by 
modifying  radically  the  light  distribution  curve  that  the  desirability 


MILLAR:  LIGHTING  OF  STREETS 


443 


or  undesirability  of  uniformity  of  light  distribution  along  the  street 
becomes  a  matter  for  discussion. 

In  the  practical  lighting  of  such  streets  uniformity  of  illumina- 
tion can  be  obtained  either  by  raising  the  angle  of  maximum  dis- 


50 


45 


40 


35 


S 

3 
§ 

J» 


20 


15 


10 


VARIABILITY  OF  HORIZONTAL 


ON  STREET. 


I 


4 


INTENSI 


Mazda-Diffusing  Globe 

Magnetite  Band  Refractor 

Magnetite  Clear  Globe 

Mazda-Bowl  Refractor 


4  6  8  10.  12  14  16 

Ratio  Spacing  to  Mounting  Height 
Fig.  20. — Variation  of  horizontal  illumination  with  mounting  height. 

tribution  about  the  source  of  light  or  by  multiplying  the  number  of 
sources.  Assume  that  it  is  desired  to  increase  the  uniformity  of 
horizontal  illumination  by  substituting  for  the  diffusing  globe  equip- 


444  ILLUMINATING   ENGINEERING   PRACTICE 

ment  shown  in  Fig.  6,  the  prismatic  refractor  equipment  shown  in 
Fig.  5.  Fig.  20  shows  that  if  the  spacing  interval  is  ten  times  the 
mounting  height,  this  change  of  accessories  will  reduce  the  ratio  of 
maximum  to  minimum  horizontal  illumination  intensity  from  34  to 
7.  This  change  would  be  attended  by  certain  changes  in  the 
lighting  conditions  as  follows: 

(a)  The  proportion  of  light  utilized  would  be  decreased.  It  is  not 
altogether  a  simple  matter  to  compare  these  distribution  curves  to  derive 
the  percentage  of  light  utilized.  If  the  illuminants  are  employed  on  a 
business  street,  much  of  the  light  delivered  above  the  horizontal  by  the 
diffusing  globe  will  have  to  be  considered  as  useful.  If  the  illuminants 
are  employed  in  outlying  streets,  the  light  delivered  above  the  horizontal 
would  not  be  of  much  utility.  Considering  the  lamps  to  be  mounted 
over  the  selected  8o-foot  street,  23  per  cent,  of  the  light  in  the  lower 
hemisphere  would  be  delivered  upon  the  street  surface  if  the  refractor 
is  used  and  32  per  cent,  if  the  diffusing  globe  is  used.  Fig.  16  shows  that 
if  we  consider  the  total  light  produced,  the  diffusing  globe  delivers  a 
larger  proportion  on  the  street  if  the  street  width  on  each  side  of  the  lamp 
is  no  greater  than  1.8  times  the  mounting  height,  while  the  refractor 
delivers  a  larger  proportion  of  the  total  light  upon  the  street  if  the  ratio 
is  greater  than  1.8. 

(6)  The  maximum  intensity  delivered  upon  the  street  surface  near  the 
lamp  is  much  reduced. 

Such  reduction  is  at  all  times  likely  to  reduce  the  effectiveness  of  the 
lighting  and  especially  so  when  the  lamp  is  located  over  street  intersec- 
tions where  the  bright  pavement  near  the  lamp  is  visible  from  four 
directions. 

(c)  The  effect  of  glare  is  increased. 

The  brightness  of  the  light  source  from  65  to  75  degrees  above  the  nadir 
is  increased  about  four  fold,  this  being  due  in  part  to  higher  intensity  in 
this  zone  and  in  part  to  smaller  size  of  the  accessory. 

To  sum  up,  when  the  variability  of  horizontal  illumination  as 
measured  by  the  ratio  of  maximum  to  minimum  is  reduced  from  34 
to  7  by  substituting  a  bowl  refractor  for  a  diffusing  globe,  the 
utilized  light  flux  is  reduced,  the  maximum  intensity  delivered  upon 
the  street  near  the  lamp  is  reduced  and  the  glare  is  increased.  A 
comparison  of  two  particular  equipments  has  been  chosen  as  the 
basis  of  this  discussion.  Some  other  comparison  might  modify 
the  discussion  somewhat.  The  general  tendency,  however,  would 
be  in  the  direction  indicated  and  the  case  chosen  is  peculiarly 
appropriate  in  that  a  choice  between  these  two  forms  of  accessories 
presents  itself  in  most  present-day  designs. 


MILLAR:  LIGHTING  OF  STREETS 


445 


In  the  second  case  (numerous  small  lamps)  there  is  involved: 

(d)  Increased  expense  for  both  installation  and  operation. 

(e)  Likelihood  of  reduced  effectiveness  due  to  multi-directional  light 
and  consequent  reduction  of  contrasts. 

This  effect  was  brought  out  clearly  in  Dr.  Bell's  lecture. 

It  might  be  well  to  incur  the  foregoing  disadvantages  involved 
in  securing  uniform  illumination  if  any  important  purpose  were  to 
be  served  by  uniform  illumination.  But  it  is  a  fact  that  uniformity 
of  incident  light  is  not  necessary  to  the  unvarying  maintenance  of 
the  most  important  visibility  conditions  throughout  the  length  of 
the  street.  This  is  evidenced  by  the  following: 


0.6 


ILLUMINATION  INTENSITY  AND  BRIGHTNESS 
ASPHALT  STREET- LAM  PS  260  FT.  APART 


100   120   140   160   180   200   220   240 
Feet 


0       20        40        60 

Fig.  24. — Illumination  intensity  and  brightness. 


(/)  At  points  between  lamps  where  intensity  is  feeble,  superior  direction 
of  incident  light  tends  to  compensate. 

Figs.  21  and  22  show  two  somewhat  similar  depressions  in  the  street 
surface.  The  first  is  illuminated  principally  by  a  lamp  which  is  removed 
about  25  feet  and  which  is  behind  the  camera.  The  latter  is  illuminated 
by  a  lamp  which  is  removed  about  150  feet  and  which  is  beyond  the  de- 
pression. Unquestionably  in  driving  or  walking,  one  would  be  more 
likely  to  see  the  depression  in  the  pavement  in  the  latter  case  in  spite  of 
the  fact  that  the  intensity  is  much  less  than  that  at  the  depression  shown 
in  Fig.  21.  This  is  because  the  direction  of  the  light  is  such  as  to  produce 
in  Fig.  22  a  strong  contrast  which  reveals  the  presence  of  the  depression 
in  spite  of  the  low  intensity.  Generally  speaking,  light  delivered  at  acute 
angles  midway  between  lamps  is  more  effective  in  revealing  surface 
irregularities  than  is  light  near  the  lamps. 


446  ILLUMINATING   ENGINEERING  PRACTICE 

(g)  At  points  between  lamps  where  intensity  is  feeble,  silhouetting  is 
effective. 

For  the  principal  purpose  of  the  street  lighting,  the  discernment  of  large 
objects,  the  serviceability  of  this  illumination  is  greater  midway  between 
lamps  than  it  is  at  the  point  of  highest  intensity.  (See  Figs,  u  and  12.) 

(h)  At  points  between  lamps  where  intensity  is  feeble,  brightness  is 
likely  to  be  maintained  at  a  fair  value. 

Fig.  24  shows  illumination  data  for  Avenue  A  between  68th  and  6gth 
Streets,  New  York  City.  You  are  asked  to  observe  that  in  the  region 
between  adjacent  lamps  along  a  line  halfway  between  the  curb  and  the 
center  of  the  street  the  variability  of  illumination  as  measured  by  ratio 
of  maximum  to  minimum  is  40  to  i  for  the  horizontal  illumination  and 
8.4  to  i  for  the  vertical  illumination.  Now  note  the  curve  of  street  bright- 
ness under  this  illumination  as  measured  at  an  angle  of  3  ^  degrees  (known 
colloquially  as  "chauffeurs'  angle"  and  illustrated  in  Fig.  23).  This 
brightness  is  as  nearly  uniform  as  might  be  wished,  the  ratio  of  maximum 
to  minimum  being  2.17  to  i.  A  similar  discrepancy  between  horizontal 
illumination  intensity  and  brightness  is  encountered  when  measurements 
are  made  along  a  line  transverse  of  the  street.  The  brightness  between 
lamps  when  viewed  at  "chauffeurs'  angle"  is  many  times  as  great  as  the 
normal  brightness  when  viewed  from  directly  above.  Measurements  of 
brightness  viewed  from  above  show  that  this  value  is  substantially  pro- 
portional to  the  horizontal  illumination  intensity. 

Avenue  A  is  paved  with  asphalt.  It  is  in  the  poorer  section  of  the 
city;  its  vehicular  traffic  consists  principally  of  horse-drawn  vehicles. 
Judging  the  pavement  in  the  daytime  an  inexperienced  observer  would 
conclude  that  it  offers  but  little  specularity;  certainly  it  is  less  specular 
than  is  the  average  asphalt  street,  yet  on  this  street  variable  illumination 
intensity  is  translated  into  practically  uniform  brightness  when  the  street 
surface  is  viewed  as  in  driving. 

As  a  matter  of  fact,  little  if  any  consideration  has  been  given  by  uni- 
formity adherents  to  the  importance  of  brightness  uniformity  as  distin- 
guished from  uniformity  of  some  component  of  incident  light.  This 
aspect  of  the  matter  was  perhaps  first  emphasized  by  the  lecturer  in  1910. n 

It  is  the  lecturer's  view  therefore  that  uniformity  of  incident  light 
on  the  street  is  unnecessary  because :  (i)  with  moderately  variable 
illumination,  because  of  more  favorable  direction  of  incident  light, 
one  sees  surface  irregularities  as  well  in  the  darker  regions  between 
lamps  as  in  the  more  brightly  lighted  regions;  (2)  one  sees  large 
objects  on  the  streets  as  silhouettes  in  the  dimly  lighted  regions  even 
more  surely  than  in  the  brightly  lighted  regions  and  (3)  the  ap- 
pearance of  the  street  surface  approximates  uniform  brightness 


MILLAR:  LIGHTING  OF  STREETS 


447 


even  with  marked  diversity  of  incident  light.  These  views  have 
been  fully  confirmed  in  the  investigations  of  street  lighting  effective- 
ness conducted  during  the  past  two  years  under  the  auspices  of 

LAMPS: 

•   PONY  BROAD  CARBON  OPEN  ARC  LAMPS 

o  4  AMPERE  MAGNETITE  LAMPS  H.  E.  ELECTRODES 

INSTALLATION: 

SPACING  210  FEET.          MOUNTING  HT.  22  FT. 
AVERAGE  FOR  ENTIRE  WIDTH  OF  STREET 


AVERAGE  PER. 
CEPTION  DISTANCE 


Fig.  25. — Illumination  intensity  and  target  findings. 

the  Street  Lighting  Committees  of  the  National  Electric  Light 

Association  and  the  Association  of  Edison  Illuminating  Companies. 

An  example  of  findings  in  local  observational  tests*  will  further 

•  Courtesy  of  the  Philadelphia  Electric  Company. 


448  ILLUMINATING   ENGINEERING   PRACTICE 

illustrate  these  points.14  Fig.  25  shows  the  distribution  of 
horizontal  and  vertical  illumination  along  the  street  under  two 
systems  of  lighting.  Below  these  curves  are  the  findings  in  observa- 
tional tests.  The  targets  (see  Fig.  34)  of  the  disc  type  were  found 
by  pedestrians  quite  as  generally  between  lamps  as  in  the  more 
brightly  lighted  regions  near  lamps.  The  targets  of  the  cylindrical 
type  were  seen  at  quite  as  great  distances  when  located  between 
lamps  as  when  located  near  lamps,  yet  the  horizontal  illumination 
intensities  under  these  two  systems  varied  respectively  from  23 
to  i  and  from  17  to  i. 

It  is  the  lecturer's  view  that  strict  uniformity  of  illumination  on 
all  but  the  most  brilliantly  lighted  streets  can  be  attained  only 
with  added  expense  and  with  some  difficulty;  that  its  attainment 
is  attended  by  disadvantages;  and  that  visibility  requirements  do 
not  demand  it.  It  is  submitted  therefore  that  a  moderate  diversity 
of  intensity  along  the  street  is  a  more  reasonable  criterion. 

GLARE 

Glare  in  street  lighting  manifests  itself  principally  in  reducing 
visual  ability,  in  causing  ocular  discomfort  or  annoyance,  and  in 
rendering  an  installation  less  attractive.  In  a  general  way  it  may  be 
said  that  methods  of  reducing  glare  influence  all  of  these  manifesta- 
tions, although  in  some  cases  the  effect  upon  one  manifestation  may 
be  more  pronounced  than  upon  another.  Little  is  known  concern- 
ing the  numerical  relations  involved  in  the  reduction  of  glare  in 
street  lighting.15 

We  do  know,  however,  that  glare  is  reduced:16 

i.  If  the  power  and  brightness  of  light  sources  at  observed  angles 
are  reduced. 

The  customary  method  of  reducing  glare  consists  in  surrounding  the 
light  source  with  a  diffusing  globe  of  as  large  size  as  may  be  regarded  as 
desirable.  It  is  to  be  remembered  that  assuming  complete  diffusion  by 
the  accessory  the  brightness  of  a  globe  decreases  inversely  as  the  square 
of  its  radius,  and  that  therefore  with  the  same  lamp  a  1 6-inch  globe  will 
be  only  40  per  cent,  as  bright  as  a  lo-inch  globe. 

A  more  elaborate  method  recently  advocated  and  embodied  in  some 
modern  practice  consists  in  limiting  within  a  lower  zone  which  is  relatively 
free  from  observation,  the  greater  part  of  the  light  flux,  and  allowing  little 
or  no  light  to  emanate  from  the  source  at  angles  which  will  be  observed 
in  the  ordinary  use  of  the  street.  This  method  is  open  to  the  objection, 
(i)  that  it  requires  for  successful  application  either  excessively  great 


MILLAR:  LIGHTING  OF  STREETS  449 

mounting  heights  or  very  short  spacing  intervals,  both  involving  large 
cost;  (2)  elimination  or  reduction  of  flux  at  angles  which  are  very  useful 
when  the  characteristics  of  specular  pavement  are  considered;  (3)  as  ap- 
plied thus  far  this  method  usually  carries  with  it  so  great  a  reduction 
in  the  light  delivered  upon  building  fronts  as  to  make  the  effect  unsat- 
isfactory on  streets  of  commercial  importance. 

2.  If  the  visible  region  immediately  contiguous  to  the  light  source 
is  made  bright. 

This  is  a  little  apprehended  effect.  It  is  probably  associated  with  the 
ocular  characteristic  described  elsewhere  according  to  which,  under  con- 
ditions of  twilight  vision,  retinal  sensitiveness  is  increased  if  the  area  of 
the  stimulus  is  increased.  One  reason  why  the  exposed  arc  of  the  magne- 
tite lamp  has  been  found  to  be  a  less  serious  source  of  glare  than  might 
have  been  anticipated  is  doubtless  the  presence  of  a  reflector  immediately 
adjacent  to  the  arc.  When  mounted  in  streets  any  light  source  presents 
a  less  serious  source  of  glare  if  it  is  seen  against  a  background  of  a  light- 
colored  building  than  it  does  if  its  background  is  in  darkness. 

3.  If  the  surfaces  viewed  are  made  bright. 

In  street  lighting  the  surface  viewed  is  generally  the  street  itself.  This 
may  be  increased  in  brightness  by  reason  of  more  powerful  lighting  or  by 
reason  of  a  higher  albedo,  or,  under  certain  conditions,  by  reason  of  in- 
creased specularity.  The  effect  of  glare  from  a  given  source  is  diminished 
if  the  street  surface  is  rendered  brighter  in  any  of  these  particulars. 

4.  If  the  visual  angle  between  light  sources  and  the  observed  sur- 
face is  increased. 

The  general  application  of  this  is  to  be  found  in  a  demand  for  increased 
mounting  heights  as  light  sources  become  more  powerful  and  more  bright. 
On  curved  roadways  this  finds  application  if  a  lamp  is  mounted  over  the 
inner  curb,  the  visual  angle  between  it  and  an  object  in  the  distance  be- 
yond the  curb  is  likely  to  be  small  and  the  glare  reduces  visibility  markedly. 
If  the  lamp  is  mounted  over  the  outer  curb,  the  visual  angle  between  it 
and  the  roadway  beyond  the  curb  is  increased  and  the  glare  is  less  serious 
(Fig.  32). 

5.  If  a  large  portion  of  the  field  of  view  is  illuminated. 

Also,  if  the  general  field  of  view  contains  many  lighted  surfaces,  the 
effect  of  glare  will  be  less  than  if  large  portions  of  the  field  of  view  are  left 
in  darkness. 

STREET  PAVEMENTS 

The  light  reflecting  qualities  of  street  pavements  both  as  respects 
reflection  coefficient  and  specularity  are  of  prime  importance  in  the 
street  lighting  problem.     A  street  pavement  which  is  naturally 
29 


45 O  ILLUMINATING   ENGINEERING   PRACTICE 

light  in  color  and  which  can  be  kept  from  darkening  seriously  under 
use  obviously  is  capable  of  enhancing  greatly  the  effectiveness  of  the 
street  lighting  provided  by  any  system.  Most  pavements  darken  in 
use,  especially  under  automobile  traffic,  where  oil  drippings  bring 
early  discoloration.  This  would  result  in  rendering  ineffective  the 
most  powerful  of  street  lighting  systems  if  it  were  not  for  the  saving 
fact  that  in  practically  all  such  cases  the  pavement  takes  on  a  con- 
siderable degree  of  specularity,  especially  under  automobile  traffic. 
Fig.  26  is  a  view  of  the  wooden  block  pavement  of  Columbus  Circle, 
New  York  City.  The  background  reveals  the  polish  resulting 
from  automobile  use.  The  foreground  consists  of  the  same  pave- 
ment which  is  not  traversed  by  automobiles  and  is  not  specular. 

The  rapidly  increasing  use  of  automobiles  is  exerting  a  marked  in- 
fluence on  this  aspect  of  the  street  lighting  problem  and  specularity 
of  street  pavement  must  now  be  taken  into  account  in  practically 
all  streets  that  are  lighted  artificially. 

The  higher  spots  of  the  pavement  take  on  a  polish  and  become 
small  mirrors  on  the  street  surface.  In  driving  one  views  the  pave- 
ment at  an  angle  of  perhaps  from  i  to  5  degrees.  Each  street  lamp 
may  be  seen  in  many  of  these  small  mirrors,  the  result  being  a  broken 
streak  of  light  along  the  street  not  unlike  moonlight  on  the  water. 
It  should  be  noted  that  it  is  the  distant  lamps  and  not  the  nearby 
lamps  which  are  seen  reflected  in  these  little  mirrors.  If  a  large 
number  of  distant  lamps  are  within  view  at  a  given  time,  especially  if 
they  are  distributed  across  the  street,  the  result  will  be  many  streaks 
of  light  side  by  side,  all  contributing  to  render  the  pavement  bright. 
Generally  speaking,  specular  pavements  are  dark  in  color,  and  under 
diffused  light,  as  in  the  daytime,  appear  incapable  of  reflecting  light 
advantageously  as  compared  with  other  pavements.  At  night, 
however,  specular  pavements  usually  exhibit  characteristics  which 
promote  visibility.  Figs.  27  and  28  for  example  are  comparisons 
between  suburban  roads,  one  roadway  in  each  case  being  specular 
and  the  other  a  dirt  road  which  does  not  reflect  light  specularly. 
While  the  lighting  systems  are  not  strictly  comparable,  yet  both 
comparisons  indicate  the  fact  of  favorable  reflection  from  the  specu- 
lar pavements  and  unfavorable  reflection  from  the  mat  surface 
pavements.  Fig.  29  is  a  view  of  a  fairly  wide  street  lighted  by  lamps 
which  are  mounted  over  curbs.  It  will  be  observed  that  there  are 
many  such  streaks  of  light  along  the  sides  of  the  street,  but  that  the 
center  of  the  street  appears  dark.  This  installation,  by  the  way, 
was  designed  to  produce  uniform  illumination. 


.  26.— Showing  effect  of  automobile  traffic  (background)  in  polishing  pavement. 


Fig.  27. — Mat  and  specular  road  surfaces. 

(Facing  Page  450.) 


Fig.  28. — Mat  and  specular  road  surfaces. 


Fig.  29. — Uniform  intensity  of  incident  light.     Variable  brightness. 


Fig.  30. — Driveway  illuminated  by  three  rows  of  lamps. 

(Facing  insert  Figs.  28  and  29. ) 


Fig.  31. — Two  views  of  a  drive,  without  and  with  lamps  concealed. 


MILLAR:  LIGHTING  OF  STREETS  451 

On  the  other  hand,  Fig.  30  is  an  illustration  of  one  driveway  of  a 
wide  street  illuminated  by  three  rows  of  lamps.  The  streaks  of 
light  are  in  this  case  distributed  to  better  advantage  across  the  street, 
creating  a  general  appearance  of  uniformity  which  was  lacking  in 
Fig.  29.  The  characteristics  of  street  pavements  here  introduce 
a  condition  which  makes  the  skillful  location  of  light  sources  a  most 
important  factor,  affecting  the  value  of  the  street  illumination  to 
the  detriment  of  intensity  considerations. 

Fig.  31  offers  two  views  of  a  drive  in  Central  Park,  New  York, 
lighted  by  lamps  which  are  mounted  n  feet  over  each  curb  and 
spaced  along  each  curb  at  intervals  of  about  75  feet  staggered.  At 
the  top  the  view  shows  usual  lighting  conditions.  The  view  below 
shows  the  results  when  the  lighting  is  modified  by  covering  the  lamps 
with  white  pasteboard  reflectors  which  limit  the  light  below  an 
angle  of  65  degrees.  These  effectually  conceal  the  lamps  from  view 
and  increase  materially  the  light  on  the  street  within  the  illuminated 
area.  They  leave  the  pavement  relatively  dark  between  lamps. 
As  the  pavement  reflects  specularly  the  downward  lighting  is  not  so 
effective  as  it  would  be  otherwise.  The  reflectors  eliminate  glare 
but  at  the  same  time  make  it  impossible  to  avail  of  the  advantageous 
reflecting  qualitities  of  the  pavement  by  intercepting  all  of  the  light 
which  would  be  reflected  specularly  to  the  user  of  the  drive.  It  is 
possible  that  if  this  pavement  reflected  diffusely,  the  covered  lamps 
might  provide  superior  lighting.  With  the  specular  pavement  they 
undoubtedly  provide  an  inferior  lighting. 

LAMP  LOCATIONS 

In  locating  lamps  on  the  principal  business  streets  of  a  city,  a 
standard  arrangement  is  usually  desired  and  required  and  but  little 
deviation  is  warranted.  The  precise  location  of  the  lamps  is 
relatively  unimportant  as  far  as  illumination  is  concerned. 

In  secondary  streets  where  the  lamps  are  likely  to  be  rather 
inadequate,  it  is  often  desirable  to  mount  them  well  out  from  the 
curb  either  on  suspensions  or  on  mast-arm  posts.  Usually  such 
streets  do  not  have  many  trees,  and  the  sidewalk  lighting  does 
not  suffer  as  a  result  of  the  central  mounting. 

In  residence  streets,  where  trees  are  likely  to  abound,  the  loca- 
tion of  the  lamps  becomes  much  more  important,  the  more  so  since 
fewer  lamps  are  employed  and  the  utmost  must  be  made  of  the 
materials  at  hand.  So  far  as  the  roadway  is  concerned,  it  is  usually 


452  ILLUMINATING   ENGINEERING   PRACTICE 

difficult  to  improve  upon  a  location  over  the  middle  of  the  street  low 
enough  to  escape  serious  interference  from  trees  and  high  enough 
to  avoid  serious  glare.  In  such  lighting,  however,  the  sidewalk  is 
likely  to  be  neglected.  If  the  lamps  are  mounted  low  over  the  curbs, 
in  order  to  keep  the  light  well  beneath  the  limbs  of  the  trees,  the 
sidewalk  is  likely  to  be  taken  care  of  to  a  somewhat  better  degree, 
but  the  roadway  lighting  is  likely  to  be  ineffective.  Abroad  to 
some  extent  a  combination  of  the  two  lamp  locations  has  been 
found  effective,  large  lamps  being  mounted  over  the  middle  of  the 
streets  at  intersections,  small  supplementary  lamps  being  mounted 
low  over  the  curbs  between  street  intersections.  Similar  arrange- 
ments have  been  tried  in  this  country. 

In  outlying  districts,  parks,  etc.,  lamp  locations  are  usually  some- 
what optional.  Here  the  illuminating  engineer  has  an  opportunity 
to  exhibit  his  skill  as  an  engineer  and  as  an  artist.  By  studying 
the  topography  and  curvature  of  the  roadway,  by  making  due 
allowance  for  glare,  and  by  taking  full  advantage  of  pavement 
specularity,  the  skillful  engineer  may  so  locate  his  lamps  as  to  obtain 
with  a  given  expenditure  much  more  effective  street  lighting  than 
could  be  had  with  perfunctory  location  of  the  lamps.  An  excellent 
illustration  of  the  importance  of  lamp  location  under  such  condi- 
tions is  afforded  by  the  two  views  in  Fig.  32.  In  the  one  a  lamp  is 
mounted  over  the  inner  radius  of  a  curve  in  an  automobile  driveway. 
There  is  a  great  deal  of  light  on  the  pavement  at  the  curve,  but  the 
roadway  beyond  is  obscured.  The  glare  is  very  serious.  In  the 
other  view  the  lamp  has  been  moved  to  the  outer  curb  of  the  curved 
roadway.  There  is  less  light  upon  the  pavement  in  the  foreground, 
but  the  curb  can  be  seen  readily.  The  distant  roadway  may  be 
seen  readily  as  a  result  of  specular  reflection  from  the  next  lamp, 
which,  by  the  way,  is  located  at  a  distance  of  about  900  feet. 

SUMMARY 

Summing  up  the  foregoing  comments  on  the  design  of  street 
illumination,  it  will  be  noted  that  the  simple  method  of  calculating 
flux  on  the  street  leads  to  the  ready  establishment  of  relations  in- 
volving the  location  of  the  lamp,  its  height  and  its  equipment.  Not 
only  must  the  total  flux  delivered  upon  the  street  surface  be  taken 
into  account,  but  the  amount  of  light  delivered  upon  building  fronts 
is  important.  Moreover,  the  variability  of  the  illumination  along 
the  street  requires  careful  thought,  especially  where  low  intensities 


MILLAR:  LIGHTING  OF  STREETS  453 

prevail.  The  means  for  reducing  glare  are  indicated,  the  character- 
istics of  street  pavements  are  illustrated  and  the  importance  of 
this  factor  is  emphasized.  The  possibilities  of  improvement  by 
skilful  lamp  location  is  the  last  point  mentioned. 

^COMPARISON  AND  TESTS 

As  street  illumination  is  generally  supplied  under  a  contract  be- 
tween a  municipality  and  a  public-service  corporation,  there  is  an 
ever-present  desire  to  provide  some  means  of  proving  the  adequacy 
of  the  service  rendered.  In  the  past  too  much  emphasis  has  perhaps 
been  placed  in  this  connection  upon  the  candle-power  of  the  lamps 
or  upon  the  illumination  intensity.  It  is  evident  that  a  street-light- 
ing service  must  include  such  important  elements  as  reliability  and 
continuity  of  operation,  good  maintenance  of  lamps,  poles,  lines, 
etc.,  and  a  satisfactory  attitude  on  the  part  of  the  contracting  com- 
pany as  well  as  reasonably  good  maintenance  of  the  candle-power 
of  the  lamps.  Lamps  may  be  shown  to  be  of  adequate  candle-power 
and  yet  the  service  in  general  may  be  unsatisfactory.  On  the  other 
hand,  the  lamps  at  times  may  not  be  quite  up  to  par  in  candle-power 
and  yet  the  street-lighting  service  as  a  whole  maybe  eminently  satis- 
factory. It  is  desired  therefore  to  deprecate  the  tendency  of  the 
past  to  over-emphasize  this  one  phase  of  street-lighting  service  to 
the  exclusion  of  other  equally  important  features. 

Nevertheless  for  engineering  or  political  reasons  the  demand 
recurs  for  a  measure  of  the  iUuminating  value  of  street-lighting 
systems.  It  has  been  the  writer's  privilege  during  the  past  six 
years  to  be  closely  identified  with  efforts  which  have  been  put  forth 
in  this  country  to  solve  the  problem  of  providing  a  satisfactory 
measure  of  street-lighting  values  for  this  purpose,  and  the  statements 
on  this  subject  which  follow  are  largely  based  upon  the  experience 
which  he  has  had  in  the  conduct  of  investigations  in  this  field  for 
the  Street  Lighting  Committees  of  the  National  Electric  Light 
Association  and  of  the  Association  of  Edison  Illuminating  Companies. 

The  problem  of  testing  street  illumination  is  divided  naturally 
into  two  parts.  The  first  has  to  do  with  means  of  determining 
whether  or  not  a  stipulated  lighting  service  is  being  rendered;  the 
second  has  to  do  with  the  determination  of  the  relative  illuminating 
value  of  two  different  street-lighting  systems. 

The  first  of  these  is  by  far  the  simpler.  A  contract  or  specifica- 
tion for  street  lighting  under  which  tests  are  to  be  performed  ought 


454  ILLUMINATING   ENGINEERING   PRACTICE 

to  include  a  description  of  the  lamps  and  accessories  to  be  em- 
ployed and  a  statement  of  their  photometric  values,  including  the 
total  flux  of  light,  the  candle-power  distribution  curve  and  a  range 
of  toleration  above  and  below  the  standard  within  which  the  lamps 
may  be  allowed  to  fluctuate.  Test  of  fulfilment  of  this  part  of 
the  contract  then  consists  in  determining  the  total  flux  of  light  of 
the  lamps.  This  may  be  accomplished  either  by  determining  the 
operating  electrical  values  of  the  lamps,  removing  them  from  the 
circuit  and  sending  them  to  a  laboratory  in  their  operating  condi- 
tion, or,  where  practicable,  in  subjecting  them  to  test  in  situ  by 
bringing  an  integrating  sphere  photometer  to  the  street  for  the 
purpose  (see  Fig.  33).  These  methods  are  not  simple.  Such  tests 
do  not  need  to  be  made  often,  but  in  the  event  of  a  serious  question 
arising  concerning  the  adequacy  of  the  service,  they  afford  means 
which  experience  has  shown  to  be  most  reliable  for  determining 
accurately  the  illuminating  values  in  terms  of  the  contractual 
provision. 

The  history  of  attempts  to  arrive  at  a  satisfactory  method  of 
testing  street  illumination  is  a  record  of  confusion,17  and  it  now 
appears  that  much  of  the  confusion  has  arisen  as  the  result  of  a  vain 
attempt  to  adopt  some  method  which  would  at  once  prove  fulfil- 
ment of  contractual  obligation  and  indicate  the  usefulness  of  the 
illumination.  Thus  the  1907  "Committee  to  Consider  Specifica- 
tions for  Street  Lighting"  of  the  N.  E.  L.  A.  sought  a  measure  of 
the  illuminating  value  and  finally  recommended  the  mean  normal 
illumination  produced  by  a  lamp  in  the  street  at  the  height  of  the 
observer's  eye  and  at  a  distance  of  not  less  "than  200  and  not  more 
than  300  feet  from  a  point  immediately  below  the  lamp,  as  com- 
pared with  the  illumination  provided  by  a  standard  lamp  under  like 
conditions.  It  would  appear  likewise  that  the  Joint  Committee  on 
Street  Lighting  Specifications,  which  has  labored  recently  in  Eng- 
land, was  actuated  by  a  desire  to  combine  both  purposes  when  they 
recommended  the  minimum  horizontal  illumination  as  a  measure 
of  street  lighting.  The  whole  problem  is  immensely  simplified 
if  the  purpose  of  proving  fulfilment  of  contract  is  divorced  once  and 
for  all  from  the  purpose  of  comparing  relative  illuminating  effective- 
ness of  different  street-lighting  systems.  Considering  exclusively 
the  latter  problem,  let  it  be  noted  that  a  difficulty  which  existed 
until  recently  has  been  the  lack  of  any  method  for  definitely  com- 
paring the  illuminating  effectiveness  of  street-lighting  systems. 
There  has  been  no  way  to  determine  whether  the  notion  of  minimum 


Fig.  32a. — Good  lighting  of  curve- 


Fig-  33- — Integrating  sphere  photometer  in  service  tests. 


Fig.  34. — Target  painted  similar  to  street  surface. 


MILLAR:  LIGHTING  OF  STREETS  455 

illumination  or  of  average  vertical  illumination  provides  the  nearer 
approach  to  a  real  measure  of  effectiveness.  There  has  been  no 
test  for  such  proposed  measures.  It  has  been  my  privilege,  with 
the  aid  of  associates  at  the  Electrical  Testing  Laboratories,  to  devise 
and  with  the  assistance  of  the  Street  Lighting  Committees  which 
have  been  named  to  put  into  effect  certain  methods  calculated  to 
furnish  a  means  for  determining  street-lighting  effectiveness  in 
several  important  respects  and,  therefore,  for  testing  these  several 
proposed  measures  of  effectiveness.  These  methods  are  described 
in  detail  elsewhere.18  They  consisted  first  in  classifying  the  pur- 
poses of  street  illumination  through  inquiry  and  consultation  into 
the  following  three  principal  purposes: 

Discernment  of  objects  on  the  street. 
Discernment  of  surface  irregularities. 
Esthetic  qualities. 

Second,  in  devising  tests  to  obtain  relative  discernment  values  for 
large  objects  on  the  street  and  for  small  objects  on  the  street  surface 
and  in  recording  opinions  of  qualified  observers  in  respect  to  the 
aesthetic  qualities.  The  means  for  measuring  discernment  which 
were  finally  adopted  consisted  in  determining  the  maximum  dis- 
tances at  which  automobilists  could  see  targets  painted  similar 
to  the  street  surface  (see  Fig.  34)  and  in  determining  the  percentage 
of  small  targets  similarly  painted  which  pedestrians  could  find  in 
walking  through  the  street.  By  carefully  eliminating  variables  and 
standardizing  conditions,  comparative  discernment  values  were 
obtained  for  any  two  systems  compared  upon  the  same  street  at 
one  and  the  same  time  by  a  given  group  of  observers  (see  Fig.  34). 
It  is  the  writer's  opinion  that  these  observational  tests,14  when 
impartially  conducted,  provide  a  closer  approximation  of  the  real 
illuminating  values  of  street-lighting  systems  than  have  heretofore 
been  available,  and  are  the  only  reasonably  adequate  means  thus  far 
developed  for  testing  the  validity  for  proposed  measures  of  street 
lighting  effectiveness.  With  the  results  of  the  Street  Lighting 
Committees'  investigations  before  us,  it  is  possible  to  put  these  pro- 
posed measures  to  the  test  and  to  ascertain  whether  or  not  they 
afford  reliable  measures  of  lighting  effectiveness.  From  the  mass 
of  data  available  on  this  subject,  I  have  selected  a  few  striking  in- 
stances illustrating  the  weaknesses  of  the  several  proposed  measures ; 
these  are  presented  in  the  following  table. 


456 


ILLUMINATING   ENGINEERING   PRACTICE 


Description 

Proposed 
measure  of 
lighting 

Ratio  A  to  B 

Comment 

Proposed 
measure 

a 

If 
Jh 

0 

System  A 

System  B 

250-c.p.       lamps, 

25o-c.p.  lamps, 

Flux  in  low- 

0.72 

1.03 

With  system  B  all  light  was 

light       diffusing 

refractor,     24 

er      hemi- 

concentrated   in    the    lower 

globes,    24    feet 

feet^overjnid- 

sphere. 

hemisphere.       Much    of    it 

over    middle    of 

dlejof  street, 

was  redirected  at  angles  well 

street,    150   feet 

i5O_feet^apart. 

up    toward    the    horizontal. 

apart. 

The    greater    bulk    of    such 

light,    of    course,    fell  upon 

houses,  trees  or  lawns  and 

did  not  contribute  materially 

to  the  street  lighting.     As  a 

result  substantially  the  same 

total  quantity  of  light  was 

delivered    upon    the    street 

from  system  A  and  system  B. 

2SO-c.p.    lamps, 

2SO-c.p.  lamps, 

Intensity 

0.41 

1.03 

The  refractor  equipment  re- 

light    diffusing 

refractor,      24 

10°    below 

ceives  an  especially  favora- 

globes,   24    feet 

feet  over  mid- 

horizontal. 

ble  rating   in   terms    of    in- 

over   middle    of 

dle   of   street, 

tensity  10°  below  horizontal. 

street,    150   feet 

1  50  feet  apart. 

It  is  very  evident  from  the 

apart. 

comments  made  above  that 

there  is  no  such  difference  in 

illuminating    value    as    this 

form  of  rating  would  indi- 

cate. 

125-c.p.    lamps, 

250-c.p.  lamps, 

Minimum 

2.6 

0.95 

light     diffusing 

light  diffusing 

horizontal 

globes,     1  8    feet 

globes,  24  feet 

illumina- 

over   middle    of 

over  middle  of 

tion. 

street,     75     feet 

street,  150  feet 

apart. 

apart. 

Each  of  these  three  proposed  measures  which  has  been  prominently 
urged  upon  attention  in  recent  years  is  shown  by  these  single  in- 
stances to  be  in  valid,  as  judging  by  the  observational  tests  which  have 
been  made.  It  does  not  seem  necessary  to  cite  other  instances  or  to 
justify  the  observational  tests  as  a  basis  for  judging  the  proposed 
measures.  It  would  seem  to  be  evident  that  each  of  the  latter 
fails  to  measure  real  lighting  effectiveness  and  that  each  offers  a 
basis  of  rating  of  which  advantage  can  easily  be  taken  to  secure  a 
higher  rating  for  illuminants  of  no  greater  illuminating  value.  Both 
the  rating  in  terms  of  intensity  10  degrees  below  the  horizontal  and 
the  rating  in  terms  of  minimum  horizontal  illumination  have  to  do 
with  light  at  a  particular  angle,  and  the  rating  may  easily  be  in- 

*  Automobilists*  and  pedestrians'  tests  combined  with  equal  weight. 


MILLAR:  LIGHTING  OF  STREETS  457 

fluenced  by  altering  such  light  without  changing  the  illuminating 
value.  Safer  and  more  reliable  but  still  inadequate  measures  are 
afforded  by  the  average  horizontal  illumination  and  the  average 
vertical  illumination  on  the  street.  All  of  these,  however,  fail  to 
give  any  credit  for  light  directed  above  the  horizontal,  which  in 
modern  street-lighting  practice,  holds  a  very  definite  value.  The 
total  flux  of  light  is  a  fairly  reliable  measure,  comparing  favorably 
with  the  best  of  the  others,  but  inadequate  in  that  it  fails  to  dif- 
ferentiate between  desirable  and  undesirable  distribution  character- 
istics. As  previously  stated,  experience  in  the  work  of  this  Com- 
mittee results  in  the  view  that  the  most  useful  single  method  of 
rating  is  to  be  had  by  combining  with  a  statement  of  the  total  light 
flux  or  the  mean  spherical  candle-power  a  candle-power  distribution 
curve.  The  two  should  then  be  interpreted  according  to  best  judg- 
ment, the  distribution  characteristic  being  considered  in  the  light 
of  the  facts  first,  that  where  it  is  desirable  to  illuminate  building 
fronts,  a  certain  proportion  of  the  light  should  preferably  be  directed 
above  the  horizontal;  and  second,  best  lighting  effects  in  general 
are  obtained  with  a  moderate  diversity  of  illuminating  intensity 
along  the  street,  avoiding  on  the  one  hand,  that  degree  of  non- 
uniformity  which  results  in  unlighted  areas  between  the  lamps  and, 
on  the  other  hand,  that  degree  of  uniformity  which  tends  to  reduce 
contrast  and  definition. 

Measures  which  have  been  proposed  cannot  be  relied  upon  to 
afford  an  accurate  and  final  test  of  street-lighting  effectiveness. 
They  may  show  the  amount  of  light  produced  and  something  of  its 
distribution.  While  these  are  important  factors,  yet  they  do  not 
give  wholly  complete  information.  No  appraisal  of  a  street-lighting 
installation  can  be  made  reliable  until  in  addition  to  these  facts 
information  is  available  including  a  complete  description  of  the 
illuminants  and  their  accessories,  the  candle-power  distribution 
characteristic,  the  location  of  the  lamps,  including  height  and 
spacing,  and  the  brightness  of  the  light  source  in  directions  in  which 
it  is  likely  to  be  viewed.  If  all  of  this  information  were  available, 
one  could  form  a  good  idea  of  the  merits  of  a  street-lighting  system 
for  general  purposes,  but  its  effectiveness  when  installed  on  a  par- 
ticular street  could  not  be  approximated  unless  in  addition  informa- 
tion were  available  including  a  photograph  and  description  showing 
the  buildings  and  trees  along  the  street,  and  indicating  in  a  general 
way  the  extent  and  character  of  traffic  and  the  criminal  hazard  in 
that  locality. 


458  ILLUMINATING   ENGINEERING   PRACTICE 

If  these  statements  are  correct,  it  must  be  evident  that  there  has 
not  yet  been  devised  any  thoroughly  satisfactory  measure  of  street 
lighting  which  can  be  employed  to  show  street  lighting  effectiveness. 
Through  test  data  and  capable  observation  the  relative  effectiveness 
of  two  different  systems  of  lighting  may  be  established,  but  more 
than  this  cannot  be  said  at  the  present  time. 

For  proving  contractual  obligations  the  selection  of  a  suitable 
measure  is  simpler  and  nothing  seems  indicated  which,  upon  the 
whole,  is  quite  so  generally  satisfactory  as  a  measurement  of  the 
total  flux  of  light.  Another  question  arises  in  this  connection, 
however,  which  is  not  so  simple  to  dispose  of.  This  is  the  proper 
sampling  of  lamps  in  order  to  secure  reliable  indication.  It  should 
be  taken  for  granted  that  the  purpose  of  testing  is  to  secure  represen- 
tative and  reliable  information  regarding  the  service,  and  that  the 
purpose  of  including  test  provisions  in  specifications  for  street  light- 
ing is  to  hold  the  service  up  to  a  high  standard,  reducing  to  a  mini- 
mum the  number  of  individual  lamps  which  are  of  inadequate  illumi- 
nating power  and  providing  an  additional  incentive  to  keep  the 
average  illuminating  power  at  a  reasonably  high  value.  Starting 
with  this  assumption  it  is  regarded  as  good  practice  to  select  a 
particular  area  of  district  for  investigation  to  secure  representative 
sample  lamps  from  such  a  district  and  to  provide  for  reliable  tests 
of  such  samples.  The  selection  of  samples  ought  to  be  made  with- 
out prejudice  and  without  a  knowledge  of  the  condition  of  the  lamps 
which  are  chosen  for  test.  Where  lamps  are  of  the  incandescent 
type,  the  testing  work  is  simplified  and  the  following  sample  sched- 
ule may  be  expected  to  provide  a  reasonable  sampling. 

SAMPLING  SCHEDULE 

Number  of  lamps  in  district  Minimum  number  or  per  cent. 

investigated.  lamps  to  be  tested. 

Less  than  300  130  lamps 

300-  499  10  per  cent. 
500-2499  7 .  5  per  cent 

2500-9999  5.0 

The  lamps  ought  to  be  tested  in  their  operating  condition  if 
practicable,  either  by  bringing  an  integrating  sphere  to  the  lamps  in 
the  street  or  removing  the  lamps  after  ascertaining  their  operating 
values  and  having  them  tested  in  a  suitable  laboratory.  The  average 
illuminating  power  of  the  samples  tested  ought  to  be  regarded  as 
applicable  only  to  the  district  investigated. 


MILLAR:  LIGHTING  OF  STREETS  459 

Bibliography 

1  Report  of  Committee  on  Street  Lighting.     Transactions  National  Electric 
Light  Association,  Technical  Section,  1914,  page  589. 

2  Report  of  Special  Committee  on  Commercial  Aspects  of  Municipal  and 
Highway  Lighting.     National  Electric  Light  Association,  May,  1916. 

3  Report  of  Committee  on  Electric  Advertising  and  Decorative  Street  Lighting. 
Transactions  National  Electric  Light  Association,  Vol.  II,  1912,  page  188. 

4  C.  A.  B.  HALVORSON. — "Ornamental  Luminous  Arc  Lighting  at  New  Haven." 
General  Electric  Review,  1912,  page  221. 

&  WALDEMAR  KAEMPFFERT. — "Ornamental  Street  Lighting."  Published  by 
the  National  Electric  Light  Association,  Commercial  Section,  1912. 

6  F.  A.  VAUGHN.— "A  Practical  Application  of  the  Principles  of  Scientific 
Street  Lighting."     Transactions  Illuminating  Engineering  Society,  1910,  page 
282. 

7  L.  B.  MARKS.— "The  Invention  of  the  Enclosed  Arc  Lamp."     Sibley  Journal 
of  Engineering,  Vol.  XXII,  Oct.,  1907. 

8C.  P.  STEINMETZ.— "The  Magnetite  Arc  Lamp."  Electrical  World,  Vol. 
XLIII,  1904,  page  974. 

•I.  LANGMUIR  and  J.  A.  ORANGE. — "Tungsten  Lamps  of  High  Efficiency." 
Proceedings  American  Institute  of  Electrical  Engineers,  Vol.  XXXII,  1913,  page 

1935- 

10  "Refractor  for  Street  Lighting."     Electrical  World,  Vol.  LXIV,  1914,  page 

439- 

11  P.   S.   MILLAR. — "Some   Neglected   Considerations   Pertaining   to   Street 
Lighting."     Transactions  Illuminating  Engineering  Society,  1910,  page  653. 

12  P.  S.  MILLAR. — "An  Unrecognized  Aspect  of  Street  Illumination."     Trans- 
actions Illuminating  Engineering  Society,  Vol.  V,  1910,  page  546. 

13  A.    J.    SWEET. — "An   Analysis   of   Illumination   Requirements   in    Street 
Lighting."     Journal  of  the  Franklin  Institute,  1910. 

14  P.  S.  MILLAR. — "Tests  of  Street  Illumination."     Transactions  Illuminating 
Engineering  Society,  Vol.  XI,  1916,  page  479. 

15  A.  J.  SWEET. — "Glare  as  a  Factor  in  Street  Lighting."     Electrical  Review 
and  Western  Electrician,  Vol.  XL VI,  1915,  page  439. 

16  P.  S.  MILLAR. — "Effective  Illumination  of  Streets."  Transactions  American 
Institute  of  Electrical  Engineers,  Vol.  XXXIV,  1915,  page  1429. 

17  J.    W.   LIEB,    Chairman. — "Report   of    Committee   on   Street  Lighting." 
Transactions  National  Electric  Light  Association,  1913,  page  357. 

18  "Report  of   Street  Lighting  Committee."  Transactions  National  Electric 
Light  Association,  1914,  page  589,  and  1915,  page  710;  also  P.  S.  MILLAR. — 
"Tests  of  Street  Illumination."  Transactions  Illuminating  Engineering  Society, 
1916,  page  479. 


THE  LIGHTING  OF  STREETS— PART  II 

BY   C.   F.   LACOMBE 

The  subject  of  street  lighting  in  this  lecture  course  has  been  divided 
between  Mr.  P.  S.  Millar  and  myself,  and  as  the  subject  has  been 
summarized  in  the  syllabus  of  the  course  by  the  Committee  on 
Lectures,  the  main  subjects  assigned  to  me  will  be  taken  up  without 
further  introduction. 


REQUIREMENTS  OF  CITY  LIGHTING 

The  problem  of  lighting  a  city  is  to  distribute  the  illumination  in 
proportion  to  the  streets,  within  the  funds  available,  from  that  of 
the  most  congested  thoroughfare  to  that  of  a  sparsely  settled  suburb ; 
the  maximum  requirement  being  the  lighting  of  great  squares  and 
street  intersections  under  conditions  of  heavy  congestion  of  traffic; 
and  the  minimum  being  that  necessary  for  policing  the  city  and  the 
prevention  of  accidents.  One  should  therefore  study  the  classes 
of  streets  existing  in  cities  of  various  grades.  We  may  arrange  the 
grades  of  cities  and  classes  of  streets  about  as  follows: 


Grades  of  cities,  by 
population 


Class  of  street 


Description  of  use 


I — 500,000  and  over 

II — 250,000  to  500,000 

III — 100,000  to  250,000 

IV — Less  than  100,000 


Special  or  Class  AA 
Class  A 

Class  B 
Class  C 

Class  D 
Class  E 
Class  F 

Class  G 


Very  important.  Crossing  of 
great  streams  of  traffic. 

Important  streets,  greatly  used 
at  night. 

Well  used  streets. 

Ordinary  night  use,  best  resi- 
dence streets. 

Ordinary  residence. 

Suburban  residence. 

Parkways  or  boulevards  and 
suburban  roads. 

Connecting  country  thorough- 
fares, State  or  County  roads. 


In  all  cities  certain  streets  pass  from  one  class  to  another  in  their 
course  and  some  care  is  necessary  in  classifying  them  for  lighting 

461 


462  ILLUMINATING    ENGINEERING    PRACTICE 

intensities;  the  different  grades  of  streets  can  perhaps  be  best 
identified  by  some  examples: 

Class  A  A. — Parts  of  Wabash  Avenue  and  Dearborn  Street, 
Chicago;  Broad  Street,  Chestnut  and  Walnut  Streets,  Philadelphia; 
Times  Square  and  portions  of  Broadway  and  Fifth  Avenue,  New 
York. 

Class  A. — Parts  of  these  same  streets  contiguous  to  the  most 
congested  sections;  such  as  streets  within  the  Loop,  Chicago;  Fifth 
Avenue,  Pittsburgh;  Broadway  from  6oth  Street  to  72d  Street, 
New  York;  parts  of  Walnut,  Twelfth  and  Market,  Chestnut  and 
Broad  Streets,  Philadelphia. 

Class  AA  streets  ra/rely  exist  outside  the  very  largest  cities  of 
Grade  I  in  the  country. 

Class  A  streets  represent  the  most  important  streets  in  the  usual 
city  between  250,000  and  500,000  inhabitants  of  Grade  n,  and  the 
White  Ways  of  smaller  cities.  For  instance,  parts  of  Pennsylvania 
Avenue,  Washington;  Grand  Avenue,  Milwaukee;  Main  Street, 
Rochester;  Seventh  and  Third  Avenues,  New  York. 

Class  B  streets  are  those  probably  greatest  in  number  in  all 
cities  of  any  size  in  the  country.  These  are  well  used,  often  have 
street  railways  on  them,  with  wholesale  or  retail  stores,  and  usually 
toward  their  farther  ends  develop  into  residence  streets,  for  instance: 
Broad  Street,  Newark,  and  upper  Broadway,  New  York. 

Class  C  streets  are  streets  of  ordinary  night  use,  except  as 
they  may  lead  to  amusement  centers  or  contain  street-car  lines. 
Examples  are:  Commonwealth  Avenue,  Boston;  Park  Avenue, 
New  York;  Michigan  Avenue,  Chicago. 

Class  D  streets  are  usual  residence  streets  of  good  quality 
throughout  cities  generally.  In  the  larger  cities  the  houses  are 
in  blocks  of  buildings,  but  in  smaller  cities  this  class  of  streets  gener- 
ally develops,  even  near  the  center,  into 

Class  E  or  suburban  residence  streets  with  detached  houses, 
and  usually  full  of  trees. 

Class  F  streets,  or  special  boulevards  and  parkways,  have  been 
developed  of  late  by  demands  of  more  rapid  transportation,  which 
has  been  made  possible  by  motor  cars,  and  in  this  way  the  country 
has  been  brought  in  close  contact  with  the  city.  Usually  no  street- 
car traffic  exists  on  these  roads  and  they  are  used  almost  exclusively 
by  automobiles  running  at  a  speed  of  20  miles  an  hour,  or  over. 
Examples  of  this  are:  the  Shore  Boulevard  from  Boston  to  Lynn, 


LACOMBE:  STREET  LIGHTING  463 

the  Ocean  Parkway  to  Coney  Island,  the  Parkway  Systems  of 
Chicago  and  Boston. 

For  the  same  reason,  increased  ability  to  travel  at  high  speeds, 
these  boulevard  systems  may  be  said  to  be  further  extending  into 
cross-country,  county,  state  and  national  highways  which  can  be 
classified  as  Class  G  roads.  Examples  of  this  are  certain  roads  in 
New  Jersey:  for  instance,  between  Jersey  City  and  Paterson;  certain 
roads  in  Eastern  New  York;  Nahant  Road  near  Nahant,  Mass.; 
Lincoln  Highway  between  Jersey  City  and  Newark,  and  the  same 
highway  in  Salt  Lake  County,  Utah.  Comparatively  little  has  yet 
been  done  in  lighting  such  highways,  but  the  prospect  is  encouraging. 

Assuming  that  these  grades  of  streets  cover  the  general  scale  of 
street  lighting,  they  will  receive  different  amounts  of  lighting  in- 
tensity, somewhat  in  accord  with  the  necessities  of  vision,  but  the 
dominant  factor  is  the  amount  of  money  devoted  to  street  lighting 
by  various  municipalities.  The  question  of  appropriations  really 
decides  the  type  of  lighting  that  can  be  planned  for  a  given  city, 
assuming  always  it  is  properly  proportioned  to  the  use  of  the  respec- 
tive streets.  It  appears  that  at  present  these  appropriations  are 
usually  too  small  and  the  amount  and  intensity  of  lighting  too  low 
for  the  reason  that  within  the  last  fifteen  years  automobile  traffic  has 
developed  in  its  entirety,  and  the  congestion  in  night  centers  has 
increased  in  proportion  to  the  population  and  to  the  increase  of 
transportation  facilities.  In  this  regard  it  is  well  to  remember  that 
while  the  development  of  better  illuminating  appliances  and  the 
demands  for  better  lighting  have  increased  the  intensity  of  interior 
illumination  probably  five  or  six  fold  in  the  last  fifteen  years,  general 
street  illumination  has  increased  but  little. 

PHENOMENA  OF  VISION 

The  faculty  of  vision  ranges  from  full  direct  sensation  from  re^ 
fleeted  light  to  what  is  termed  adaptation  to  darkness,  where  we 
can  distinguish  only  shades  and  contrasts.  As  you  know,  the 
retina  of  the  eye  receives  light  upon  an  arrangement  of  sensitive 
nerve  termini  known  as  rods  and  cones.  In  this  arrangement  the 
cones  are  rather  at  the  center  of  the  retina  and  the  rods  with  scattered 
cones  radiate  toward  the  periphery.  The  cones  are  supposed  to 
give  us  the  sensations  of  color,  and  detail  we  have  with  direct  vision 
by  reflection,  such  as  we  get  in  daylight  or  under  high  artificial 
illumination.  As  the  intensity  of  the  light  diminishes,  the  cones 


464  ILLUMINATING   ENGINEERING   PRACTICE 

begin  to  lose  their  sensibility,  we  slowly  lose  the  sense  of  color,  dis- 
tinctness and  so  on,  and  our  vision  is  restricted  largely  to  the  sensa- 
tion given  by  the  rods.  These  rods  retain  their  sensitiveness;  in 
fact,  this  sensitiveness  really  increases  with  darkness  until  in  full 
adaptation  it  has  become  very  acute.  Be  this  as  it  may,  however, 
rod  vision  is  poor  vision.  In  consequence,  as  Dr.  Bell  pointed  out 
in  the  Johns  Hopkins  lectures  of  1910,  there  is  a  physiological  divid- 
ing line  between  that  illumination  by  which  we  can  see  well  and 
with  ease,  and  one  by  which  we  can  only  see  forms  or  contrasts  and 
shadows.  It  is  the  first  class  of  vision  which  requires  the  most  ex- 
pensive lighting,  and  which  naturally  we  prefer ;  but,  unfortunately, 
on  account  of  insufficient  appropriations,  and  so  on,  we  generally 
have  to  deal  with  the  lower  grade  of  vision,  and  the  difficulties  in 
deVeloping  illumination  of  this  class  produce  most  of  our  problems. 

Referring  again  to  the  grades  of  streets,  direct  vision,  as  in  day- 
light, is  required  on  Class  AA  streets  and  areas,  and  to  a  large  degree 
on  Class  A  and  B  streets,  and  particularly  at  intersections  of  streets 
of  this  character.  Powerful  lighting  is  necessary  where  street-car 
lines  cross  each  other  and  turn  in  various  directions,  and  where 
intermingling  streams  of  pedestrians  are1  persistent  and  continuous.. 
Police  statistics  have  shown  that  the  greatest  number  of  street 
accidents  occur  at  places  where  traffic  crosses  or  changes  from 
one  direction  to  another.  In  consequence,  where  such  streets  cross, 
each  of  them  contributing  streams  of  traffic,  high  illumination  is 
required  for  safety  if  nothing  else.  If  such  lighting  is  provided  in 
these  places  one  has  a  degree  of  visual  acuity  approaching  that  of 
daylight,  and  feels  the  same  sense  of  safety  in  that  one  can  note  the 
color,  speed  and  detail  of  approaching  objects  and  so  direct  one's 
movements  as  to  avoid  them. 

Foreign  cities  have  developed  lighting  which  produces  direct 
vision  on  their  more  important  streets  to  a  greater  extent  than  in 
this  country.  We  have  few  examples  of  lighting  of  the  order  of 
5.25  foot-candles  average  horizontal  measurement  over  a  con- 
siderable length  of  street,  as  exists  in  London,  nor  have  we  any  such 
examples  of  high  illumination  as  is  shown  in  the  Potsdamer  Platz,  in 
Berlin,  where  an  average  of  1.75  foot-candles  is  developed  over  the 
entire  Platz. 

While  the  sense  of  vision  varies  with  the  individual,  this  direct 
vision  by  rods  and  cones  together  prevails  down  to  about  o.i  foot- 
candle;  below  that  we  begin  to  get  into  rod  vision,  not  entirely  by 
any  means  at  0.05  foot-candle,  but  completely  so  at  o.oi  foot- 


LACOMBE:  STREET  LIGHTING 


465 


candle.  When  low  orders  of  intensity  prevail  in  certain  sections  of 
a  city  and  reliance  must  be  placed  on  adaptation  of  the  eye  to  a 
certain  degree  of  darkness,  it  is  well  to  avoid  sudden  changes  to 
bright  lighting  within  the  section,  as  the  eye  adapts  itself  to  such 
changes  quite  slowly.  With  this  in  view,  we  can  approximate  a 
scale  of  illumination  for  the  various  requirements  of  the  grades  of 
streets.- 

REQUIREMENTS  OF  CITY  LIGHTING  FOR  SEVERAL  CLASSES  OF  STREETS 


Grade  of  city 

I 

500,000 
&  over 

II 
250,000 
&  over 

HI 

100,000 

&  over 

IV 
under 
100,000 

Degree  of  intensity 

Grade  of  street 

Hor.  foot  candles 

Average 

Minimum 

High.    Excess  at  street  crossings 
Giving  clear  vision 

AA 

A 

B 
C 

D 
E 
F 

AA 

A 

B 
C 

D 
E 
F 

G 

A 

White 
B 
C 

D 

E 
F 

A 

ways 
B 
C 

D 
E 

0.5 
0.35 

O.2 

0.075 
0.04 

0.02 

From  that 
D&Ewit 
o.oi     for 
roads. 

From  direc 
ing  merely 
interurbar 

0.25 

O.I 

0.05 

0.02 
O.OI 

0.003 

of   Classes 
bin  a  city  to 
interurban 

tional  light- 
T  to  that  of 
i  roads. 

Lower  but  vision  still  distinct  .  .  . 
Above  clear  moonlight  

About  clear  moonlight,  average 
higher  

Vision    by    silhouette    or    con- 
trast.    Dark  adaptation  
Vision    by    silhouette    or    con- 
trast.    Dark  adaptation  

Vision    by    silhouette    or    con- 
trast.    Dark  adaptation  

CLASS  AA. — FOREIGN  STREETS  AND  PLACES — 1913 


Horizontal  foot-candles 

Max. 

Aver. 

Min. 

Cheapside,  London. 

2.00 
9OO 

1.23 

0.72 
0.2 

0.5 
0.24 

0.12 

Regent  Street,  London  . 

Piccadilly,  Manchester. 

1.4 
0.64 
i-75 

Freidrichsstrasse,  Berlin.. 

1.0 

7.6 

Potsdamer  Platz,  Berlin  

The  general  question  now  arises  as  to  how  we  are  to  produce  the 
illumination  required  on  the  streets.     Only  a  short  time  ago  one  of 


3'J 


466  ILLUMINATING   ENGINEERING   PRACTICE 

our  greatest  problems  was  the  fact  that  for  practical  use  we  had  only 
two  classes  of  lighting  units;  one,  the  larger  unit,  typified  by  an 
arc  lamp;  the  other,  the  small  unit,  such  as  an  incandescent  or  mantle 
gas  lamp.  It  is  obvious  that  it  was  impractical  to  properly  grade 
the  lighting  just  described  with  such  limited  means. 

ILLUMINANTS  AND  LAMPS 

To-day  there  are  two  improved  types  of  arc  lamps,  namely,  the 
flaming  arc  lamps  of  two  or  three  intensities;  and  the  luminous  arc 
lamps  of  three.  In  incandescent  lamps  we  have  now  the  complete 
system  of  gas-filled  tungsten  or  "Mazda,  Type  C"  lamps  of  prac- 
tically any  output  desired  for  street  lighting  from  60  to  1500 
candle-power,  suitable  for  both  multiple  and  series  circuits.  In  gas 
lamps,  the  inverted  mantle  lamps  with  a  variable  number  of  mantles, 
and  the  vertical  single  mantle  lamps  are  valuable  and  eminently 
practical. 

In  the  brief  time  given  to  this  lecture,  it  is  impossible  to  cover 
older  types  of  lamps,  which  are  now  superseded.  As  a  matter  of 
fact,  the  enclosed  arc  lamp  and  the  vertical  mantle  gas  lamp  are  and 
will  be  in  general  use  for  a  long  period  undoubtedly,  although  they 
may  be  said  to  be  superseded  and  obsolete. 

Lamps  as  usually  equipped  for  street  purposes  in  standard  forms 
may  be  described  as  follows,  so  far  as  concerns  the  light  distribution. 

The  enclosed  carbon  arc  lamp  gives  its  maximum  ray  at  about 
40°  from  the  vertical,  the  flaming  arc  lamp  not  enclosed  gives  its 
largest  flux  nearer  the  vertical,  the  enclosed  type  with  standard  equip- 
ments gives  its  maximum  flux  along  and  to  about  30°  from  the 
horizontal,  the  luminous  arc  and  the  "Mazda  Type  C"  along  and 
usually  below  the  horizontal  at  from  10°  to  30°  or  40°,  depending 
on  the  equipment;  inverted  mantle  gas  lamps  give  their  largest 
flux  downward  and  around  the  vertical,  and  the  vertical  mantle 
lamp  around  the  horizontal  and  downward. 

STREET  LIGHTING  WITH  LARGE  AND  SMALL  UNITS 

In  a  lighting  system  with  large  units  it  is  characteristic  that  when 
they  are  closely  spaced  the  direction  of  the  rays  relatively  is  not  as 
important  as  when  spaced  far  apart,  when  such  direction  for  pro- 
motion of  perception  has  a  decided  effect  in  producing  the  necessary 
contrast.  •  Where  closely  spaced  the  equipment  would  be  such  as  to 


LACOMBE:  STREET  LIGHTING  467 

give  a  distribution  below  the  lamp  approaching  the  hemispherical. 
An  important  characteristic  in  the  case  of  arc  lamps  is  that  the 
light  is  usually  white,  either  the  violet  white  of  the  carbon  arc,  the 
clear  white  of  the  luminous  arc,  or  the  creamy  or  pinkish  white  of 
the  usual  flame  arc.  These  light  units  scintillate,  giving  the  effect 
of  brilliance,  vary  in  intensity  and  appear  alive.  The  color  effect 
produced  is  in  contrast  to  the  yellower  light  given  from  store  win- 
dows and  seems  to  belong  characteristically  to  the  street.  This 
result  is  quite  desirable. 

Large  incandescent  lamp  units  equipped  and  spaced  like  arc  lamps 
producing  the  same  general  effect  as  the  large  arc  units  of  similar 
distribution  characteristics,  but  giving  a  light  tending  toward  yellow, 
are  still  and  continuous  in  their  performance  and  in  consequence  do 
not  give  the  brilliant  lively  effect  of  the  arc  lamps.  There  is  little 
differentiation  in  color  with  the  light  from  store  windows  and, 
therefore,  the  street  seems  to  take  on  the  effect  of  one  tone  somewhat 
duller  and  more  monotonous  than  with  the  arc  system. 

Lighting  with  small  units  spaced  at  distances  proportionate  to 
those  for  large  units  has  the  same  characteristic  of  a  lighted  spot  and 
a  darker  area.  This  was  very  familiar  under  the  usual  treatment  of 
vertical  single  mantle  gas  lamps  with  their  average  candle-power  and 
distribution.  The  substitution  of  electric  lamps  of  much  higher 
intensity  broadened  the  scope  of  the  small  unit  to  a  large  extent. 
With  lamps  of  from  80  to  100  candle-power  at  greater  heights  from 
the  street,  the  illumination  was  much  increased  and  brightened. 

The  effect  of  a  number  of  smaller  lamps  on  a  block  formerly  lighted 
by  two  arcs,  one  at  each  end,  was  to  break  up  the  large  dark  area, 
decrease  the  extreme  contrast  between  maximum  and  minimum 
intensity,  and  generally  resulted  in  much  greater  visibility  and  safety. 
Where  not  too  greatly  diffused  by  enclosing  glassware,  care  being 
taken  to  reduce  glare,  and  arranged  parallel  or  on  one  side  of  the 
street  only,  the  contrast  or  unidirectional  effect  is'  maintained. 
Where  strongly  diffused,  arranged  opposite  each  other  or  staggered, 
and  with  relatively  short  spacing,  the  lighting  loses  contrast  effect, 
and  perception  is  affected  detrimentally. 

SOME  FEATURES  OF  ILLUMINATION  SYSTEMS  OF  ARC  AND 
INCANDESCENT  LAMPS 

The  lighting  system  now  made  possible  by  the  use  of  various 
intensities  in  arc  lamps  and  with  "Type  C "  incandescent  lamps,  the 


468  ILLUMINATING   ENGINEERING  PRACTICE 

latter  ranging  in  intensity  from  very  high  to  low  candle-powers, 
enables  one  to  grade  the  lighting  of  streets  as  to  their  use,  much  more 
accurately  than  was  possible  in  the  past.  The  gas-filled  incandescent 
lamps  particularly  are  available  practically  on  all  systems  of  dis- 
tribution, except  that  there  are  maintenance  conditions  which  must 
be  considered  when  they  are  used  on  the  same  circuits  with  arc 
lamps. 

Lamps  using  the  same  current  but  of  various  intensities  are 
available  on  any  series  system,  so  that  we  no  longer  have  to  provide 
special  arc  lamp  circuits  for  the  supply  of  current  to  large  units. 
With  this  lamp  we  now  have  a  series  unit  graded  in  consumption 
and  intensity  to  meet  all  conditions  necessary  to  fit  the  lighting  to 
the  street.  They  are  economical  and  efficient,  practically  inter- 
changeable, having  no  mechanical  moving  parts;  are  susceptible  of 
artistic  treatment  and  are  in  every  way  flexible  and  adaptable  to 
street  lighting  conditions. 

There  is  little  question  that  the  system  of  lighting  with  "Type 
C"  lamps  of  all  required  sizes,  will  replace  the  direct  and  alter- 
nating current  enclosed  carbon  arc  lamp  almost  entirely,  as  soon  as 
the  equipment  can  be  economically  changed.  It  is  also  true  that 
larger  units  will  probably  take  the  place  of  flaming  arc  lamps,  in 
spite  of  the  improvements  that  have  been  made  in  these  lamps  and 
their  higher  initial  efficiency.  It  is  unnecessary,  therefore,  to  devote 
any  particular  time  to  the  discussion  of  these  types  of  lighting  units. 
The  formidable  rival  of  the  "  Type  C  "  lamp  is  the  luminous  arc  which 
is  somewhat  more  efficient.  It  is  available  in  three  current  ratings, 
4,  5,  and  6  amp.,  with  standard  fixtures  of  several  forms.  It  gives 
a  very  brilliant  white  and  scintillating  light  of  more  accurate 
color  value.  It  is  also  susceptible  of  ornamental  treatment  and  by 
the  use  of  refractors  can  meet  conditions  of  almost  any  required 
spacing.  Its  high  initial  candle-power  enables  it  to  be  widely  dif- 
fused and  yet  develop  strong  lighting  on  the  street.  It  is  par- 
ticularly practicable  for  business  streets  where  not  only  the  street 
but  the  building  fronts  should  be  well  illuminated. 

An  advantageous  feature  of  the  magnetite  arc  lamp  for  street 
lighting  is  the  brilliant  white  light  it  gives.  A  unit  of  this  character 
is  distinctive  of  the  street  itself,  the  light  produced  being  in  great 
contrast  to  that  given  by  store  lighting.  Compared  with  incandes- 
cent lamps,  the  luminous  arc  lamp  differs  in  that  it  operates  mechani- 
cally, is  limited  to  large  units  and  cannot  be  used  directly  on  alter- 
nating current  circuits  but  requires  the  use  of  current  rectifiers. 


LACOMBE:  STREET  LIGHTING  469 

For  the  last  year  there  has  been  a  close  rivalry  between  the  two 
systems;  both  have  definite  characteristics  which  in  specific  prob- 
lems will  lead  to  a  choice  of  one  or  the  other.  For  the  general  re- 
quirements of  illumination  in  the  average  city,  however,  it  appears 
that  the  lower  initial  investment,  when  combined  with  the  extreme 
adaptability,  the  ease  of  operation  and  satisfactory  service  perform- 
ance of  the  "Type  C"  incandescent  lamps,  make  them  the  more 
general  choice. 

In  view  of  the  availability  of  graded  units  of  illumination,  in  both 
arc  and  incandescent  lamps,  the  development  of  the  lighting  on  the 
streets  requires  more  careful  adjustment  than  in  the  past,  when  we 
had  only  two  units  of  illumination,  one  large  and  one  small.  One 
must  also  carefully  study  the  development  of  such  a  graded  system 
with  reference  to  first  cost  as  well  as  of  operation. 

One  of  the  greatest  obstacles  to  the  improvement  and  increase 
of  street  lighting  is  the  cost  of  the  equipment  and  its  installation  on 
the  streets;  the  most  careful  attention  should  be  paid  to  this,  and 
where  it  can  be  kept  down  without  loss  in  appearknce  or  in  the  effi- 
ciency of  the  illumination,  it  should  be  done.  The  utilization  of 
present  equipment  so  far  as  is  possible  to  produce  good  results,  will 
secure  the  greatest  economy  in  first  cost. 

DEVELOPMENT  OF  ILLUMINATION  ON  THE  STREET 

Assuming  the  use  of  graded  units,  if  we  should  use  magnetite 
lamps  for  the  large  units,  the  circuits  and  locations  of  these  lamps 
would  naturally  be  in  the  central  section  of  the  city  and  on  important 
radial  streets  leading  therefrom,  the  smaller  units  of  the  gas-filled 
type  would  cover  those  portions  directly  enveloping  the  central 
section  and  the  various  residence  and  suburban  districts  with  sepa- 
rate circuits  therefor.  In  the  simplest  form,  assuming  that  "Type 
C"  unit  alone  is  used,  the  general  design  of  lighting  the  city  would 
be  about  as  follows :  The  various  streets  of  the  night  center  or  centers, 
usually  easily  determined,  would  receive  the  most  brilliant  lighting 
from  large  units  spaced  regularly  and  closely  along  the  streets. 
Special  intersections,  where  streams  of  traffic  meet  and  diverge, 
should  be  very  adequately  lighted  with  double  lighting  at  the  inter- 
sections and  close  spacing  along  streets.  Where  such  intersections 
become  open  squares  or  plazas,  excellent  effects  can  be  obtained  from 
tall  standards  with  lamps  40  to  45  feet  from  the  ground.  In  such 
cases,  with  proper  reflectors  for  throwing  all  the  light  below  the 
horizontal,  a  high  intensity  can  be  obtained  over  the  whole  area. 


470  ILLUMINATING   ENGINEERING   PRACTICE 

The  light  need  be  diffused  only  slightly,  if  at  all,  and  full  efficiency 
can  be  obtained  without  danger  of  glare.  It  is  in  these  sections  of  a 
city  that  one  should  see  clearly  by  direct  reflection.  From  this 
central  section,  avenues  and  streets  will  lead  to  the  residence  and 
other  sections,  and  should  receive  the  next  lower  grade  of  lighting, 
"B,"  amply  sufficient  for  fast  moving  street-car  or  automobile 
traffic.  This  lighting  may  properly  be  so  arranged  as  to  be  brightest 
nearest  the  centers  and  decrease  to  Class  C  as  the  distance  increases 
and  the  traffic  diminishes.  This  can  be  done  most  easily  and  cheaply 
by  increasing  the  spacing  between  lamps,  retaining  at  least  one  per 
street  intersection.  Where  these  streets  intersect  other  streets  of 
the  same  character,  the  lighting  should  be  reinforced  at  the  inter- 
section. This  grading  of  street  lighting  is  further  aided  by  the 
ability  we  now  have  to  decrease  the  candle-power,  and  by  the  light- 
directing  devices  now  available.  Generally  outside  of  the  main 
night  center  of  the  city  other  local  centers  will  be  found  much  used 
at  night,  which  must  receive  augmented  lighting  for  a  few  blocks. 
In  such  cases  the  lighting  should  usually  be  sufficient  to  produce 
direct  vision  by  reflection.  As  a  general  rule  in  business  districts, 
the  equipment  should  be  of  a  type  which  will  light  the  building 
fronts  as  well  as  the  streets.  In  residence  districts  this  should  be 
avoided  above  the  first  story  in  sections  where  houses  are  in  blocks, 
and  further  minimized  in  suburban  districts.  In  the  latter  sections 
the  average  resident  objects  to  almost  any  form  of  visible  light  source. 

In  every  city  will  also  be  found  the  wholesale  business  and  finan- 
cial section  which  is  usually  deserted  at  night  but  requires  ample 
lighting  for  police  purposes,  about  Class  C.  As  such  sections  are 
usually  treeless  they  may  best  be  lighted  by  large  units  at  street 
intersections  and  alley  entrances.  Where  blocks  are  short  and  mid- 
block  alleys  exist,  mid-block  lamps  may  be  of  a  smaller  size  and  with 
different  equipment. 

The  residence  sections  of  a  city  take  varied  treatment.  In  almost 
all  cases,  silhouette  lighting  must  be  depended  on  in  these  sections. 

The  density  of  population,  the  character  of  the  houses,  and  the 
trees  each  has  a  strong  effect  in  determining  the  lighting  of  residence 
streets.  In  the  residence  sections  of  the  city  where  the  houses  are 
practically  continuous,  the  illumination  intensity  should  be  main- 
tained at  about  moonlight  value — Class  "D."  As  one  leaves  these 
sections,  however,  and  reaches  the  suburban  residence  district,  with 
detached  houses  becoming  further  and  further  apart,  the  lighting  in- 
tensity would  naturally  diminish  to  Class  "E."  In  this  case  where 


LACOMBE:  STREET  LIGHTING  471 

trees  are  few  in  number,  large  lamps  at  street  intersections  placed 
fairly  high,  with  directing  refractors  give  good  results  and  may  be 
spaced  at  considerable  distances  without  intermediate  lamps.  Where 
in  suburbs  the  spacing  can  be  moderately  short,  even  more  agreeable 
results  may  be  obtained  by  the  use  of  reflectors  and  diffusing  globes. 
Where  heavy  foliage  exists,  small  lamps  at  comparatively  short 
distances  apart  on  standards  low  enough  to  allow  the  light  to  spread 
under  the  trees  gives  the  most  satisfactory  results  to  the  residents. 

In  the  suburban  sections  where  overhead  construction  is  usual, 
very  satisfactory  lighting  can  be  obtained  by  the  smaller  incandescent 
units  placed  on  each  line  pole  or  on  every  other  line  pole,  due  regard 
being  had  for  the  street  intersections.  If  available,  the  use  of  the 
poles  of  other  lines  than  those  of  the  lighting  company  for  street 
lamps  in  the  residence  district,  would  be  valuable  in  such  cases, 
particularly  on  those  streets  having  trolley  lines  and  acting  as 
arteries  or  feeders  leading  from  the  outskirts  to  the  center  of  the 
city. 

In  general,  a  low  order  of  intensity  may  be  used  throughout 
suburban  residence  sections  sufficient  for  police  purposes,  and  yet 
fairly  below  the  intensity  of  moonlight.  Unidirectional  lighting 
in  such  sections  is  particularly  valuable  as  full  use  of  the  silhouette 
effect  is  necessary. 

Boulevards  may  be  treated  like  residence  streets  with  either  the 
large  or  the  small  unit.  Where  there  is  little  interference  with  light- 
ing and  the  boulevard  is  so  wide  that  trees  do  not  meet,  and  center 
suspension  cannot  be  used  for  aesthetic  reasons,  large  lamps  on  higher 
standards  with  long  arms  bring  the  light  source  well  over  the  road- 
way and  give  excellent  results.  Where  the  reverse  conditions  pre- 
vail smaller  lamps  should  be  used.  Where  the  boulevard  is  parked, 
smaller  lamps  attractively  mounted  and  properly  equipped  give 
satisfactory  results.  If  center  plots  are  available,  the  boulevard 
being  quite  wide,  the  cheapest  and  most  efficient  lighting  will  be 
obtained  by  large  lamps  properly  arranged  in  these  plots.  If  the 
traffic  is  sufficient,  and  in  any  case  at  important  intersections,  the 
center  plot  lamps  would  properly  be  supplemented  by  lamps  ar- 
ranged along  the  outer  lines  of  the  roadways.  When  boulevards  are 
extended  and  become  interconnecting  roads,  passing  temporarily 
from  city  conditions,  the  lighting  must  be  graded  in  accordance  with 
the  amount  of  traffic.  Originally  lighted  to  the  degree  of  "D" 
or  "E"  streets,  they  may  change  to  Class  "F"  conditions,  and  the 
lighting  be  diminished  accordingly  with  less  expensive  equipment. 


472  ILLUMINATING   ENGINEERING   PRACTICE 

Interconnecting  country  roads,  county  roads  or  highways,  except 
within  town  limits,  are  rarely  lighted  at  this  time.  A  movement 
in  this  direction  is  beginning,  however,  and  should  be  encouraged. 

To  make  this  type  of  lighting  popular  it  must  be  very  inexpensive 
in  so  far  as  equipment  is  concerned  and  low  in  operating  cost.  In 
consequence  it  usually  consists  of  lamps  mounted  on  line  poles. 
These  lamps  are  placed  at  distances  varying  from  300  to  900  feet 
and  more  apart.  It  is  obvious  that  the  lighting  at  such  distances 
apart  is  largely  directional  in  character.  Four-ampere  luminous  arc 
lamps,  from  600  to  900  feet  apart,  properly  equipped  with  refractors 
and  reflectors,  give  about  the  best  lighting  of  this  type  in  use  at  this 
time,  and  objects  are  quite  visible  in  silhouette.  This  condition 
exists  even  up  to  spacing  1 200  feet  or  1 500  feet  apart.  If  the  smaller 
incandescent  lamps  are  used  with  refractors  they  should  not  be 
spaced  at  distances  over  500  feet  apart  for  the  loo-c.p.  size. 

ELECTRICAL  DISTRIBUTION  SYSTEMS 

In  the  lighting  of  streets,  the  electrical  distribution  system  is 
important,  from  its  bearing  on  the  question  of  first  cost  and  also  of 
operation.  Distribution  systems  can  be  generally  stated  to  be 
either  series  or  constant  current  systems,  and  multiple  or  constant 
potential  systems,  while  combinations  of  these  in  one  system  are 
sometimes  used.  The  second  or  multiple  system,  usually  low  ten- 
sion, is  generally  limited  to  the  central  or  business  area  of  a  city 
where  the  density  of  consumers  is  at  the  maximum.  Energy  for  the 
lighting  is  taken  directly  from  the  general  supply  net  work.  Except 
in  the  largest  cities  this  system  is  not  generally  in  use  for  street 
lighting,  and  elsewhere  the  series  system  is  practically  universal. 
Originally  it  was  used  for  open  direct-current  series  arc  lamps  operat- 
ing from  arc-lighting  generators.  Speaking  broadly,  with  the  ad- 
vent of  the  enclosed  arc  lamps  came  the  alternating-current  series 
circuits  with  series  transformers,  improved  later  by  the  constant 
current  regulator,  and  then  with  the  development  of  the  luminous 
or  magnetite  lamp,  came  the  mercury  are  rectifier.  To-day  the 
alternating  constant-current  series  system  in  one  of  its  forms  is  used 
in  most  localities,  as  it  is  available  for  carbon  or  flaming  arc-lamp 
circuits  or  "Type  C"  incandescent  lamps  and,  with  rectifiers,  can 
be  used  for  luminous  arc  lamps. 

With  this  system  the  energy  is  distributed  from  central  stations  or 
sub-stations  feeding  the  various  circuits.  Two  methods  seem  to  be 


LACOMBE:  STREET  LIGHTNG  473 

in  use.  In  the  first,  constant-current  regulating  transformers  change 
the  constant-voltage  alternating  current  to  constant-current  series 
alternating,  being  so  regulated  that  the  sizes  of  the  lamps  may  be 
varied  independently  of  each  other.  In  the  other,  series  transformers 
are  used  to  supply  the  energy  to  the  series  circuit  with  special  taps 
on  the  primary  and  secondary  of  these  transformers  for  regulating 
the  current  within  the  limitations  of  the  transformer,  in  accordance 
with  the  number  of  lamps  on  the  circuit.  In  some  systems  a  reac- 
tance is  added  in  series  with  the  lamp  so  that  in  case  a  lamp  goes 
out,  constant  current  will  be  maintained  throughout  the  circuit. 
Each  system  has  its  advantages,  particularly  under  special  circum- 
stances. The  automatic  regulation  of  the  constant-current  regulat- 
ing transformer  is  more  accurate  and  less  awkward  in  adjustment 
than  the  series  transformer  system  with  taps.  On  account  of  the 
importance  of  the  street  lighting  to  the  night  life  of  a  city,  it  is  wise 
to  have  the  lighting  circuits  originate  at  one  point,  as  with  the 
constant  current  regulator  system,  where  it  is  under  the  observation 
of  an  attendant,  as  this  would  tend  toward  better  operation  and 
quicker  repair  in  case  of  interruption  to  service. 

So  far  as  illumination  is  concerned,  the  question  of  the  location 
of  the  lamps  is  relatively  of  great  importance  in  the  amount  of 
expense  that  may  attach  to  the  first  cost  of  the  system,  on  account 
of  the  expense  of  lamp  posts,  their  erection,  and  the  connection  of 
the  lamps  to  the  system.  For  instance,  in  overhead  systems  line 
poles  can  be  used  as  lamp  poles,  particularly  if  at  every  street  inter- 
section there  was  at  least  one  pole  in  a  suitable  position.  This 
should  be  provided  for  in  the  construction  of  new  lines.  In  such 
case  the  first  cost  of  lighting  units  is  at  a  minimum.  The  objec- 
tions to  overhead  lines  are  the  well-known  ones  of  obstructions  on 
sidewalks,  ugliness  and  interference  with  trees.  These  objections 
may  be  greatly  minimized  if  proper  precautions  are  taken,  careful 
preparations  made,  permits  obtained  in  advance,  the  poles  painted 
so  as  to  be  unobtrusive,  and  the  structure  maintained  in  first-class 
condition.  The  trimming  of  trees  may  also  be  accomplished  if 
carefully  negotiated,  but  it  is  best  to  conduct  such  operations  with 
the  authorities  having  jurisdiction,  usually  the  Park  Department  of 
a  city.  Attention  is  drawn  to  these  points  for  the  reason  that  a 
part  of  the  demand  for  underground  construction  and  its  conse- 
quent expense  is  caused  by  the  use  at  times  of  somewhat  arbitrary 
methods  and  the  neglect  of  neat  and  workmanlike  construction. 

Where  underground  construction  is  warranted,  or  one  is  compelled 


474  ILLUMINATING   ENGINEERING   PRACTICE 

to  use  it,  the  question  of  equipment  cost  becomes  even  more  im- 
portant, the  relative  expense  is  greater  and  every  effort  should  be 
made  to  utilize  any  available  equipment  and  to  locate  the  lamps  so 
that  the  minimum  expense  be  incurred.  The  cables,  conduits, 
manholes,  hand-hole  boxes,  street  openings  and  settings  and  founda- 
tions of  heavy  iron  poles  involve  a  heavy  first  cost.  Where  iron 
trolley  poles  exist,  as  they  usually  do  in  downtown  districts,  very 
advantageous  results  can  be  obtained  by  utilizing  them,  with  either 
the  parallel  or  the  staggered  arrangement  of  lamps.  By  this  sensible 
and  economical  method  a  number  of  quite  successful  installations 
have  been  made  throughout  the  country,  by  which  the  street  lighting 
has  been  greatly  improved  at  a  minimum  cost. 

Where  underground  construction  must  be  extended  into  the  outer 
sections  of  the  city  to  avoid  pole  lines,  armored  cable  may  be  used 
at  much  less  expense  than  standard  underground  construction. 
Where  iron  lamp  posts  are  available,  such  as  in  the  substitution  of 
electric  lighting  for  gas  lighting,  they  may  be  used  to  advantage, 
with  modern  diffusing  appliances,  in  economy  of  first  cost. 

LOCATIONS,  SPACING  AND  HEIGHT 

This  brings  us  naturally  to  the  question  of  location,  spacing  and 
height  of  lighting  units.  A  very  careful  study  and  survey  of  the 
streets  to  be  lighted  should  be  made  before  lamps  are  located,  keep- 
ing in  mind  the  kind  of  illumination  to  be  used,  the  various  grades 
of  streets  and  the  results  to  be  obtained.  Full  field  notes  should  be 
made  covering  the  characteristics  of  each  street,  its  use,  character 
and  direction  of  general  traffic;  the  type  and  position  of  buildings, 
particularly  of  special  buildings;  the  color,  type  and  reflection  char- 
acteristics of  the  pavement  and  buildings;  curves  in  streets,  alleys; 
parking,  if  any;  special  open  areas  along  streets,  special  street  inter- 
sections, and  streets  containing  street-car  lines,  and  particularly 
intersections  of  such  streets.  The  existing  lamp  posts,  distribution 
system,  trolley  poles  and  construction  of  pavements  are  also  of 
great  importance.  These  data  should  be  transferred  to  a  large 
street  map  for  record. 

So  many  different  conditions  may  appear  and  the  results  desired 
are  so  varied,  that  it  is  impossible  to  lay  down  any  general  rule  of 
procedure  other  than  in  the  general  description  already  given,  except 
to  emphasize  the  importance  of  studying  the  streets  and  designing 
the  lighting  in  accordance  with  field  conditions,  even  to  the  extent 


LACOMBE:  STREET  LIGHTING  475 

of  making  trial  installations  in  the  most  important  places.  A  care- 
ful study  of  accurate  field  notes  in  connection  with  the  general 
description  of  the  laying  out  of  lighting  already  given,  will  generally 
cause  the  situation  to  clear  up  and  definite  lines  of  procedure  will 
develop. 

If  one  had  complete  control  of  height,  spacing  and  location  of 
lamps,  theoretically  almost  any  desired  lighting  could  be  obtained, 
uniform  or  non-uniform,  with  minimum  glare.  Mr.  Millar's  tests 
for  the  National  Electric  Light  Association  and  the  Association  of 
Edison  Illuminating  Companies,  have  shown  that  for  visibility, 
uniform  lighting  on  streets,  particularly  of  a  low  order,  is  not  the 
best,  thereby  exploding  an  old  theory.  In  view  of  the  results  he 
has  obtained,  one  should  work  toward  lighting  giving  reasonable 
contrasts,  that  is,  lighting  which,  predominating  in  one  direction, 
creates  contrasts.  Such  contrasts  alternating  between  each  light 
source  thus  create  a  moderate  diversity  of  intensity  between  the 
extremes  of  uniformity  and  non-uniformity.  We  rarely  have 
complete  control  of  height  and  location,  so  that  heights  and  spacing 
in  actual  practice  must  be  considered.  In  general  the  height  at 
which  lamps  are  placed  is  limited  by  the  expense  of  the  equipment 
and  the  difficulties  in  getting  at  lamps  at  considerable  heights  for 
cleaning  and  renewal.  From  20  to  25  ft.  is  the  general  limit  for 
large  units,  and  10  to  16  ft.  for  the  smaller  ones.  Within  such 
limits,  the  higher  the  better,  as  wider  light  distribution  is  obtained, 
the  high  lighting  under  the  lamp  being  decreased  while  the  minimum 
normal  illumination  within  the  usual  radius  is  only  slightly  affected. 
At  these  heights  the  larger  lamps  are  generally  out  of  the  direct  line 
of  vision  which,  of  course,  is  an  advantage.  It  is  rare  to  find  large 
units  at  less  than  18  ft.  above  the  surface  of  the  street  except  in 
ornamental  and  well  diffused  lighting  systems. 

Larger  lamps  can  be  used  with  the  higher  posts  and  in  conse- 
quence there  would  be  fewer  posts.  It  will  be  noted  that  the 
generally  available  heights  are  limited  within  small  variations,  and 
hence  the  spacing  is  the  dominant  factor  in  determining  the  size  of 
the  unit  with  which  the  desired  illumination  is  to  be  obtained.  The 
limit  of  practical  spacing  where  large  lamps  are  used,  has  been  some- 
what increased  lately  by  reason  of  the  introduction  of  refractors 
which  distribute  the  lighting  flux  at  a  further  distance  from  the  post. 
In  certain  cases,  therefore,  the  distances  between  posts  may  be  in- 
creased from  that  of  older  practice  and  the  use  of  some  intermediate 
lamps,  with  their  extra  installation  cost,  be  avoided. 


476  ILLUMINATING   ENGINEERING   PRACTICE 

Concerning  the  relative  effect  of  height  and  spacing  on  the  hori- 
zontal illumination  of  the  street  within  the  usual  working  limits, 
it  can  be  stated  that  with  a  spherical  or  uniform  candle-power  distri- 
bution curve  below  the  horizontal,  a  change  in  height  from  19.5  to 
26  ft.  will  decrease  the  mean  horizontal  illumination  by  14  per  cent., 
and  would  increase  the  minimum  illumination  by  20  per  cent.; 
whereas,  assuming  a  fixed  height,  the  mean  horizontal  illumination 
is  practically  inversely  proportional  to  the  distance  from  the  lamp. 
The  maximum,  of  course,  varies  very  little  as  the  spacing  is  increased, 
but  the  minimum  decreases  very  rapidly,  and  in  consequence  the 
uniformity  of  the  lighting  becomes  less  and  less. 

Many  arrangements  of  locating  lamps  on  streets  are  in  use,  vary- 
ing with  their  importance.  Where  large  units  are  utilized  on  streets 
of  ordinary  use,  such  as  C,  D,  and  E,  they  are  usually  suspended  at 
the  center  of  street  intersections,  or  on  brackets  from  poles  at  such 
points.  On  more  important  streets,  however,  where  center  sus- 
pensions have  many  structural  objections,  lamps  are  usually  placed 
on  posts  arranged  in  parallel  along  the  curb,  and  staggered,  or  else 
placed  parallel  along  the  curb  and  opposite  each  other.  This  prob- 
lem frequently  comes  up  in  connection  with  "White  Way"  lighting, 
and  at  times  it  is  quite  difficult  to  tell  which  is  most  desirable.  As  a 
matter  of  fact,  parallel  and  opposite  lamps  do  not  give  as  uniform 
lighting  along  the  center  of  the  street,  although  similar  in  each  space, 
as  is  given  by  the  parallel  and  staggered  arrangement.  The  ar- 
rangement of  lamps  opposite  each  other  is  usually  admired  for  its 
symmetry  of  location,  but  it  is  more  expensive  than  the  usual  ar- 
rangement. In  general,  except  where  artistic  requirements  dominate, 
the  staggered  location,  except  where  closely  spaced,  is  preferable. 
At  street  intersections  under  the  parallel  and  opposite  arrangement 
along  any  one  street,  lamps  are  not  usually  placed  at  street  corners, 
but  some  little  distance  back  from  the  house  line,  so  located  as  to 
apply  properly  certain  zones  of  light  on  the  intersection  from  each 
of  the  units. 

The  parallel  and  opposite  arrangement  further  requires  that  the 
intersecting  street  must  be  independently  lighted  and  hence  this 
system  becomes  more  expensive  in  first  cost  and  operation,  so  much 
so,  in  fact  that  it  is  rarely  used  on  city  streets  except  for  display 
lighting  or  where  trolley  poles  are  available  at  small  expense.  The 
parallel  and  staggered  system  usually  involves  the  use  of  two  stand- 
ards at  each  street  intersection  at  diagonal  corners,  not  exactly  at 
the  corners  but  approximately  on  the  house  lines.  The  intermediate 


LACOMBE:  STREET  LIGHTING  477 

lamps  are  then  placed  alternately  on  opposite  sides  of  the  street  at 
curb  lines,  the  spacing  depending  on  the  intensity  of  lighting  desired, 
length  of  block,  location  of  alleys,  and  so  on.  With  this  arrangement 
the  intersecting  streets  are  taken  care  of,  the  intensity  required  ob- 
tained, and  contrast  and  unidirectional  lighting  are  maintained 
without  the  use  of  as  many  lamps  and  standards,  and  consequently 
at  less  expense  in  first  and  operating  cost. 

The  position  of  the  lamps,  whether  placed  along  the  curb  or  sus- 
pended in  the  center  of  the  street,  has  an  effect  on  the  general  ap- 
pearance of  the  street.  Under  the  first  type  of  lighting,  the  street 
appears  narrower  than  under  the  second.  Under  metropolitan 
conditions,  however,  it  would  be  very  difficult  to  suspend  lamps  over 
the  center  of  the  street,  and  hence  they  are  usually  placed  on  the 
curb.  A  row  of  lamps  opposite  each  other  along  the  curb,  or  stag- 
gered along  the  curb,  arranged  regularly  and  carefully  coordinated 
with  the  curb  line  as  to  height,  distance  from  curb,  and  so  on,  both 
give  the  street  more  or  less  the  same  effect  of  parallel  lines  of  light 
along  the  curb,  with  a  long  open  field  of  vision,  for  at  a  little  distance 
staggered  lamps  at  night  have  about  the  same  appearance  as  lamps 
placed  opposite. 

The  theoretically  desirable  position  for  large  units  in  the  metro- 
politan districts  would  be  suspended  above  the  center  of  the  street 
at  a  good  height,  using  powerful  lamps  and  varying  the  illumination 
intensity  by  closer  or  wider  spacing.  While  this  arrangement  is  real- 
ized abroad  by  suspension  from  the  buildings  or  from  posts  on  isles 
of  safety,  it  is  rarely  possible  in  this  country. 

EFFECT  OF  PAVEMENTS  AND  BUILDINGS 

The  effect  of  lighting  a  city  street  of  one  of  the  higher  classes 
depends  very  largely  on  the  characteristics  of  the  street,  its  pave- 
ments and  its  buildings,  for  in  such  streets  it  is  usually  desirable  to 
light  the  building  fronts.  A  straight  street  with  light-colored 
smooth  pavement,  broad  sidewalks,  and  buildings  of  a  light  color 
is  most  favorable  to  lighting,  allowing  long  symmetrical  lines  of 
lamps,  the  light  from  which  can  be  diffused,  and  which  will  be 
reflected  specularly  by  the  pavement  and  the  buildings.  A  note- 
worthy instance  of  this,  brought  about  by  the  cooperation  of  mer- 
chants and  authorities,  is  Regent  Street  in  London,  which  is  lighted 
by  powerful  lamps  giving  white  light  erected  on  isles  of  safety  in  the 
center  of  the  street.  The  street  is  paved  with  asphalt,  has  broad 
sidewalks,  and  the  stores,  which  vary  from  3  to  6  or  7  stories  in 


478  ILLUMINATING   ENGINEERING   PRACTICE 

height,  were  by  agreement  painted  a  light  color,  the  result  being  a 
brilliantly  lighted  and  effective  street,  although  there  is  little  auxil- 
iary window  lighting.  Pavements,  as  a  matter  of  fact,  have  a 
greater  influence  in  the  appearance  of  a  lighted  street,  than  have 
buildings.  Specular  reflection  from  the  pavement  is  very  useful, 
particularly  with  lower  grades  of  illumination  where  it  is  important 
to  develop  the  silhouette  effect.  In  most  instances  we  see  objects 
against  the  background  of  the  pavement  rather  than  against  the 
buildings,  particularly  the  more  distant  objects. 

The  lighter-colored  the  pavement  the  more  the  reflection  obtained, 
the  lighter  the  street  appears,  and  the  better  the  general  effect. 
A  given  lighting  system  in  a  street  with  a  dark  dirty  pavement, 
houses  back  from  the  street,  would  look  dull  and  dim;  whereas, 
under  favorable  conditions  it  would  be  entirely  adequate.  If 
municipal  affairs  could  be  so  coordinated  that  the  road  surface 
would  be  always  light  in  color  the  appearance  of  the  streets 
at  night  would  be  improved  and  less  intensity  would  be  required 
to  produce  the  desired  effect. 

GLARE 

The  blinding  effect  of  glare  is  one  of  the  most  perplexing  prob- 
lems in  illumination  work.  Practically  it  cannot  be  entirely  elimi- 
nated; it  occurs  in  daylight  and  even  in  moonlight.  Its  effects  are 
almost  independent  of  distance  and  it  would  seem  that  nature  in- 
tended that  the  eye  should  be  subjected  to  a  certain  amount  of  it. 

With  artificial  lighting  it  can  be  relieved  by  removing  the  source 
from  the  line  of  vision  by  a  considerable  angle,  say  25°.  This  result 
is  accomplished  by  raising  the  lamp  above  the  observer.  Or,  it  can 
be  relieved  by  enlarging  the  source  of  light  by  diffusing  globes 
and  large  diffusing  reflectors.  Refractors  are  also  used  to  deflect 
the  rays  from  those  angles  near  the  line  of  vision.  Little  contrast 
between  the  source  of  illumination  and  its  background  also  aids  in 
reducing  glare.  Recent  instances  of  these  methods  of  minimizing 
glare  for  large  units  may  be  quoted : 

One  instance,  three  magnetite  lamps  with  diffusing  globes  of  some 
absorption  are  to  be  placed  on  the  top  of  3o-ft.  trolley  poles. 

A  magnetite  lamp  is  placed  in  a  large  sectional  diffusing  globe  at 
1 6  feet  from  the  sidewalk. 

In  one  case  looo-c.p.  "Type  C"  lamps  were  placed  in  refractors  for 
redirecting  the  rays,  and  suspended  at  a  height  of  30  feet  above  the 
street. 


LACOMBE:  STREET  LIGHTING  479 

Another  instance,  looo-c.p.  or  i5oo-c.p.,  "Type  C"  lamps  were 
equipped  with  band  refractors  and  placed  15  feet  above  the  street 
in  large  lanterns  with  diffusing  glass. 

With  small  units  at  low  height  and  short  spacings  it  is  even  more 
difficult  to  prevent  glare;  obviously  the  source  of  light  is  nearer  the 
line  of  vision,  yet  the  flux  is  small  and  if  diffused  too  much  becomes 
too  weak  to  produce  the  lighting  effect  desired.  With  the  "Type 
C"  lamp,  however,  the  small  source  of  light  is  of  such  intense  bright- 
ness that  the  light  must  be  diffused,  redirected,  or  the  source  raised 
as  far  as  possible  from  the  line  of  vision.  In  many  cities,  diffusing 
globes  are  placed  around  these  lamps  where  they  are  used  in  connec- 
tion with  former  gas  lamp  posts.  In  other  cities,  lamps  of  loo-c.p. 
are  placed  at  intervals  of  from  24  to  1 5  feet  in  domed  reflectors  with 
the  filament  so  focussed  that  one  can  only  get  the  lessened  glare 
effect  of  the  large  white  disc  of  the  reflector.  This  arrangement  was 
found  not  to  produce  excessive  glare.  In  another  arrangement  a 
25o-c.p.  lamp  was  placed  at  a  height  of  16.5  feet  with  diffusing  globe 
of  low  absorption  value.  In  another  instance,  ico-c.p.  lamps  were 
placed  in  bowl  refractors  at  a  height  of  15  ft.  in  attempting  to  pre- 
vent glare,  but  the  effect  was  to  make  the  lighting  look  dull.  It  is 
believed  that  an  arrangement  of  a  100  c.p.  lamp  in  reflectors  with 
band  refractors,  the  source  of  illumination  being  above  the  lower 
edge  of  the  reflector  and  the  bowl  of  the  lamp  being  frosted,  would 
give  the  brilliant  effect  of  the  bright  light  source  without  injurious 
glare. 

ACCESSORIES 

In  one  form  or  another,  reflectors,  refractors  or  diffusing  globes 
are  in  general  use  on  all  lamps,  for  the  reason  that  it  is  desired  to 
redirect  the  rays  of  the  lamp  towards  the  surface  to  be  illuminated. 
Diffusing  globes  are  used  often  with  reflectors  where  it  is  desired  to 
throw  most  of  the  diffused  light  downward.  Globes  alone  are  used, 
sometimes  of  special  design,  where  a  part  of  the  light  is  to  be  used  in 
lighting  the  upper  parts  of  the  buildings.  Reflectors  should  be 
carefully  designed  in  connection  with  the  lamp  and  the  position  of 
the  light  source,  so  that  this  source  is  at  the  proper  focussing  point 
in  connection  with  the  reflector  for  the  light  distribution  desired. 
Where  this  is  done,  and  with  the  addition  of  a  diffusing  globe  in  case 
of  a  light  source  of  high  intrinsic  brilliancy,  the  effect  of  a  large  light 
source  is  obtained  with  fairly  well  distributed  illumination.  Where 
the  sources  of  light  are  spaced  far  apart,  refractors  add  much  to 


480  ILLUMINATING   ENGINEERING   PRACTICE 

their  effectiveness,  the  action  of  the  refractor  being  to  redirect  e 
rays  of  light  emitted  by  the  light  source  into  prescribed  direct  ^s 
for  which  the  refractor  is  designed,  usually  at  from  10  to  15  de<  a 
below  the  horizontal.  This  will  increase  the  illumination  HIK  ay 
between  lamps,  diminishing  it  near  the  lamps,  thereby  reducing  the 
spot  effect.  Refractors,  which  have  been  in  use  abroad  for  a  com- 
paratively long  time,  came  into  use  in  this  country  with  improvem  :nts 
of  the  luminous  arc  and  the  "Type  C"  lamp,  in  which  the  position 
of  the  light  source  does  not  vary. 

POSTS  AND  MOUNTINGS 

Among  the  many  other  details  necessary  to  successful  street 
lighting,  one  must  consider  posts  and  mountings,  particularly  under 
city  conditions,  where  the  more  simple  standard  apparatus,  such  as 
are  used  for  overhead  circuits  on  wooden  poles,  is  not  desirable 
except  in  so  far  as  the  lamp  itself  is  concerned.  It  is  necessary 
that  the  posts  and  mountings  of  lighting  units  should  be  as  attract- 
ive as  possible  in  appearance  in  the  daytime;  in  any  case  they  should 
be  neat,  well  painted  and  workmanlike  in  effect,  and  kept  so. 
Posts  for  metropolitan  use  are  usually  built  of  steel  and  iron,  or  iron 
and  concrete,  treated  more  or  less  ornamentally,  and  developed  in 
many  forms. 

Lamps  are  supported  on  top  of  the- posts,  hung  in  lyre  tops,  or 
are  placed  in  diffusing  glass  globes  and  in  specially  designed  lanterns, 
for  use  either  on  pole  tops  or  on  brackets,  and  one  or  more  brackets 
with  a  lamp  on  each  bracket  are  often  used.  In  all  these  forms,  one 
of  the  principal  considerations  should  be  the  ease  with  which  the 
lamp  can  be  reached  for  operation  and  maintenance.  When  lamps 
are  set  at  from  22  to  25  ft.  in  height  where  pavements  are  smooth 
and  traffic  is  dense,  they  are  frequently  attended  to  by  men  on  tower 
wagons.  As  this  plan  is  expensive,  the  lamps  should  be  so  arranged 
that  they  can  be  lowered  to  the  street  when  possible  to  do  so.  Auto- 
matic hangers  are  frequently  used;  they  have  the  advantage  of 
detaching  the  lamp  from  the  circuit  while  it  is  lowered  to  the  street 
for  attention. 

The  expense  of  the  post  and  lantern  itself  varies  over  wide  limits, 
depending  on  the  elaboration  of  artistic  design  and  finish.  On 
metropolitan  streets  of  the  AA,  A  and  B  class,  use  is  made  *f  iron 
and  steel  posts,  costing  from  $35  to  $125  and  above  depenuing  on 
the  strength  required,  height  and  ornamentation.  A  full  standard 


LACOMBE:  STREET  LIGHTING  481 

ar...  ornamental  equipment  for  luminous  arc  lamps  has  been  de- 
\r'  ed  and  used  with  good  effect  in  many  cities.  The  reinforced 
r-te  post  has  proved  to  be  serviceable  and  cheap.  It  is  par- 
ticuv.oiy  adapted  to  artistic  treatment  at  a  minimum  expense,  and 
has  .  sen  used  with  great  success  in  parks  and  parkways  particularly 
with  the  smaller  lighting  units.  A  very  attractive  post  can  be  ob- 
tained for  this  service  for  about  $9.  A  successful  attempt  at  a  tall 
conclete  post  has  recently  been  made  in  a  Western  city,  where  re- 
irforted  posts  30  ft.  in  height,  with  reinforced  concrete  brackets, 
'  ave  been  constructed.  Each  of  these  posts  with  lamps  cost  about 
$115  installed.  With  the  exception  of  this  one  case,  the  estimates 
given  above  do  not  include  the  cost  of  setting,  or  of  lamps.  The 
cost  of  setting  depends  on  variable  conditions;  with  the  larger  and 
taller  posts  concrete  foundations  are  used,  and  the  necessary  excava- 
tion under  congested  city  conditions,  is  extremely  costly  in  many 
cases.  It  is  desirable,  so  far  as  is  reasonable,  to  use  standard  designs 
of  posts  and  lanterns,  thereby  avoiding  excessive  first  cost.  Where 
special  posts  and  lanterns  are  designed  for  artistic  effect,  the  first 
cost  is  largely  increased  and  the  operating  costs  also,  for  renewal 
parts  have  to  be  specially  made  and  are  expensive. 

In  general,  where  large  units  are  employed,  it  is  desirable  to  use 
brackets  and  bring  them  well  out  over  the  roadway  of  the  street, 
in  order  to  put  the  light  where  it  is  needed  for  the  greatest  traffic. 
With  the  ordinary  foliage  encountered  on  most  avenues  and  streets  in 
the  central  part  of  a  city,  long  brackets  will  be  found  very  desirable, 
with  lamps  at  20  ft.  or  higher,  in  order  to  bring  the  light  out  beyond 
the  foliage.  This  arrangement  also  serves  to  place  the  lamps  at  a 
point  where  they  not  only  light  the  roadway  but  throw  a  considerable 
portion  of  the  light  under  the  trees  and  on  the  sidewalk. 

About  the  usual  limit  in  length  for  big  units  under  metropolitan 
conditions  is  from  8  to  10  ft.  and  in  the  suburbs  where  overhead 
construction  can  be  used  from  10  to  12  ft.  In  small  units  at  heights 
of  from  14  to  15  ft.,  4-ft.  brackets  are  quite  sufficient  to  improve 
materially  the  lighting  in  the  roadway  and  yet  throw  the  light  under 
the  trees. 

The  housing  of  the  light  source  itself;  in  other  words,  the  lantern, 
must  be  weatherproof,  designed  for  its  type  of  lamp,  to  allow  ease 
in  repair,  cleaning,  and  the  renewal  of  glassware.  It  should  be  de- 
signed 9~>r  the  best  light  distribution  and  be  attractive  in  appearance. 
The  standard  apparatus  to-day  generally  meets  these  conditions. 
Special  lanterns,  like  special  posts,  are  expensive  in  first  cost  and 
31 


482  ILLUMINATING    ENGINEERING   PRACTICE 

maintenance,  and  they  are  warranted  only  where  artistic  design  is 
required  to  harmonize  with  that  of  the  post.  They  are  rarely 
justified  except  for  points  where  great  artistic  merit  is  desired. 

GRADED  ILLUMINATION  AND  RESULTS  ON  CERTAIN  STREETS 

In  establishing  a  certain  grade  of  illumination  on  a  street,  a  de- 
termination of  the  average  and  minimum  illumination  having  been 
made,  the  size  and  number  of  lamps  can  be  established  quite  easily 
by  the  flux  method.  Manufacturers  now  supply  candle-power  dis- 
tribution curves  of  the  complete  unit,  including  globes,  reflector  or 
refractor.  They  show  the  spherical  or  hemispherical  candle-power 
and  the  total  or  the  downward  useful  lumens.  From  these  data 
can  be  determined  the  number  and  size  of  lamps  required  per  block 
or  unit  area  to  obtain  the  average  foot-candles  (lumens  per  square 
foot)  desired.  The  location  of  the  lamps  must  then  be  made  after 
a  study  covering  the  many  local  factors  that  determine  this.  The 
locations  should  be  finally  checked  by  inspection,  particularly  on 
important  streets  where  many  factors  may  effect  the  desired  result. 
With  these  points  established,  the  minimum  and  maximum  illu- 
mination can  be  determined  usually  by  the  point-to-point  method. 

When  very  low  average  illumination  is  to  be  established,  the  de- 
termination of  the  minimum  illumination  may  be  all  that  is  neces- 
sary. Some  typical  installations  illustrating  the  grades  of  street 
lighting  given  earlier  in  this  lecture  may  be  described  as  follows: 

The  so-called  'Times  Square"  or  "Longacre  Square,"  New  York, 
may  be  cited  as  a  good  example  of  an  area  where  Class  AA  lighting 
is  desirable.  The  " Square"  is  formed  by  the  two  triangles  meeting 
at  their  apexes  and  extends  from  43d  to  46th  Street.  Two  double- 
track  street  railways  cross  each  other  at  an  acute  angle  and  great 
intersecting  streams  of  traffic  of  street  cars,  motors,  carriages  and 
pedestrians  occur  until  very  late  at  night.  A  certain  illumination  is 
obtained  until  quite  late  from  electric  signs,  etc.,  and  from  street 
lamps  on  the  edge  of  the  Square;  we  assume  this  to  average  o.i  foot- 
candles,  and  that  it  was  desired  to  bring  up  the  average  of  the  Square 
to  0.5  foot-candles.  It  is  noted  that  the  sidewalks  around  the  Square 
are  well  lighted  and  the  area  over  which  the  lighting  is  to  be  increased 
is  in,  the  middle  beyond  the  sidewalks.  Three  ornamental  poles,  one 
placed  at  the  apex  near  46th  Street,  one  near  44th  Street,  on  an  isle 
of  safety,  and  one  on  the  sidewalk  at  center  near  43d  Street,  equipped 
with  four  brackets  suspending  the  lamps  at  45  ft.,  would  be  the  best 


LACOMBE:  STREET  LIGHTING  483 

height  and  location.  Four  lamps  per  post  of  about  1500  mean  hemi- 
spherical candle-power  each,  so  equipped  with  reflectors  as  to  give 
about  9000  lumens  within  75°  from  the  vertical  would  produce  the 
desired  result.  Auxiliary  but  smaller  lamps  would  be  placed  on 
posts  at  45th  Street.  This  is  the  apex  of  each  triangle  and  the  large 
posts  would  be,  respectively,  300  ft.  to  the  north  and  240  ft.  and  560  ft. 
to  the  south  of  this  point.  A  standard  type  of  "Type  C"  lamp  and 
pendant  fixture  with  reflector  will  fill  the  requirements.  At  the 
height  stated  the  glare  is  negligible. 

A  type  of  street  of  the  A  Class,  which  would  be  a  "White  Way" 
street  in  a  smaller  city,  having  an  average  illumination  of  about  0.35 
foot-candles  and  a  minimum  of  0.15  would  be  equipped  about  as 
follows:  On  a  street  50  ft.  wide  4-amp.  luminous  arc  lamps,  one  per 
post  staggered  and  65  ft.  apart,  18  ft.  high,  with  diffusing  globe  and 
high  efficiency  electrodes,  would  give  2500  lumens  per  lamp,  which 
would  produce  the  required  average.  Standard  globes,  mountings 
and  posts  can  be  obtained  for  this  equipment. 

Class  "B"  lighting  would  be  produced  on  a  street  95  ft.  wide 
with  75o-watt  "Type  C"  lamps  at  18.5  ft.  from  the  ground,  with 
standard  lantern  and  diffusing  globe,  arranged  staggered  100  ft. 
apart  between  streets  and  60  ft.  apart  at  street  intersections,  this 
arrangement  giving  by  measurement  an  average  of  0.25  foot-candle 
with  a  minimum  of  0.044. 

Class  "C."  This  illumination  can  be  obtained  from  100  c-p. 
"Type  C"  lamps,  120  ft.  apart  on  a  street  45  ft.  wide,  lamps  placed 
on  poles  along  one  side,  15  ft.  in  height,  equipped  with  slightly  coned, 
radial  wave  reflectors  with  filament  of  lamp  carefully  focussed  in 
reflector,  the  resulting  average  illumination  being  0.075,  with  a 
minimum  of  0.015  foot-candle. 

Class  "D"  lighting  is  about  that  needed  for  a  boulevard  street  of 
high  class  with  many  trees.  A  very  effective  installation  giving 
about  this  illumination  would  be  one  with  25o-c.p.  lamps  in  orna- 
mental fixtures  with  reflector  and  diffusing  globes,  mounted  about 
15  ft.  in  height,  placed  parallel  and  opposite  along  a  street  100  ft. 
wide  and  1 10  ft.  apart.  Concrete  posts  like  those  in  the  Parkways  of 
Chicago  would  be  attractive  in  this  case.  Another  method  would 
be  placing  i5o-c.p.  lamps  with  light  diffusing  globes  at  a  height  of 
14  ft.,  no  ft.  apart  along  the  curb,  staggered  on  a  street  60  ft.  wide. 
.  Class  "E"  lighting  is  about  that  produced  by  the  familiar  vertical 
mantle  gas  lamp  9  ft.  10  in.  high  when  in  first-class  condition,  spaced 
90  ft.  apart,  staggered  on  a  street  60  ft.  wide. 


484  ILLUMINATING   ENGINEERING   PRACTICE 

Class  "F"  intensities  depend  largely  on  the  amount  of  money 
that  can  be  devoted  to  them.  The  minimum  is  given  for  interurban 
conditions;  it  is  very  low  and  when  within  a  city  the  illumination 
should  be  raised  to  Class  "E"  or  "D. "  Such  lighting  as  designated 
by  "F"  can  be  obtained  with  400  c.p.  "Type  C"  lamps  with  80° 
refractors  22  ft.  high,  center  suspension,  500  ft.  apart. 

Class  "  G"  streets  or  country  roads  so  far  have  been  lighted  mainly 
for  directional  effect  only  and  have  already  been  described. 

It  is  well  to  note  in  connection  with  the  general  fashion  of  "  White 
Way"  lighting  with  its  increased  intensities  over  those  of  a  few 
years  ago,  that  such  lighting  has  benefited  only  small  portions  of  a 
city's  streets  and  that  the  balance  suffers  from  the  appropriation  of 
funds  for  this  purpose  only.  Usually  little  effort  has  been  made 
to  improve  properly  the  remaining  lighting  in  the  municipality 
since  improved  appliances  became  available. 

It  is  necessary  to  explain  that  the  intent  of  this  lecture  has  been 
to  cover  briefly  the  utilitarian  side  of  street  lighting.  The  most 
attractive  side,  that  of  ornamental  and  artistic  street  lighting  and 
fixtures,  could  not  be  touched  on  in  the  time  allotted.  However, 
it  is  proper  to  emphasize  the  desirability  of  such  illumination,  and 
while  it  is  usually  expensive  and  limited  to  certain  streets  in  the 
larger  cities,  every  effort  should  be  made  to  attain  it,  even  in  more 
commonplace  installations.  A  pleasing  effect  does  not  depend  on 
expense  alone,  and  good  taste  may  be  exercised  with  simple  standard 
forms  as  well  as  with  more  expensive  ones. 

CONTRACTUAL  RELATIONS 

The  increase  and  development  of  good  street  lighting  depends  to  a 
very  important  extent  on  the  contractual  relations  between  the 
public  utility  and  the  municipality,  and  these  relations  should  be 
cooperative,  harmonious  and  progressive  to  develop  this  work  to  its 
full  growth  and  usefulness  which  have  not  yet  been  attained.  The 
provisions  of  a  contract  between  the  two  parties  should  cover  fully 
the  complete  illumination  service,  the  equipment  and  investment 
necessary  thereto,  and  the  remuneration  therefor.  It  should  pro- 
vide for  the  continuous  production  of  a  stated  quantity  of  light  at 
the  points  desired  for  certain  hours  in  a  given  period  of  time,  and  also 
the  service  by  which  the  quality  of  the  illumination  is  maintained. 
It  should  also  be  flexible  and  provide  for  increases  or  decreases  in 
number  of  units  and  changes  in  type  of  units  and  appurtenances  with 


LACOMBE:  STREET  LIGHTING  485 

the  normal  development  of  the  art.  The  full  details  of  such  a  con- 
tract cannot  be  given  here.  The  legal  form  is  usually  provided  by 
legal  advisors  of  a  city,  and  will  embody  the  statutory  or  local  legal 
requirements.  The  specifications  covering  the  work  of  the  lighting 
contractor  should  include  the  following  requirements  and  conditions: 

CONTRACT  REQUIREMENTS 

I.  General. — (a)  Equipment  to  be  first-class,  efficient  and  safe. 

(b)  Contractor  to  be  responsible  for  injury  or  damage  and  to 
indemnify  the  city  against  patent  infringements. 

(c)  Contractor  to  exercise  skill  and  foresight  in  carrying  out  the 
provisions  of  the  contract. 

//.  Work  to  be  Performed. — (a)  Requires  the  furnishing  of  all 
lamps,  supports,  connections,  appurtenances,  electric  energy,  repairs 
and  all  service  for  operation  and  maintenance. 

(b)  States  the  respective  numbers  of  each  size  and  kind  of  lamp 
to  be  furnished  at  the  beginning  of  the  contract,  including  technical 
description  of  the  same  with  its  equipment  and  operating  supplies, 
if  any.     The  rated  energy  and  illumination  performance  when  prop- 
erly operated  to  be  described  and  submitted. 

(c)  States  the  respective  lamps  to  be  supplied  with  energy  from 
underground  and  from  overhead  circuits,  with  the  type  and  method 
of  support.     A  list  of  locations  should  be  furnished,  preferably  with 
a  map  indicating  the  kind  of  service  connection  and  support. 

(d)  As  the  locations  and  kind  of  supports  are  usually  prescribed 
or  approved  by  the  city,  this  clause  would  provide  for  submitting 
the  necessary  samples,  photographs  or  maps  to  the  city  within  a 
specified  time  and  getting  its  approval  of  these  items. 

///.  Operation. — Recites  the  conditions  of  the  operation  and 
maintenance  of  the  lamps  and  all  their  appurtenances,  practically 
covering  the  lighting  service,  including  the  uniformity  of  lighting, 
elements  of  the  same,  regulation,  trimming  or  replacement,  cleaning, 
painting  and  prompt  repairs.  This  clause  should  require  complete 
operation  in  accordance  with  the  best  modern  practice. 

IV.  Testing. — Should  cover  the  tests  to  be  made  by  city  of  the 
fulfilment  of  the  contract  requirements  as  to  the  light  given  by 
the  unit.  This  is  usually  required  either  in  the  terms  of  energy  or 
illumination  or  both;  tests  to  be  made  either  on  the  streets  or  in  the 
laboratories  or  both.  This  would  usually  be  accompanied  by  clauses 
covering  the  quality  of  service  and  inspection  thereof. 


486  ILLUMINATING   ENGINEERING   PRACTICE 

V.  Increase  or  Decrease  in  Number  of  Lamps. — -(a)  In  case  of 
additional  lamps,  would  require  compliance  with  previous  specifi- 
cations as  to  type  of  supports,  connections,  lamp  units  and  so  on, 
and  locations  as  specified  by  the  city. 

(b)  Should  cover  the  installation  of  additional  lamps  and  damage 
and  allowance  for  delay. 

(c)  Would  cover  limitations  as  to  distance  of  extensions  from  the 
present  circuits  on  either  the  underground  or  the  overhead  circuits, 
without  cost  to  the  city.     Would  also  cover  the  payment  by  the 
city  if  the  lamps  are  placed  at  distances  greater  than  the  limits 
agreed  upon. 

(d)  Defines  the  right  of  the  city  to  discontinue  lamps  entirely  or 
change  from  overhead  to  underground  circuits,  depending  on  the 
rates  for  the  lamps  on  each  circuit  or  with  provisions  for  refunding 
to  the  company  the  unamortized  cost  of  the  equipment. 

(e)  Would  state  the  conditions  in  long-term  contracts  as  to  the 
lamps  and  equipment  ordered  late  in  the  life  of  the  contract  and  the 
unamortized  cost  of  the  same  toward  the  end  of  the  contract  term, 
if  it  was  not  renewed. 

(/)  Requirements  to  be  stated  covering  deductions  as  liquidated 
damages  for  outages. 

(g)  Provision  covering  arrangements  by  which  another  type  of 
improved  lamp  may  be  tried  and  adopted  if  desirable  to  both  parties. 
This  clause  would  also  provide  method  of  estimating  the  cost  of 
such  change  relative  to  the  cost  of  the  original  equipment. 

(h)  Provision  should  be  made  for  covering  arbitration  of  disputes 
between  the  parties  to  the  contract,  or  of  amendments  to  the 
contract. 

(i)  Conditions  to  be  stated  covering  payments  by  the  city  under 
the  contract  in  accordance  with  the  local  statutes  governing  this 
process. 

It  will  be  understood  that  this  list  of  contract  provisions  is  general 
and  will  require  considerable  amplification  and  change  in  many 
instances.  It  is  understood,  of  course,  that  every  contract  stands 
on  its  own  bottom  and  no  general  rules  can  be  laid  down  to  cover 
local  policies  and  conditions. 

PRESENT  PRACTICE   IN    CONTRACTS 

When  one  considers  the  many  varied  circumstances  that  have 
existed  in  the  development  of  the  electric  lighting  industry  since  its 


LACOMBE:  STREET  LIGHTING  487 

inception  and  the  various  political  forms  of  government  under  which 
contracts  have  been  made,  it  is  not  strange  that  these  contracts 
vary  very  much  in  certain  provisions,  and  that  no  standard  form 
seems  to  exist,  even  at  this  late  date.  It  is  interesting  therefore  to 
observe  the  data  obtained  by  an  inquiry  on  this  subject  and  others, 
instituted  by  The  Milwaukee  Electric  Railway  and  Light  Company 
in  1915.  Through  the  courtesy  of  the  company  a  partial  summary 
of  it  can  be  submitted  to  you,  the  limited  time  requiring  that  only 
the  most  important  and  usually  disputed  points  be  touched  on. 

It  might  be  deduced  from  the  data  collected  that  contracts  were 
divided  into  two  great  classes  depending  on  the  theory  on  which 
the  contract  is  based.  One  class  obviates  specific  clauses  covering 
removals,  changes  of  equipment,  types  of  units,  etc.,  by  providing 
a  margin  or  factor  of  safety  in  its  rates  to  generally  cover  these 
points.  The  other  class  takes  up  special  charges  in  detail  and  pro- 
vides for  them  specifically  outside  of  the  rates  for  the  illumination 
service  itself.  Data  were  obtained  from  about  128  different  com- 
panies in  cities  varying  from  20,000  to  over  200,000  in  population, 
but  omit  Chicago,  Philadelphia,  and  portions  of  Greater  New 
York. 

From  the  data  it  appears  that  the  length  of  term  of  contract 

In  21  cases  was  under  5  years 

In  44  "  "                5      " 

In  7  "  "  over  5   "    and  under  10 

In  48  "         10  " 

In  4  "  "  over  10  " 

In    4  "  there  was  no  contract. 

As  to  the  question  of  the  right  of  a  city  to  increase  the  number  of 
lamps,  there  was  no  restriction  in  96  cases.  As  to  the  right  of  remov- 
ing lamps  from  one  location  to  another,  there  was  no  restriction, 
except  that  in  a  little  more  than  half  the  cases  the  companies  bore 
all  the  costs,  while  in  the  balance  the  city  generally  paid  the  costs. 

As  to  ordering  lamps  changed  from  overhead  to  underground 
circuits,  in  over  half  the  cases  the  city  had  no  right  to  do  so.  In 
39  cases  the  city  had  the  right,  but  it  was  rare  that  the  company 
was  remunerated  for  such  change. 

No  provision  was  made  for  tests  of  illumination  or  energy  in  58 
cases,  nearly  half  the  total  number.  In  34  cases  the  city  may  test ;  in 
the  majority  of  these  cases  the  tests  were  for  energy  consumption 
only. 

Referring  to  the  distance  of  the  location  of  a  new  lamp  from  the 


488  ILLUMINATING   ENGINEERING   PRACTICE 

nearest  circuit,  it  was  found  in  72  cases  that  there  were  no  limits. 
In  49  cases  it  was  limited  by  various  distances.  Where  the  distance 
to  the  location  of  the  new  lamp  exceeds  the  limit,  in  36  cases  no  pro- 
vision was  made  to  cover  excess  cost,  but  in  20  cases  the  city  paid 
the  cost  for  the  excess  distance. 

The  discontinuance  of  lamps  is  allowed  to  a  minimum  number  in 
45  cities;  in  35  cities  the  city  may  discontinue  as  much  as  it  pleases, 
in  22  cases  no  provision  is  made,  and  in  n  cases  discontinuance  of 
lamps  is  not  allowed  at  all. 

Provisions  for  substitution  of  improved  lamps  are  found  in  the 
contracts  of  only  29  cities.  In  27  of  these,  the  conditions  on  which 
such  changes  may  be  made  vary  from  no  additional  charge  to  the 
full  cost  in  excess  of  the  original  contract  requirements. 

Outage  regulations  and  penalties  are  enforced  in  only  40  cities. 

Contracts  are  subject  to  revision  as  to  price  by  agreement  in 
14  cities. 

Contract  rates  are  subject  to  regulation  and  amendment  by 
national,  state  and  municipal  authorities  as  follows: 

In  1 8  cases  by  the  municipality. 

In  55  cases  by  public  service  commissions. 

In    5  cases  by  the  state  authorities. 

In    4  cases  by  the  state  and  municipal  authorities. 

In    i  case  by  Congress. 

The  queries  just  mentioned  are  those  which  might  cause  more  or 
less  serious  disputes  between  the  parties  to  a  street-lighting  contract, 
and  this  extremely  wide  variation  in  practice  is  difficult  to  explain. 
It  certainly  shows  a  state  of  looseness  in  contracts  which  under  usual 
business  conditions  would  produce  needless  disputes,  but  this  gener- 
ally does  not  seem  to  have  been  the  case. 

MEASURE  OF  ILLUMINATION  SERVICE 

Among  the  most  frequently  discussed  questions  in  contract 
requirements  is  that  of  a  satisfactory  measure  of  the  illumination 
service  rendered  to  the  city.  It  is  a  complicated  matter  and  from 
the  municipal  standpoint  in  actual  practice  should  be  handled  on 
administrative  engineering  lines  based  on  correct  technical  data. 

Good  illumination  service  implies  two  things.  One  is  a  satis- 
factory light-giving  source,  and  the  other  is  the  attention  given  to 
it  or  the  service  which  keeps  the  source  of  illumination  at  its  maxi- 
mum efficiency,  and  provides  for  its  continuous  and  regular  opera- 


LACOMBE:  STREET  LIGHTING  489 

tion.  These  two  should  be  taken  together  in  measuring  the 
result. 

The  older  types  of  lighting  units  such  as  the  carbon  arc  lamp 
varied  and  were  irregular  in  intensity,  so  that  much  difficulty  was 
encountered  in  establishing  a  satisfactory  measure  of  illumination. 
It  was  very  difficult,  if  at  all  practical,  to  transport  the  units  to  the 
laboratories  and  duplicate  street  conditions.  It  was  also  difficult 
to  handle  smaller  and  fragile  units,  such  as  mantle  gas  or  vacuum 
tungsten  lamps.  The  more  modern  illuminants  are  comparatively 
rugged,  vary  little  in  actual  operation  and  may  be  tested  with  greater 
ease  and  more  definite  results.  A  satisfactory  specification  can  now 
be  drawn  covering  such  lighting  units  as  the  luminous  arc  or  the 
"Type  C"  incandescent  lamp,  either  of  which,  with  specific  equip- 
ment, will  develop  a  predetermined  distribution  of  intensity  from 
the  light  source  when  a  specific  amount  of  energy  is  delivered  at  the 
lamp  and  the  variations  of  this  may  be  fixed  within  close  limits 
with  modern  regulating  appliances.  A  diagram  showing  the  candle- 
power  distribution  with  the  mean  spherical  candle-power  and  the 
flux  data  for  a  given  lamp  and  equipment  should  be  required  and  this 
from  time  to  time  may  be  used  as  a  check  on  the  fulfillment  of  the 
contract. 

On  the  usual  series  circuits  a  simple  system  of  recording  ammeters 
operated  and  owned  by  the  city  would  check  the  energy  used 
daily  and  a  reasonable  inspection  force  can  determine  whether  other 
service  conditions  were  fulfilled.  With  limiting  requirements  as 
to  the  economical  life  of  incandescent  lamps,  in  view  of  loss  in 
candle-power,  inexpensive  street  or  laboratory  tests  by  the  nearest 
university  or  laboratory,  on  units  taken  at  random,  would  afford 
a  practical  check  on  the  illumination  service  rendered  and  be 
within  the  means  of  almost  any  city  spending  a  reasonable  sum  for 
its  lighting.  In  large  cities,  involving  various  types  of  units  and 
service,  an  elaboration  of  this  system  with  the  aid  of  a  testing 
laboratory  would  accomplish  satisfactory  results.  In  such  cases  the 
inspection  service  becomes  even  more  important,  as  usually  in  such 
cities  there  are  a  number  of  light  sources  which  vary  largely  in  accord- 
ance with  the  attention  given  them,  such  as  mantle  gas  lamps. 
These  vary  from  so  many  causes  that  the  inspection  of  the  results  of 
the  attention  given  to  the  unit  and  service  given  by  it  is  most  im- 
portant, for  the  illumination  shown  by  tests  of  these  lamps  on  the 
street  varies  within  a  considerable  percentage  even  when  they  are 
well  maintained. 


490  ILLUMINATING  ENGINEERING   PRACTICE 

UTILIZATION   OF  IMPROVEMENTS 

Another  important  provision  of  a  lighting  contract  of  any  length 
is  a  clause  providing  a  means  of  utilizing  improved  lighting  appliances. 

General  dissatisfaction  will  occur  when  the  people  of  one  city 
see  in  other  cities  considerable  improvements  in  lighting  which  they 
cannot  have  on  account  of  unsatisfactory  contract  conditions.  It  is 
wise  public  policy,  therefore,  for  many  reasons  to  include  in  a  long- 
term  contract  a  clause  which  will  allow  for  the  trial  and  possible 
substitution  of  improvements  in  lighting  units  and  adjustments  of 
costs.  Such  a  clause  should  provide  for  a  thorough  service  trial 
and  conservative  methods  of  change;  for  if  lighting  systems  had  been 
changed  as  rapidly  as  the  successive  leading  improvements  have 
taken  place  in  the  past  few  years,  a  heavy  financial  loss  would  have 
ensued. 

You  will  note  from  the  statistics  already  given  of  the  length  of  term 
of  contracts,  that  103  out  of  128  were  for  5  years  or  over  in  length. 
This  is  due  undoubtedly  to  the  general  business  advantages  of  long- 
term  contracts.  Formerly  there  was  little  if  any  possible  revision 
during  the  term  of  the  contract,  but  conditions  have  changed.  A 
large  number  of  these  contracts  are  now  subject  to  possible  revi- 
sion as  to  rates  or  improvements  during  the  life  of  the  contract,  a 
few  by  provisions  of  the  contract  itself,  the  larger  number  by  public 
service  commissions  which  on  their  own  initiative,  or  on  request 
by  the  city,  may  investigate  and  revise  certain  contract  conditions. 

STATE  CONTROL  OF  CONTRACTS  AND  EFFECT 

State  control  of  public  utilities  has  a  decided  effect  on  contracts 
for  street  lighting.  With  well-administered  companies  it  removes 
possible  competition.  Such  a  company  normally,  therefore,  will 
continue  to  perform  the  street-lighting  service  and  its  equipment 
therefore  will  be  available  for  its  whole  economic  or  useful  life.  To 
this  extent,  therefore,  the  former  value  of  a  long-term  contract  is 
lessened. 

Rates  for  street  lighting  have  also  been  affected.  Where  this 
question  has  been  investigated  by  public  service  commissions  the 
trend  of  their  decisions  generally  follows  the  theory  of  cost  of 
service  with  a  reasonable  rate  of  return.  In  many  cases  they  have 
obtained  the  approved  contract  rate  through  an  inventory  and  cost 
apportionment  precisely  similar  in  method  to  that  used  in  estab- 
lishing rates  for  commercial  business. 


LACOMBE:  STREET  LIGHTING  491 

Generally  this  has  been  done  without  allowing  any  marked  dis- 
crimination in  favor  of  a  municipality.  Free  service  particularly 
is  condemned  and  is  apportioned  as  part  of  the  cost  of  street  light- 
ing. The  commissions  rarely  interfere,  however,  in  a  street-light- 
ing contract  so  long  as  its  terms  are  reasonable  and  encourage  a 
practical  and  liberal  policy  of  adjustment  during  the  life  of  the 
contract.  They  usually  allow  rates  for  energy  lower  than  published 
tariff  rates  for  various  reasons,  among  them  being  the  facts  that  the 
city  is  a  single  customer,  generally  requires  no  meters,  contracts  for 
long  periods,  and  its  uses  of  energy  develop  a  favorable  load-factor. 

Apparently,  therefore,  the  question  of  rates  for  street  lighting  in 
the  future  under  public  service  commissions,  will  tend  toward  cost 
of  service  plus  a  reasonable  return,  both  of  which  depend  to  a  certain 
degree  on  the  size  and  length  of  the  contract,  the  service  required, 
and  the  general  business  of  the  company  in  the  community  in  which 
it  operates.  The  most  advanced  form  of  a  contract  based  on  cost 
and  return  is  that  which  has  been  recently  adopted  in  the  state  of 
Wisconsin  and  known  as  the  "  Indeterminate  Contract."  This 
has  been  fully  described  in  the  technical  literature  of  the  day.  In 
brief,  it  is  a  contract  where  the  company  becomes  the  agent  of  the 
city,  carrying  out  its  wishes  at  cost  and  receiving  for  its  remunera- 
tion, in  addition  to  ah1  costs  including  amortization  of  equipment, 
only  a  reasonable  return  on  the  business  involved.  The  initial 
investment  basis  and  rate  of  return  is  established  by  agreement  or 
by  a  decision  of  the  Public  Service  Commission  and  the  items  of  all 
accounts  are  rendered  to  the  city  yearly.  This  form  of  contract 
should  encourage  the  use  of  higher  intensities  of  illumination,  for 
under  it  a  larger  illuminant  may  be  used  in  place  of  a  smaller  one 
with  a  relatively  small  increase  of  the  cost  of  the  service. 

REDUCTION  OF   COST  AND  INCREASE  IN  USE   OF  LIGHT 

Summing  up  the  situation,  it  may  be  said  that  the  general  trend 
of  the  various  factors  bearing  on  rates  for  street  lighting,  such  as 
improved  and  more  efficient  units  and  state  control,  is  toward  low 
rates.  This  in  itself  encourages  a  more  liberal  use  of  street  lamps 
and  better  lighting.  This  greater  volume  of  business  tends  to  offset 
the  lower  return  per  unit. 

There  is  a  large  field  of  needed  improvement  in  street  lighting 
in  this  country,  not  only  in  the  normal  increase  in  numbers  of  lamps 
but  in  the  increased  intensity  really  required  by  modern  condi- 


4Q2  ILLUMINATING   ENGINEERING   PRACTICE 

tions.  Its  development  may  be  greatly  accelerated  when  costs  are 
decreasing,  and  a  wise  business  policy  will  urge  the  advantages  of 
improved  lighting  where  favorable  prices  occur. 

GOOD  PUBLIC  POLICY 

A  careful  far-seeing  public  policy,  after  providing  for  good  ser- 
vice with  all  that  that  requires,  will  proceed  to  the  education  of  the 
public  in  the  possibilities  of  the  illumination  of  the  city  and  the 
ability  of  the  contractor  to  furnish  such  illumination  at  reasonable 
rates. 

This  function  naturally  devolves  on  the  public  utility.  With 
this  in  view,  it  is  not  enough  to  restrict  one's  efforts  to  the  require- 
ments of  the  day.  The  relations  between  the  city  and  company 
should  be  such  as  to  make  it  possible  for  the  company  to  show  the 
city  the  improvements  in  the  art  as  they  occur,  and  to  explain 
how  the  city  can  best  use  its  available  funds  in  the  increase  and 
improvement  of  the  street  lighting  so  as  to  attract  people  and  busi- 
ness to  it.  This  effort  should  be  continuous  and  of  the  same  general 
persistence  used  for  commercial  customers,  and  the  utility  should 
remember  that  it  has  a  dual  responsibility  in  the  improvement  and 
welfare  of  the  city,  for  what  helps  the  city  helps  the  company. 

Even  if  this  were  done  without  profit  or  with  a  relatively  low  rate 
of  return,  the  effort  would  be  justified  from  the  increased  business 
derived  from  the  general  improvement.  It  is  difficult  to  imagine 
a  better  method  for  the  advancement  of  street  illumination  or  for 
retaining  the  good  will  of  a  community. 


RAILWAY  CAR  LIGHTING 

BY  GEORGE  H.  HULSE 

The  proper  lighting  of  railway  cars  has  always  offered  special 
problems,  both  in  regard  to  the  methods  employed  in  producing 
the  energy  for  lighting,  and  in  the  application  of  the  light  sources 
to  obtain  proper  illumination. 

As  methods  of  lighting  have  been  improved,  the  new  methods  have 
been  applied  to  the  lighting  of  cars  with  such  modifications  as  the 
special  conditions  make  necessary.  Oil  lighting  superseded  can- 
dles, gas  displaced  oil  lighting,  and  for  a  time  completely  dominated 
the  field,  but  at  the  present  time,  as  in  other  places,  the  field  is 
divided  between  gas  and  electricity. 

GAS  LIGHTING 

Practically  all  cars  using  gas  light  employ  oil  gas  as  the  illuminant. 
As  the  storage  space  available  is  limited,  it  is  necessary  to  carry  the 
gas  under  pressure  in  order  to  have  a  sufficient  supply  on  the  car, 
and  also  to  have  a  gas  of  comparatively  high  illuminating  value. 
Coal  gas,  of  low  candle-power  primarily,  loses  at  least  50  per  cent, 
of  its  illuminating  value  when  compressed  to  a  point  high  enough 
to  give  sufficient  storage.  Oil  gas  has  not  only  a  much  higher  candle- 
power  uncompressed,  but  when  compressed  to  ten  atmospheres, 
loses  only  10  per  cent,  of  its  illuminating  power. 

Oil  gas  is  made  by  the  distillation,  or  "cracking"  of  petroleum 
oil  in  cast  iron  or  clay  retorts,  or  in  steel  generators  filled  with  fire 
brick  checker  work.  A  fixed  gas  is  formed  which  has  for  its  prin- 
cipal ingredients  methane  and  heavy  illuminants  with  a  very  small 
amount  of  hydrogen.  It  has  a  heating  value  when  compressed  of 
1250  heat  units  per  cubic  foot.  After  passing  through  proper 
washing  and  purifying  apparatus,  the  gas  is  compressed  to  1 2  atmos- 
pheres in  store  holders,  from  which  it  is  carried  to  the  railroad  yards 
by  suitable  pipe  lines.  The  car  holders  are  filled  from  these  pipe 
lines.  The  car  equipment  (see  Fig.  i)  consists  of  one  or  more  welded 
steel  holders  to  contain  the  gas  supply,  two  filling  valves,  a  pres- 
sure gauge,  a  regulator  for  reducing  the  holder  pressure  to  that  at 

493 


494 


ILLUMINATING   ENGINEERING    PRACTICE 


which  the  lamps  operate,  the  pipe  line  for  carrying  the  gas  from 
the  holders  to  the  lamps,  and  the  lamps  or  burners.  All  fittings 
for  both  the  low-pressure  and  high-pressure  piping  are  especially 
designed  for  the  work.  The  pressure  regulator  is  placed  under  the 
car  near  the  holders  so  that  the  amount  of  high-pressure  piping  is 
small  and  none  of  it  is  inside  the  car. 

The  pressure  regulator  reduces  to  the  proper  pressure  and  main- 
tains this  pressure  constant  with  the  varying  amounts  of  gas  used. 

At  the  beginning  oil  gas  was  burned  in  regenerative  lamps  with 
a  cluster  of  from  two  to  four  burners  of  the  union  jet  type.  All 
lamps  used  at  the  present  time  are  fitted  with  incandescent  mantles. 

One  of  the  early,  and  probably  the  earliest  completely  successful 
application  of  the  inverted  mantle  was  to  car  lighting.  This  was 


Fig.  I. — Diagram  of  Pintsch  gas  equipment  using  mantle  lamps. 

due  to  the  fact  that  while  the  development  of  the  inverted  mantle 
for  general  use  had  to  contend  with  low  and  varying  pressure,  in 
car  lighting  a  sufficient  and  uniform  pressure  was  at  all  times 
available. 

In  this  country  two  sizes  of  mantle  are  used,  one  which  gives 
28  candle-power,  with  a  gas  consumption  of  0.8  cubic  feet  per  hour, 
the  gas  pressure  being  i  pound  per  square  inch.  The  use  of  this 
size  mantle  is  limited  to  bracket  lamps,  and  a  few  installations  in 
which  four  of  these  mantles  are  employed  in  a  cluster  for  center 
lighting. 

The  other  size  of  mantle,  which  is  used  for  center  lamps  gives 
90  candle-power,  with  a  gas  consumption  of  2  cubic  feet  per  hour 
at  2  pounds  pressure  per  square  inch. 

The  mantles  used  are  of  a  special  form  and  composition  to  with- 


HULSE:  RAILWAY  CAR  LIGHTING  495 

stand  the  rigors  of  railway  service,  and  give  three  months  average 
life  in  service. 

There  are  upward  of  85  gas  plants  for  the  manufacture  of  oil 
gas  in  the  United  States  and  Canada.  Gas  is  delivered  to  the  car 
holders  and  charged  for  at  a  uniform  rate,  the  amount  of  gas  sup- 
plied being  measured  by  the  increase  of  gauge  pressure. 

The  holders  are  made  exact  size  and  the  contents  of  a  holder  can 
always  be  determined  by  multiplying  its  capacity  by  the  gauge  pres- 
sure in  atmospheres.  This  feature,  besides  furnishing  a  means  of 
measuring  the  amount  of  gas  supplied,  is  important  for  determining 
the  hours  of  lighting  which  a  holder  contains  and  also  for  the  purpose 
of  car  interchange. 

Cars  using  oil  gas  are  dependent  upon  stationary  plants,  but  this 
has  not  been  found  to  be  a  disadvantage,  principally  because  the 
time  required  to  charge  a  car  is  so  short.  It  can  be  done,  if  neces- 
sary, at  a  division  station  stop. 

ELECTRIC  LIGHTING 
Three  methods  of  electric  lighting  for  railway  cars  are  in  use: 

1.  The  head-end  system. 

2.  Straight  storage. 

3.  Axle-driven  generators. 

The  Head-end  System. — In  the  head-end  system  use  is  made  of  a 
generator  driven  by  a  steam  engine  at  the  head  of  the  train,  either 
in  the  baggage  car  or  on  the  locomotive.  Electrical  energy  is  car- 
ried back  from  the  generator  to  the  cars  to  be  lighted  by  means  of  a 
train  line  on  the  car  roof  and  connectors  between  the  cars. 

In  this  country  the  generator  of  a  head-end  system  is  usually 
installed  in  the  baggage  car,  and  is  driven  by  a  steam  turbine,  steam 
for  its  operation  being  brought  from  the  locomotive  through  suit- 
able hose  connections.  A  very  few  equipments  are  in  service 
with  the  generating  set  mounted  on  the  locomotive,  but  this  entails 
heavy  installation  cost,  since  several  locomotives  may  be  used  in 
hauling  one  train  over  its  trip. 

As  the  steam  supply  is  shut  off  when  the  locomotive  is  detached 
from  the  train  it  is  necessary  to  have  a  storage  battery  on  one  of 
more  of  the  cars  to  supply  light  during  such  time  as  the  locomotive 
is  detached  at  terminals  or  division  points. 

The  head-end  system  gives  efficient  and  economical  results,  but 


496  ILLUMINATING   ENGINEERING   PRACTICE 

its  great  disadvantage  is  that  light  can  only  be  used  when  a  car  is 
in  a  train  with  a  generator  equipment.  If  the  cars  are  equipped  with 
batteries  to  supply  light  during  such  times  as  the  locomotive  is  dis- 
connected, the  proper  arrangements  for  charging  the  batteries  entail 
a  sacrifice  of  simplicity  and  economy. 

Straight  Storage. — -In  the  straight  storage  system  each  car  is 
equipped  with  a  set  of  storage  batteries  of  sufficient  capacity  to 
supply  energy  to  the  lamps  for  the  desired  trip.  As  ordinarily  ap- 
plied, the  equipment  is  simple,  consisting  of  lamps,  storage  batteries 
and  charging  receptacles,  with  necessary  wiring.  At  terminal  yards 
the  batteries  are  charged  with  energy  obtained  from  a  stationary 
power  plant.  The  lamps  operate  directly  from  the  batteries,  no 
voltage  regulator  being  used. 

This  system  of  lighting  would  be  ideal  if  it  were  not  for  the  fact 
that  the  charging  of  the  batteries  consumes  too  much  time.  Gen- 
erally cars  are  not  available  in  one  location  long  enough  to  receive 
proper  charge. 

The  cost  of  equipping  a  railroad  yard  with  the  proper  charging 
lines  is  considerable. 

Car  lighting  systems  dependent  upon  stationary  plants  are  fea- 
sible as  shown  by  the  oil  gas  system,  but  the  time  required  for  charg- 
ing must  not  interfere  with  car  service. 

Axle  Driven  Generators. — In  this  system  the  car  axle  is  used  to 
drive  a  generator  which  supplies  energy  for  the  lamps  in  the  car, 
and  for  charging  a  storage  battery  which  supplies  energy  to  the 
lamps  when  the  car  is  running  below  a  certain  speed. 

The  equipment  consists  of  the  following: 

A  generator  mounted  either  on  the  car  body  or  truck  with  some 
form  of  driving  system  between  the  car  axle  and  the  generator,  a 
storage  battery  to  maintain  the  light  when  the  speed  of  the  generator 
falls  below  that  at  which  it  gives  the  proper  voltage,  regulating 
apparatus  to  govern  the  output  of  the  generator  at  varying  speeds, 
to  give  the  proper  charge  to  the  storage  battery,  and  to  maintain 
constant  voltage  at  the  lamps,  and  some  means  of  keeping  the  polar- 
ity of  the  battery  charging  current  constant  when  the  direction  of 
the  movement  of  the  car  is  reversed. 

Various  systems  have  been  devised  to  meet  these  conditions  and 
a  large  number  are  in  successful  operation  on  railway  cars.  The 
best  practice  is  exemplified  by  an  equipment  in  which  the  generator 
is  mounted  on  the  car  underframe,  the  generator  controlled  for  out- 
put at  varying  speeds  by  a  carbon  pile  rheostat  in  its  field  circuit, 


HULSE:  RAILWAY  CAR  LIGHTING  497 

which  give  a  constant  current  output  until  the  battery  approaches 
full  charge,  when  the  control  is  automatically  changed  to  constant 
voltage,  thereby  preventing  overcharge  of  the  battery.  The  voltage 
at  the  lamps  is  held  constant  by  an  automatic  carbon  pile  rheostat 
placed  between  the  battery  and  the  lamps. 

Of  the  three  different  systems  of  electric  car  lighting,  the  axle- 
driven  generator  system  is  and,  no  doubt,  will  continue  to  be  the 
one  most  used.  This  system  renders  the  car  absolutely  independent 
of  a  stationary  plant  and,  in  spite  of  its  seeming  complexity,  is  the 
only  one  of  the  three  systems  capable  of  general  application  to  cars. 
A  properly  designed  axle  system  is  superior  to  head-end  or  straight 
storage  system  as  it  renders  the  car  available  for  use  in  any  territory 
and  does  not  necessitate  lay-overs  at  charging  plants. 

CAR  ILLUMINATION 

Adequate  and  proper  illumination  of  passenger  cars  of  various 
types  presents  difficulties  not  met  with  in  other  lines  of  illuminating 
engineering.  Car  construction  limits  to  a  considerable  extent  the 
location  of  the  lighting  fixtures,  which,  in  combination  with  the 
seating  arrangements  makes  ideal  illumination  conditions  hard  to 
realize.  It  is  practically  impossible  to  have  the  lighting  fixtures 
out  of  the  range  of  vision,  and  very  adequate  screening  of  the  light 
source  is  necessary.  In  addition  to  this  the  constant  motion  of  the 
car  makes  it  necessary  to  have  a  greater  amount  of  illumination 
than  is  required  in  places  where  this  condition  does  not  exist. 

There  has  been  in  the  past  a  tendency  to  sacrifice  proper  illumina- 
tion results  in  favor  of  the  appearance  of  lighting  fixtures,  and  also 
to  look  for  an  appearance  of  light  in  the  car,  rather  than  for  proper 
illumination;  but  these  mistakes  are  rapidly  being  corrected,  and  I 
believe  that  the  practice  of  proper  illumination  has  advanced  as  far 
in  car  lighting  as  in  any  other  fields. 

Passenger  Coaches. — The  passenger  coach  is  the  type  of  car  that 
is  used  in  greatest  numbers,  and  its  proper  lighting  is  most  important. 
It  is  the  dividend  paying  car,  and  carries  the  great  bulk  of  the  travel- 
ing public. 

Aside  from  the  general  illumination  necessary,  the  principal  use 
of  artificial  illumination  in  a  coach  is  for  reading,  and  the  lighting 
system  should  be  so  designed  as  to  give  proper  illumination  on  the 
reading  plane,  which  is  45°  to  the  horizontal  and  at  right  angles  to 
the  center  line  of  the  car. 
32 


498 


ILLUMINATING   ENGINEERING   PRACTICE 


Owing  to  car  design  and  construction,  two  methods  of  lighting  are 
available,  as  regards  the  placing  of  the  lamps.  They  may  be  hung 
from  the  roof  in  a  single  row  down  the  center  line  of  the  car,  or  a 
row  may  be  placed  on  each  side  deck,  directly  over  the  seats.  An 


-DATA     ON      I  L 


TEST  No..jS.a>  ....... 

EFFICIENCY      AND     UNIFORMITY 


ILLUMINATION   33  IN.   ABOVE    FLOOR   OF  CAR 


45°  reading  plan 


Horizontal  plane 


>0"U.' 


Per  cent,  of  total  light  delivered  on -horizontal  plane 

Mean  variation  from  average,  45°  reading  plane 

Lowest  four  values  (excluding  station  20),  45°  reading  plane 

a.sof.c  z.<oo\  c.  a.aof.c.-s.aa  f.c. 

LIGHT  OUTPUT  OF  ONE  UNIT 

Total  light,  lamp  alone  := 7QO  lumens 

Lamp  with  reflector: — 

Distribution 100°  to  180°=  G4.E  lumens 

SO"  to  100°  =236.5  lumens 
Oe  to    50°=E94-.Slumen» 
Total 


&&$._  LIGHTING 

CEMIEBL  DUCK  S...SKAT  SPACING 
Type  of  Car  _7O  ft  COACH  

Description  of  Unit  "Pr.l5inat.IC 


Co. 


Screening  Angle  of  Reflecto 
Coefficient  ol  Reflector  of 
lining  =  £6.3  % 


ILLUMINATION    CUKVE.4.5..°..R£ADlN(i.PLANE 
Aisle  scat  illumination  shown  by  full  line  Window  sent  illuminttion  shown  bj  dotted  line 

For  Cm  Plan  and  Teil  Station  Locations  see  flan  fro. i. 

.scc.  of   Ry.   ECec.   Engr 


6       7      6      9      10      II      IZ      13      14     15     16 

STATIONS 


Fig.  2. — Illumination  results  in  coach  with  gas  mantle  center  lamps. 

elaborate  series  of  tests  made  a  short  time  ago  demonstrated  that 
equally  good  illumination  results  can  be  obtained  with  either  type  of 
installation.  Practical  considerations,  however,  make  for  center 
lighting  on  account  of  the  fewer  number  of  fixtures  used  and  the 
lesser  number  of  lamps  and  reflectors  to  maintain.  Another  con- 


HULSE:  RAILWAY  CAR  LIGHTING 


499 


sideration  is  that  with  side  lighting  shadows  are  likely  to  be  cast  by 
a  passenger's  head  which  will  interfere  with  the  proper  illumination 
of  his  own  or  another  person's  paper.  This  occurs  with  center  light- 
ing only  when  people  are  standing  in  the  aisles. 

TEST  No — 31 

DATA     ON      ILLUMINATION      EFFICIENCY      AND      UNIFORMITY 


ILLUMINATION   33  nt.    ABOVE   FLOOR   OP   CAR 


FlffTglC 


w^. 

^TH 

1.73 

a.oe 

2.<Z>I 

2.ee 

2^> 

LIGHTING 
SFACIXG 


Tvpe  of  Car  fOft 


Per  cent    of  total  light  delivered  on  hon 
Mean  variation  from  average.  45°  reading  plane 
Lowest  four  values  (excluding  station  20),  45 
l.5O(.c 


f.c.  1.59 


LIGHT  ODTTOT  OF  Out  Uurr 

Total  light,  lamp  alone :=    •• ! 

I^amp  with  reflector:— 

Distributkn 100°  to  180°  =     S^.llumeus 

50*  to  100°=     Vl-Olnmens 


Description  of  Unit  Prismatic  — 
*Rgf  \gctor  •  "B  owl  Unit  _  ' 
Hnlophang  Wk^.No.ia31Q, 
-_JaCLwatt.Gi3aTnngsten  Lamps 
Base  Contact  of  Electric  Lamp_l3(fc 
above  top  of_BEELECTOR 
Screening  Angle  of  Reflector  "•  •_•".;;_ 
Coefficient  of  Reflector  of  car  head- 
lining =  26.3  •/.  _ 


ILLUMINATION  CURVE  451  JVC AB.LNJ&. PLANE 
Aid*  mt  illnmin«tion  ibowu  by  fall  lint  Window  *e>t  i 

Far  C*r  Pl^n  **4  Ttst  Station  Loca!:mt  srt  f.'e*  A-».        i- 
Cenyrlght  1t<4  by  u»  Anoc.  of  Ry.  E:«c.  E««ra. 


i  jhown  by  dotted  line 


Fig.  3. — Illumination  results  in  coach  with  electric  center  lamps,  enclosing  bowl  type. 

The  following  are  some  of  the  results  obtained  in  the  test  of  which 
I  spoke  above: 

Fig.  2  shows  results  obtained  with  center  lamps  using  Pintsch 
gas,  with  mantle.  The  reflector  unit  is  a  prismatic  reflector  having 
a  prismatic  bowl  under  it  to  give  proper  light  distribution.  The 


5oo 


ILLUMINATING  ENGINEERING  PRACTICE 


prismatic  reflector  is  covered  on  the  outside  by  an  opal-glass  en- 
velope, adding  to  the  appearance  and  serving  to  keep  the  prismatic 
glass  clean.  This  arrangement  gives  very  uniform  lighting;  the 
illumination  of  the  aisle  and  window  sittings  being  practically  equal. 

TEST  No.       7 

D-ATA    ON    ILLUMINATION    EFFICIENCY    AND    UNIFORMITY 


ILLUMINATION   33  IN.   ABOVE   FLOOR  OF  CAR 


-^^T 

£.!i 

M,..,. 

Ai.l« 

H..i,. 

45°  reading  pUne 

I.-33 

2.€>1 

a/30 

Horuont. 

plane 

3.0  a 

•4.1  fo 

3.30 

Per  cent   of  total  light  delivered  on  horizontal  plane  4-6-7% 
Mean  variation  from  average,  45°  reading  plane  16.0% 
Lowest  four  values  (excluding  station  20),  45°  reading  plane  
1.72  f,c  -Z-Otri  cZJO-bif  2.O5f  c 

LIGHT  OUTPUT  OF  ON 
Total  light,  lamp  alone  : 
Lamp  with  reflector :- 


400 


..1006  to  180°=  56.4-lumens 

50"  to  100°=  72-4-lumens 

0°  to    50°  =219. 7  lumens 

Total  =3483  lumens 


-.-ELECTRIC LI  GHTI  NO 

CENTE.RDECK  Z.  .SEAT  SPACING 

Description  of  Unit      Cl.e.3  r 

P.ci.s.m  aii  c...Re/f  i  g  CT  o  r 
-HQlo.ph.anc NO.  I.B22.6, 

_.5O  watt.G-3.OTungsten   Lamps 
Base  Contact  of  Electric  Lamp    i'/ft 

above  top  of „  REFLECTOR 
Screening  Angle  of  Reflector     3"2>  ° 
Coefficient  of   Reflector  of  car  head- 


Fig.  4. — Illumination  results  in  coach  using  electric  center  lamps,  with  open-mouth  pris- 
matic reflectors. 


Fig.  5  shows  the  construction  of  the  lamp  used  in  the  foregoing 
test. 

Fig.  3  shows  test  with  a  similar  type  of  fixture  using  the  5o-watt 
train  lighting  electric  lamps.  The  illumination  is  considerably 


Fig.  5  — Lamp  used  in  test,  Fig.  2.  Fig.  6. — Lamp  used  in  test,  Fig.  4. 


Fig.   7. — Lamp  used  in  test,  Fig.  8. 


(Facing  page  500.) 


Fig.   8. — Interior  of  dining  car. 


F'g-  9- — Interior  of  dining  car  with  indirect  center  lighting,  and  direct  side  lighting. 


HTJLSE:  RAILWAY  CAR  LIGHTING 


lower  than  in  the  preceding  test  and  less  uniform,  there  being  more 
difference  between  the  aisle  and  the  window  sittings. 

Fig.  5  shows  the  result  with  center  lighting,  using  prismatic  open- 
mouth  reflectors,  and  Fig.  6  shows  the  type  of  unit  used  in  this  test. 


TEST  No. 
DATA     ON      ILLUMINATION      EFFICIENCY      AND      UNIFORMITY 


ILLUMINATION   33  w.    ABOVE   FLOOR   OF   CAR 


4S«r<«U»c1>U~ 


Z^5 


^OA 


LIGHTING 
«AT  SPACIKC 


Type  of  Car  ..7Q.  f* 


Per  cent,  of  toul  light  delivered  on  horizontal  pUne 

linn  variation  from  -average,  45°  read  ng  plane 

Lowest  four  value*  (excluding  station  20).  45°  reading  plane 


-4<V3% 
I  8.8* 


Description  of  Unit  Medium  Den- 
sity Opal-  M*fh 


LIGHT  Otrmrr  or  Ottm  Uxrr 

Total  light,  lamp  alone  :  — 

Lamp  with  reflector : — 

Distribution 


40Olnr 


100°  U>180l>=<78ul 
50*  to  100°  =|  I  e.felumens 
0*  to    50*=|57«felnmens 
Total 


_5O i  watt;fi£3OTungsten  Lamps 
B«se  Contact  of  Electric  LampjSfa'L 
above  top  of  gF  Fl  FCTOQ 
Screening  Angle  of  Reflector  . 
Coefficient  of  Reflector  of  car  head- 


ILLCMINATION    COBVB  45      KEAD1N&    PLANE 
showa  bj  foil  lioe                                                                                  Window  ml  illmnin 
Far  Cfr  Plan  tmJ  Tat  Stotia*  Locttient  tee  plan  Aa.__JL 

CopyHght 


i  shown  by  dotted  hue 


Fig.   10. — Illumination  results  in  coach  using  electric  center  lamps  with  medium  density 
opal  open-mouth  reflectors. 

Fig.  10  shows  the  results  obtained  with  medium  density  opal 
reflectors. 

Fig.  ii  shows  the  results  obtained  with  heavy  density  opal 
reflectors. 


502 


ILLUMINATING    ENGINEERING   PRACTICE 


Fig.  7  shows  the  type  of  unit  used  in  the  preceding  test. 

Fig.  12  is  interesting  as  it  shows  side  lighting  with  clear  pris- 
matic reflectors.  The  wattage  is  the  same,  as  with  the  center  lamps, 
and  the  results  compare  very  closely. 

TEST  No.     H    

DATA     ON     ILLUMINATION      EFFICIENCY     AND     UNIFORMITY 


ILLUMINATION   33  IN.   ABOVE   FLOOR  OF  CAR 

Window 

£«« 

Mem  oa 

Only 

Car 

45«  reading  plane 

2.01 

2-74 

2.2>2> 

Horizontal  plane 

J5.  26 

4-.sis 

2>.51 

Per  cent   of  total  light  delivered  oh  horizontal  plane  4, 
Mean  variation  from  average,  45°  reading  plane  J 
Lowest  four  values  (excluding  station  20),  45°  reading  plane  

9.7* 

_  EL,E.CTR1C_  LIGHTING 

.CENTER  DECK  ..?....SBAT  SPACING 
Type  of  Car    7O  ft.  CoacVt 

Description  of  Unit  .  He.ftV  >/..D«n- 

si4j/  Opal-  Open-mouhRef. 


LIGHT  OUTPUT  OF  ONB  UNIT 

Total  light,  lamp  alone := 

Lamp  with  reflector: — 

Distribution 


. . .  100°  to  180°=  27  2>  lumens 

50°  to  100°=    72. 0  lumens 

0"  to    50°==  216.4lumens 

T<ftal  =  5 15-7  lumens 


..50.  ..  watt.fi^3O.Tungsten   Lampsj( 
Base  Contact  of  Electri 

above  top  of 

Screening  Angle  of  Reflector.. 
Coefficient  of  Reflector  of  car  head- 
lining=  g£-3    % 


ILLUMINATION    CURVE  j£r5._R.EAP!NS    PLANE 

Aisle  teat  illumination  shown  by  fall  line  Window  seat  illumination  shown  by  dotted  line 

For  Car  Plan  and  Test  Station  Locations  ste  plan  fro.     _    J, 

Copyright  1914  by  the  Aiioc.  of   Ry.  Elec.   Engn. 


A      k      k     A     A      i      i      Jr£'-°*''-°"i| 
JJJJJJJJJJJJJJJJMM 


Pig.   n. — Illumination  results  in  coach  using  electric  center  lamps  with  heavy  density  opal 
open-mouth  reflectors. 


Fig.  13  shows  results  with  side  lighting  with  medium  density 
opal  reflectors,  and  this  test  compares  very  closely  with  center  lamp 
test,  using  the  same  class  of  reflector. 

These  tests  show  that  by  using  units  best  adapted  for  car-lighting 


HULSE:  RAILWAY  CAR  LIGHTING 


503 


service,  illumination  of  about  2.5  foot-candles  can  be  obtained  on 
the  reading  plane  by  spacing  the  center  units  6  feet  apart  and  using 
lamps  with  an  output  of  approximately  390  lumens  per  lamp.  The 
same  results  can  be  obtained  in  side  lighting  by  placing  the  units 

TEST  No.  _.-**. 

DATA     ON      ILLUMINATION      EFFICIENCY      AND      UNIFORMITY 
ILLUM 1NATION    33  IN.    ABOVE   FLOOR   OP  CAR 


HoruoaUl  plant 


1.85 


__HALF  DECK  .2..SKAT  SPACING 
Type  of  Car  7CLf»  COQCb 

ofUnit  Pri 


Per  cent,  of  total  light  delivered  on  hor 

Mean  variation  from  average,  45°  reading  plane 

Lowest  four  values  (excluding  station  20),  4,  ~ 


Afel 


.  ...    5°  reading  plane 

toz  f.c  i.oa  i  c  i.o4f  cj.oz 


LIGHT  OUTPUT  OF  ONE  Unr 
Total  light.  lamp  alone  :  =  - .  • 
Lamp  with  reflector  :— 

Distribution 


\ZQ  lumens 


100°  to  180°=  V5-Z  lumens 
50°  to  100°  =  ZO-  Alumens 
0°  to  S0"=  64.7l 

Total  =e&.3.  lumens 


VAQ\QpV.ang       No.  \S221 
__\S  wartjajb^Tungsten   Lamps 
Base  Contact  of  Electric  LampJ/fc.  ..... 

above  top  of  gfFVECIQg 
Screening  Angle  of  Reflector  ____  3.<&.* 

CotiScient  of  Reflector  of  car  head- 


JJJJJJJJJJJJJJJJJJJU 


Fig.   12. — Illumination  results  in  coach  using  electric  side  lamps  with  open-mouth  prismatic 

reflectors. 


6  feet  apart,  and  using  lamps  with  an  output  of  220  lumens  per  lamp. 
This  allows  a  depreciation  of  40  per  cent,  in  the  efficiency  of  the  light- 
ing system  before  the  illumination  drops  to  1.5  foot-candles. 

Direct  lighting  with  a  reflector  of  medium  or  heavy  density  is  most 


504 


ILLUMINATING   ENGINEERING   PRACTICE 


satisfactory  for  a  coach,  as  it  is  much  more  efficient  than  either  an 
indirect  system  or  a  direct  system  using  a  light  density  reflector 
since  the  walls  and  ceilings  cannot  be  kept  in  proper  condition  to 
reflect  any  appreciable  amount  of  the  light  which  falls  on  them.  The 


DATA     ON     ILLUMINATION     EFFICIENCY 


TEST  No. _„.-!*. ? 

AND      UNIFORMITY 


ILLUMINATION   33  IN.   ABOVE   FLOOR  OF  CAR 


•i=n 

Oily 

E£" 

45°  reading  plane 
Horizontal  plane 

\.OA 

1.20 

\.\-z 

l.^V5 

\.V9 

T3T~ 

Per  cent,  of  total  light  delivered  on  horizontal  plane 

Mean  variation  from  average,  45°  reading  plane 

Lowest  four  values  (exc.uding  station  20^  reading 


LIGHT  OUTPUT  OF  ONE  UNIT 

Total  light,  lamp  alone  := 

Lamp  with  reflector  :— 

Distribution 


;....  I2O  lumens 

.100"  to  180°=  25.4lumens 

50s  to  100"=  »e.e  lumens 

0°  to    50°  =Adfe  lumens 

Total  =98.3  lumens 


....                         S...SKAT  SPACING 
Type  of  Car  VQ«jCO<Xcb L 

Description  of  Unit      Medium  _ 

JQejii  J.ty-O.paL.'B.eilecijor  • 
.SiifiiyLCcu. NO.  .9.0  U ... 

15  watt.<3.rl.8.£Tungsten    Lamps 

Base  Contact  of  Electric  Lamp....Q * 

above  top  of._REF-LECrK>R. 

Screening  Angle  of  Reflector S7C° 

Coefficient  of  Reflector  of  car  head- 
lining =.. £.fc.3.J/o 


ILLUMINATION  CURVE  45°  REACllHGL- 
i  by  full  line 

For  Ctr  Plan  and  Tat  Station  Locations 


,  M*t  illumination  ihcwn  by  dotted  line 


A  Jc  Jc          Jc  Jc  Jc          Jc 


•f 


Pig.  I3. — Illumination  results  in  coach  using  electric  side  lamps  with  medium  density  opal 
open-mouth  reflectors. 


direct  system  with  a  reflector  which  properly  screens  the  light  source 
also  affords,  with  the  darker  ceilings  and  walls,  spaces  of  low  illumina- 
tion for  the  eye  to  rest. 

Dining  Cars. — Dining-car  lighting  is  in  a  class  apart  from  that  of 


HULSE:  RAILWAY  CAR  LIGHTING 


505 


other  classes  of  cars,  and  it  is  in  the  dining  car  that  most  of  the 
novelties  in  lighting  are  used.  Obviously  the  table  is  the  most 
important  item  in  the  car,  and  a  high  illumination  must  be  con- 
centrated on  each  table,  although  a  fairly  high  general  illumination 
has  been  found  necessary  so  that  the  car  will  present  a  cheerful 
appearance  to  the  person  entering  it.  Installations  with  high 
intensity  on  the  tables  and  low  general  illumination  have  not  been 
found  satisfactory.  Good  general  illumination  can  be  obtained 
from  center  fixtures  mounted  on  the  center  deck,  either  direct  or 
indirect  lighting  being  used.  For  table  illumination,  fixtures  should 
be  mounted  over  each  table,  and  no  more  satisfactory  type  of  unit 
has  been  developed  than  that  which  uses  a  concentrating  reflector 
and  redirecting  plate  under  it  to  give  the  proper  distribution  with 
maximum  light  on  the  table  top. 


Fig.   14. — Dining-car  lamp. 

Fig.  8  shows  a  dining  car  equipped  with  this  type  of  fixture. 

Fig.  14  shows  the  construction  of  the  table  lamp  and  Figs.  9 
and  15  types  of  indirect  center  lamps  used.  The  most  satisfactory 
dining-car  illumination  is  obtained  by  lighting  the  tables  with  this 
type  of  fixture,  and  using  a  semi-indirect  unit  for  the  center  lighting. 

Sleeping  Cars. — Sleeping  cars  require  lighting  for  general  illu- 
mination, for  reading  or  working  at  the  tables  in  the  sections,  and 
for  illumination  of  the  berths  after  they  are  made  up. 

General  illumination  is  obtained  by  center  lamps  placed  close  to 
the  ceiling  to  prevent  interference  of  the  fixture  with  the  upper  berth. 
Small  units  placed  in  the  corner  of  'each  section  provide  additional 
local  illumination  for  reading  and  to  light  the  made-up  berth. 

Figs.  1 6  and  17  show  sleepers  equipped  with  such  fixtures. 

Fig.  19  shows  the  results  of  an  illumination  test  made  on  the  car 
shown  in  Fig.  17. 


506 


ILLUMINATING   ENGINEERING   PRACTICE 


The  staterooms  of  a  sleeping  car  and  of  a  compartment  car  are 
lighted  in  the  same  way. 

Smoking  rooms  have  a  center  lamp  for  general  illumination  and 
bracket  lamps  back  of  the  fixed  seats  to  afford  proper  lighting  for 
reading. 

The  passageways  are  lighted  by  ceiling  fixtures,  using  either  an 
open-mouth  reflector,  or  a  fixture  with  reflector  and  directing  plate 
set  flush  with  the  ceiling. 


CAR J5LEEPER 

LAMP  FIXTURES 

+  USHT  ENCLOSED  CENTER  LAMPS 

I  LIGHT  BCRTH  LAMPS 

N«  OF  LAMPS 6CENTER.Z»BeRTW 

GLASSWARE 

CENTER  LAMPS  -OPAL  REFLECTOR.FK05TED  BOWL 

BERTH  LAMPS  -  PRISMATIC  GLOBE 

BULBS 8C.P  CARSON 

VOLTAGE . *S 

TOTAL  WATTS 1148 

AVERA8E  FOOT-CANOUM V»9 

MAXIMUM-  -         *i» 

MINIMUM     •  " l-Si 

EXTREME  VARIATION .__ -_' .TS 

%  VARIATION  ABOVE  MEAN Vt 

•((.VARIATION  BELOW  MEAN Z* 

SOUARE  FEET  ILLUMINATED SAO 

FOOT  CAN  bLESX FLOOR  AREA .S* 


tit 


Fig.   19. — Illumination  results  in  sleeping  car,  Fig.  17. 

The  proper  lighting  of  the  berth  section  of  the  car  after  all  the 
berths  are  made  up  is  best  accomplished  by  a  bracket  lamp  placed 
on  the  bulk-head  facing  the  aisle;  this  allows  all  the  center  lamps  to 
be  extinguished,  and  affords  sufficient  light  for  passing  through  the 
car. 

Parlor  Smoking  Cars. — This  type  of  car  presents  a  rather  difficult 
problem,  as  the  seats  are  arranged  so  that  the  occupant  faces  the 
center  of  the  car.  Best  results  are  obtained  by  the  use  of  a  few  center 


Fig.   15. — Indirect  lamps  used  for  dining-car  lighting. 


Fig.   16. — Interior  of  sleeping  car. 


(Facing  page  506.) 


Fig.  17. — Interior  of  sleeping  car. 


Fig.   1 8. — Interior  of  parlor  smoking  car. 


Fig.  20. — Interior  of  parlor  car  with  side  lighting. 


Fig.  21. — Bag-rack  portion  of  postal  car. 


(Facing  page  506.) 


Fig.  22. — Letter-case  portion  of  postal 


Fig.  23. — Observation  room  of  private  car. 


HULSE:  RAILWAY  CAR  LIGHTING  507 

lamps  for  general  illumination,  and  bracket  lamps  placed  on  the  side 
of  the  car  back  of  the  chairs  for  reading  light.  Cars  having  a  flat 
side  deck  can  be  fitted  with  reflecting  units  directly  over  the  chairs 
instead  of  bracket  lamps. 

Such  an  installation  is  shown  in  Fig.  18. 

Parlor  Cars. — The  conditions  in  this  class  of  car  are  very  similar 
to  that  in  the  passenger  coach,  and  the  same  type  of  installation  is 
used,  although  indirect  lighting  can  be  better  used  in  parlor  cars 
owing  to  the' fact  that  the  walls  and  ceilings  can  be  maintained  in 
better  reflecting  condition. 

A  parlor  car  equipped  with  side  lighting  is  shown  in  Fig.  20. 
Postal  Cars. — Postal  cars  require  greater  illumination  than  those 
of  any  other  class,  as  the  work  done  demands  constant  and  arduous 
use  of  the  mail  clerk's  eyes.  A  very  thorough  investigation  was 
conducted  some  years  ago  by  one  of  the  large  railroads  assisted  by 
various  manufacturers  and  participated  in  by  the  Standard  Car 
Committee  of  the  Post  Office  Department.  Various  types  of  in- 
stallation were  tested,  and  determinations  were  made  of  the  amount 
of  light  necessary  for  the  postal  clerks  to  work.  From  the  results 
obtained  by  these  tests  the  committee  issued  specifications  for  light- 
ing which  must  be  met  in  every  postal  car. 

Postal  cars  are  generally  divided  into  three  sections:  the  letter 
distributing  cases,  the  bag  distributing  racks,  and  the  storage  space. 
The  distributing  cases  require  high  illumination  on  the  vertical  plane 
of  the  box  labels,  and  on  a  horizontal  plane  for  reading  the  addresses 
on  the  mail.  The  bag  distributing  racks  require  high  illumination 
on  the  horizontal  plane,  for  the  labels  on  the  bag  racks.  The  storage 
end  requires  a  fair  general  illumination. 

The  following  are  the  specifications  for  illumination  of  postal 
cars  issued  by  the  Post  Office  Department: 

These  initial  values  are  set  to  give  proper  illumination  with  a  40 
per  cent,  depreciation  in  the  efficiency  of  the.installation. 
Fig.  21  shows  the  bag-rack  portion,  and 

Fig.  22  shows  letter  case  of  a  postal  car  equipped  to  meet  this 
specification. 

Private  Cars. — A  private  car  is  a  combination  of  several  types  of 
cars  and  the  lighting  is  accomplished  in  the  different  sections  some- 
what as  it  is  in  the  class  of  car  to  which  that  section  corresponds. 
The  observation  room  resembles  somewhat  the  parlor  smoker, 
and  in  this  part  of  the  car  general  illumination  is  obtained  by  center 
lighting,  with  bracket  lamps  behind  the  chairs,  or  half  deck  units 


508 


ILLUMINATING  ENGINEERING   PRACTICE 
INITIAL  VALUES   OF  ILLUMINATION  REQUIRED 


Foot-candles 

Minimum 

Maximum 

Bag  Rack  Portion: 
Center  of  Car  —  Horizontal.. 

3-75 

2.OO 

3-75 
1.66 

2.OO 

I2.OO 
12.00 

IQ.OO 
19.00 
12  .OO 

Mouth  of  Bags,  measured  18  inches  from  side  of  car  — 
Horizontal.                                                    

Letter  Cases: 
Over  table  —  Horizontal  
Face  of  Case  —  Vertical. 

Storage  Portion  

for  local  lighting.  In  private  cars,  however,  more  latitude  is  allowed 
in  fixture  design,  and  frequently  the  center  lighting  is  obtained  from 
one  large  fixture  placed  in  the  ceiling  of  the  observation  room. 

Fig.  23  shows  the  observation  room  of  a  private  car  so  equipped. 

Local  lighting  is  provided  for  the  gauges  and  speed  indicating 
devices  with  which  most  business  and  private  cars  are  equipped, 
so  that  these  instruments  may  be  read  when  the  other  lamps  in  the 
car  are  not  in  use  to  allow  track  inspection  from  this  part  of  the  car 
at  night. 

The  dining  room  of  a  private  car  is  best  lighted  by  a  single  unit 
placed  directly  over  and  throwing  a  high  illumination  on  the  table, 
with  local  lighting  for  the  buffet. 

The  staterooms  are  lighted  with  center  lamps,  with  local  lighting 
for  the  mirrors.  Where  berths  are  used,  berth  lamps  are  provided; 
and  at  the  beds  special  reading  lamps  are  attached  to  the  bed  posts. 

All  cars  having  vestibules  have  lamps  over  each  step,  directing 
light  on  the  steps.  A  flush  type  metal  reflector  is  generally  used. 

Street  and  Interurban  Cars. — The  proper  lighting  of  this  type  of 
car  has,  until  recently,  been  neglected,  both  as  to  the  amount  of 
light  furnished,  and  the  proper  application  of  the  light  sources. 
The  carbon  lamp  was  kept  in  use  for  a  considerable  time  after  the 
metallic  filament  lamp  had  displaced  it  in  almost  every  other  field. 
No  means  was  used  to  direct  the  light  to  that  portion  of  the  car 
where  it  was  to  be  used  or  to  shield  the  filament  from  the  eye.  At  the 
present  time  the  metal  filament  lamp  with  proper  reflectors  are 
being  used  exclusively. 

Two  general  arrangements  of  the  units  are  in  use.  Cars  having 
cross  seats  are  lighted  by  a  single  row  of  units  placed  on  the  ceiling 


HULSE:  RAILWAY  CAR  LIGHTING 


509 


along  the  center  line  of  the  car,  the  spacing  varying  from  4.5  to  10  ft. 
according  to  the  size  of  lamp  to  be  used.  This  spacing  is  also  depend- 
ent on  the  length  of  the  car,  as  the  lamps  are  operated  in  series  and 
must  be  in  multiples  of  five. 

When  the  seats  are  longitudinal,  along  the  sides  of  the  car,  the 
units  are  arranged  in  two  rows,  about  22  in.  from  the  side  of  the  car. 
The  same  spacing  is  used  as  with  the  center  lamps. 

The  lighting  system  is  designed  to  give  a  minimum  illumination 
on  the  plane  of  utilization  of  1.5  foot-candles  under  conditions  of 
80  per  cent,  normal  voltage,  which  means  that  at  normal  voltage  the 
illumination  will  be  about  3.75  foot-candles.  This  variation  is  a 
condition  which  will  have  to  be  corrected  before  street-car  lighting 
can  be  called  satisfactory.  Up  to  the  present  time  no  device  has 
been  produced  which  satisfies  the  operating  officials  of  this  class 
of  car  as  to  cost  and  simplicity. 

REFLECTORS  AND  GLASSWARE 

A  number  of  types  of  reflectors  and  enclosing  units  have  been  de- 
veloped for  car  lighting  uses. 

For  coach  lighting,  and  other  classes  of  cars  where  efficiency  is 
the  prime  object,  and  appearance  a  secondary  consideration,  the  open- 
mouth  reflector  is  in  almost  universal  use.  Best  results  are  obtained 
with  a  reflector  giving  the  maximum  candle-power  at  45°. 

The  following  are  the  principal  types  of  this  class  of  reflector, 
together  with  the  illumination  obtained  and  the  efficiency  using  a 
6-ft.  spacing,  giving  66%  generated  lumens  per  running  foot  of 
the  car: 


Average  illumination  on  45°  reading 
planes,  foot-candles 

Illuminating 
efficiency  on 
45°  plane 

Aisle  seats 

Window 
seats 

Average 

Prismatic  Clear  
Heavy  Density  Opal  . 

2.66 

2.41 

2.00 
I.Q4 
1.79 

2.17 
I.87 
1.65 
1-50 
1-52 

2.42 
2.14 

1-83 
1.72 
1.66 

34-2 
30-3 
25-9 
24-3 
23-5 

Medium  Density  Opal.  .  . 

Prismatic  Satin  Finish. 

Light  Density  Opal. 

Where  appearance  is  the  primary  consideration  enclosing  units  are 
used,  and  the  energy  efficiency  somewhat  sacrificed. 


ILLUMINATING   ENGINEERING   PRACTICE 


The  following  results  are  obtained  with  this  class  of  unit  under 
conditions  similar  to  those  stated  above: 


Average  illumination  on  45°  reading 
planes,  foot-candles 

Illumination 
efficiency 
on  45° 
plane 

Enclosing  units 

Aisle 
seats 

Window 
seats 

Average 

Light  Density  Opal  

1.09 

i-39 
1.44 
1-56 
1.36 
1.17 

0.97 

.09 
.24 
.24 
.11 
•13 

1.03 

1.24 

i-34 
1.40 
1.23 
1.  15 

14.6 

17-5 
19.0 
19.8 
17-4 
l6.3 

Shallow     Prismatic     Reflector 
with  Light  Density  Bowl  
Reflecting  and  Diffusing  Globes 
Semi-indirect  . 

Total  Indirect  

Bare  Lamp  

All  of  the  foregoing  are  for  electric  light.  For  gas  lighting  the 
following  results  were  obtained,  the  generated  lumens  being  130  per 
running  foot  of  car: 


Average  illumination  on  45°  reading 
planes,  foot-candles 

Illumination 
efficiency 
on  45° 
plane 

Enclosing  units 

Aisle 
seats 

Window 
seats 

Average 

Deep    Prismatic    Reflector    & 
Bowl  

3-65 
2.74 
2.08 
1.92 

3-72 
2-34 
1-52 
1.67 

3-69 
2-54 
i.  80 
i.  80 

26.8 
18.4 
I3-I 
I3-I 

Reflecting  &  Diffusing  Globes. 
Medium  Density  Opal  Globes. 
C.R.I  Diffusing  Globes  

Aluminized  metal  reflectors  are  in  very  general  use  in  postal  and 
baggage  cars  due  to  their  high  efficiency  and  durability. 


FIXTURES 

Lighting  fixtures  for  use  in  railroad  cars  require  special  design  and 
construction,  and  embody  some  features  not  found  in  fixtures  built 
for  other  purposes. 

1.  They  must  be  substantial  to  withstand  the  constant  vibra- 
tion to  which  they  are  subjected. 

2.  They  must  be  easily  removable  for  refinishing  when  the  car 
goes  through  its  regular  shopping. 

3.  The  arrangements  for  holding  the  glassware  must  be  such  that 


HULSE:  RAILWAY  CAR  LIGHTING  511 

it  can  be  easily  applied,  or  removed  for  cleaning,  but  at  the  same  time 
must  be  securely  held  so  that  there  is  no  danger  of  its  jarring  loose. 

4.  They  must  be  of  suitable  color  and  design  to  harmonize  with 
the  interior  treatment  of  the  car. 

5.  The  mechanical  design  must  be  simple  and  all  working  parts 
must  be  easily  accessible. 

The  first  condition  is  met  by  careful  mechanical  design,  sug- 
gested by  experience  in  this  class  of  work,  as  fixtures  built  for  other 
uses  are  wholly  unfit  for  use  in  railway  cars. 

The  second  feature  is  generally  covered  by  a  type  of  construc- 
tion in  which  a  plate  or  spider  is  firmly  fastened  to  the  car  ceiling, 
this  plate  forming  the  support  for  the  socket.  The  ornamental  part 
of  the  fixture  is  secured  to  this  plate,  but  may  be  removed  without 
disturbing  the  electric  connections  or  the  attachment  to  the  ceiling. 

The  arrangements  for  holding  the  glassware  in  enclosing  units 
must  be  worked  out  for  each  type  of  glass  employed.  With  a  large 
proportion  of  the  fixtures  for  electric  lighting  use  is  made  of  open 
mouth  reflectors  and  for  these,  holders  have  been  developed  which 
fulfill  the  condition  admirably.  The  ordinary  type  of  holder 
equipped  with  set  screws  was  quickly  abandoned  as  being  unsafe. 
One  of  the  best  holders  developed  consists  of  a  spring  clamp  com- 
prising a  number  of  metal  fingers  which  spring  over  and  grip  the 
neck  of  the  reflector.  In  order  to  make  the  action  of  this  spring 
clamp  positive,  a  cap  nut  is  screwed  down  against  the  spring  clamp, 
locking  the  fingers  against  the  neck  of  the  reflector  in  such  a  manner 
that  the  spring  of  the  clamp  takes  care  of  expansion  in  the  glass  and 
cushions  it  against  vibration. 

The  question  of  suitable  design  is  one  which  is  governed  to  a 
certain  extent  by  the  wishes  of  the  purchaser,  but  I  believe  that  the 
results  obtained  in  car  lighting  work  compare  very  favorably  with 
that  in  other  lines. 

Electric  lamps  are  built  so  that  the  bulbs  may  be  easily  renewed, 
and  the  sockets  and  wiring  easily  accessible.  Gas  lamps  are  made 
so  that  they  can  be  lighted  without  opening  the  bowl,  mantles  ap- 
plied without  the  mantle  being  removed  from  the  container  until 
it  is  properly  attached  to  the  lamp,  and  no  adjustments  to  the  air 
or  gas  supply  are  necessary;  in  fact  the  lamps  are  made  without  any 
means  of  adjustment. 

Much  remains  to  be  done  before  the  lighting  of  railway  cars  will 
be  all  that  can  be  desired,  but  I  know  of  no  other  field  where  more 
effort  is  being  expended  to  obtain  proper  and  adequate  illumination. 


THE  LIGHTING  OF  YARDS,  DOCKS  AND  OTHER 
OUTSIDE  WORKS 

BY  J.  L.  MINICK 

It  is  a  notable  fact  that  illuminating  engineers  generally  have  given 
only  casual  attention  to  the  field  of  lighting  in  railway  service  and  it 
is  largely  with  the  hope  of  stimulating  interest  in  this  field  that  this 
lecture  has  been  prepared.  Probably  no  other  single  industry  offers 
such  a  wide  variety  of  interesting  problems  for  solution  in  which 
practically  every  known  form  of  illuminant  may  be  used  to  advan- 
tage. This  field  is  open  to  the  illuminating  engineer  if  he  will  avail 
himself  of  the  opportunities  offered.  Many  railroads  of  importance 
have  large  engineering  organizations  but  only  a  few  employ  men 
sufficiently  well  trained  in  this  important  branch  of  science  to  solve 
properly  the  many  problems  that  constantly  arise.  Many  of  these 
problems  are  common  to  other  industries  which  have  come  within 
the  range  of  the  illuminating  engineer  and  their  solutions  are  there- 
fore well  known.  Many  others  are  peculiar  alone  to  railway  service 
and  it  is  from  among  these  that  the  material  for  this  lecture  has 
been  selected. 

American  railroads  derive  approximately  three-quarters  of  their 
gross  income  from  the  handling  of  freight  and  about  one-fifth  from 
passenger  service.  All  problems  whose  solution  will  in  any  way, 
improve,  facilitate  or  stimulate  the  movement  or  handling  of  either 
freight  or  passengers,  are  of  prime  importance  to  the  railroads  in  their 
endeavor  to  furnish  adequate  service  to  the  public.  The  lighting  of 
yards,  docks  and  other  outside  works  has  been  selected  for  review  in 
this  lecture  as  these  problems  are  intimately  connected  with  the 
handling  of  transportation  and  their  importance  is  not  generally 
well  appreciated. 

In  presenting  these  problems  it  must  be  understood  that  the 
solutions  suggested  are  not  to  be  considered  as  final  or  conclusive. 
They  represent  the  labors  and  investigations  of  only  a  limited  number 
of  engineers  during  the  past  five  or  six  years.  Improvements  in 
illuminants  and  in-  the  control  of  light  are  constantly  removing  many 
of  the  difficulties  commonly  encountered  and  the  investigations  of 

33  513 


514  ILLUMINATING   ENGINEERING    PRACTICE 

railway  committees  and  individuals  are  resulting  in  many  changes 
in  methods  of  operation  which  tend  to  simplify  the  lighting  problems. 
Few  tests  have  been  conducted  to  show  what  intensities  of  illumina- 
tion are  required  for  different  classes  of  service  and  it  is  for  this 
reason  that  data  of  this  character  are  not  presented.  The  material 
presented  deals  largely  with  the  practical  aspect  and  gives  some  con- 
ception of  the  difficulties  encountered  in  railway  service. 

FREIGHT  YARDS 

Description. — The  economical  operation  of  important  railroads 
requires  that  the  roadway  shall  be  divided  into  sections  or  divisions, 
each  of  sufficient  length  to  give  the  train  crews  a  full  day's  run  under 
normal  operating  conditions.  Shops,  roundhouses  and  other  facili- 

Direction  of  Traffic — *- 


^Grade-Sfto-e* 


--Not  less  than  Length  of  Train-- 
Receiving Yard 

Direction  of  Traffic — >• 


Grade 


Not  less  than  Length  of  Train >i 

Classification  Yard 
Fig.   i. — Diagrammatic  sketch  of  freight  yard. 

ties  are  provided  at  division  points  for  the  inspection,  maintenance 
and  repair  of  the  rolling  equipment,  also  weigh  scales  and  yards  for 
weighing  and  classifying  freight. 

Freight  originates  in  small  package  quantities,  at  stations  provided 
for  that  purpose,  and  is  here  weighed  and  loaded  for  shipment,  usually 
in  box  cars.  Full  car  loads  are  received  from  warehouses,  factories, 
mines,  etc.,  which  have  siding  connections  with  the  main  line.  All 
of  this  miscellaneous  freight,  both  small  package  and  full  cars,  is 
collected  by  "way  collection  trains"  and  transported  to  the  end  of 
the  division  for  weighing  and  classification. 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC.  515 

Two  generaltypesof  freight  yards  are  in  common  use  to-day;  one  in 
which  the  tracks  are  approximately  level,  requiring  the  constant  use 
of  locomotives  for  moving  the  cars,  the  other  in  which  the  tracks  are 
arranged  and  graded  to  permit  the  movement  of  the  cars  by  the 
force  of  gravity.  This  latter  type  is  known  as  a  "gravity"  or 
"hump"  yard,  and  since  it  is  rapidly  superseding  the  first-mentioned 
type,  this  type  of  yard  has  been  selected  for  discussion. 

Pull-in  Yard. — When  a  freight  train  arrives  at  the  end  of  a  division 
it  is  delivered  to  the  " pull-in"  yard  where  the  road  crew  turns  it 
over  to  the  yard  crew.  The  pull-in  yard  consists  of  several  tracks  or 
sidings  each  long  enough  to  receive  a  full  train,  which,  in  the  case  of 
full  car  loads,  consists  of  from  forty  to  sixty  loaded  cars  or  possibly 
as  many  as  one  hundred  and  fifty  empty  cars.  Convenient  track 
connections  permit  of  easy  access  to  the  roundhouse  and  storage 
track  for  the  locomotives  and  cabin  cars.  The  yard  crew  moves  the 
train  into  the  "receiving"  yard,  as  soon  as  possible  to  make  room  for 
the  arrival  of  other  trains. 

Receiving  Yard. — The  receiving  yard  contains  usually  two  or  three 
times  as  many  tracks  as  the  pull-in  yard  and  is  of  about  equal  length 
though  the  character  of  the  freight  handled,  local  conditions,  etc., 
play  very  important  parts  in  the  arrangement  and  length  of  tracks 
in  both  of  these  yards.  The  tracks  in  the  receiving  yard  are  graded 
slightly  in  the  direction  of  traffic  to  reduce  to  a  minimum  the  steam 
power  required  for  their  movement. 

The  work  in  both  of  these  yards  is  of  such  a  nature  as  to  require  the 
train  crews  to  be  on  their  trains  the  larger  part  of  the  time.  No  work 
of  any  account  is  performed  on  the  ground.  The  lighting  require- 
ments are  not  severe  and  usually  whatever  suffices  for  the  neck  of  the 
classification  yard  or  ladder  tracks  can  be  used  in  these  yards  to 
advantage. 

Weigh  Scales. — The  success  of  rapid  and  accurate  weighing  is 
dependent  upon  the  constant  movement  of  cars  across  the  scales  at 
uniform  speed.  Weighing  is  usually  continued  throughout  the  entire 
twenty-four  hours,  and  hence  the  artificial  illumination  provided 
must  be  such  as  to  enable  the  weigh-master  and  yard  crews  to  per- 
form their  duties  equally  well  during  all  hours  of  the  day  and  night. 

In  the  lower  end  of  the  receiving  yard  an  abrupt  change  from 
negative  to  positive  grade  takes  place,  the  rise  in  track  continuing 
until  the  rails  are  somewhat  higher  than  the  scale  platform.  From 
this  point,  commonly  known  as  the  "hump,"  they  drop  rapidly  to 
the  end  of  the  scale  platform.  The  scale  platform  also  has  a  slight 


516  ILLUMINATING   ENGINEERING   PRACTICE 

negative  grade.  In  operation  a  locomotive  is  attached  to  the  rear 
end  of  the  train  in  the  receiving  yard  to  regulate  its  speed.  The 
grade  is  such  that  the  weight  of  the  train  moving  down  the  grade 
toward  the  scales  is  sufficient  to  force  the  first  two  or  three  cars  up 
the  grade  over  the  hump,  thus  requiring  the  locomotive  to  regulate 
the  speed  only  by  applying  additional  power  as  the  train  is  decreased 
in  length. 

As  the  cars  pass  over  the  hump  a  point  is  reached  where  the  re- 
verse in  grade  causes  the  last  car  to  pull  away  from  the  train.  At 
this  point  the  car  is  cut  loose  and  allowed  to  "float"  across  the  scale 
platform.  The  grades  and  distances  are  so  arranged  and  propor- 
tioned that  all  cars,  regardless  of  size  or  weight,  move  across  the 
scales  at  approximately  the  same  velocity.  Usually  there  is  an  in- 
terval of  about  one-half  car  length  between  cars.  At  the  point  of 
cutting  the  cars  the  grades  are  such  that  the  cars  move  a  distance  of 
not  to  exceed  6  ft.  during  which  the  couplers  may  be  opened.  Dur- 
ing the  interval  of  cutting  a  visual  inspection  is  made  of  the  running 
gear,  brake  rigging,  etc. 

These  operations  require  illumination  of  fairly  high  intensities 
localized  within  a  comparatively  small  ground  area.  The  couplers 
must  be  sufficiently  well  illuminated  to  enable  the  car  cutter  to 
note  quickly  when  they  begin  to  part.  All  parts  underneath  the 
car  must  be  well  illuminated  to  enable  the  detection  of  loose,  bent 
or  broken  parts.  Usually  a  single  large  lamp  will  serve  for  this 
purpose.  Experience  shows  that  it  should  be  placed  thirty  or  more 
feet  above  the  rail  and  at  least  10  ft.  from  the  center  of  the  track 
to  give  proper  clearance  between  the  pole  and  the  sides  of  passing 
cars.  The  pole  should  be  set  opposite  the  point  where  the  cars  are 
cut.  The  distance  from  here  to  the  end  of  the  scale  platform  will 
vary  with  the  type  of  hump  used.  With  earth-fill  humps  it  may  be  as 
great  as  60  ft.,  while  with  the  more  modern  mechanical  humps,  with 
which  changes  in  elevation  are  secured  at  will  by  mechanical  means, 
it  will  vary  from  25  to  36  ft.  In  several  instances  two  lamps  have 
been  used,  one  on  each  side  of  the  track. 

Several  types  of  lamps  have  been  tried  in  this  service  but  none  of 
them  have  proven  as  satisfactory  as  the  incandescent  lamp.  The 
5 oo- watt  vacuum  type  lamp  has  been  generally  used,  though  a 
lamp  of  slightly  larger  size  would  probably  have  been  used  had  it  been 
available  at  the  time  of  making  the  initial  installation.  The  recent 
introduction  of  the  gas-filled  lamp,  with  its  increased  candle-power 
for  the  same  wattage,  provides  a  satisfactory  means  for  increasing 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC.  517 

the  illumination  without  great  expense.  The  reflector  should  be 
of  the  distributing  type  and  designed  to  give  the  maximum  inten- 
sity at  about  45  degrees.  Its  skirt  should  be  extended  to  shield 
the  lamp  from  the  range  of  vision  of  the  weigh-master  and  others 
employed  in  the  vicinity  of  the  scale  house.  All  illumination  under- 
neath the  car  must  come  from  reflection  from  the  ground.  In 
many  instances  clean  river  gravel,  crushed  oyster  shells,  white- 
wash and  other  light  colored  substances  have  been  spread  over  the 
ground  to  increase  the  reflection.  Arc  lamps  have  proved  unsatis- 
factory principally  on  account  of  the  unsteadiness  of  the  arc. 

As  previously  stated  the  grades,  dimensions,  etc.,  of  the  hump 
are  so  proportioned  as  to  give  each  car,  as  nearly  as  possible,  the 
same  momentum  as  it  passes  over  the  scale  platform.  The  average 
rate  of  movement  is  from  ten  to  fifteen  seconds  for  each  car,  from 
the  instant  the  front  wheels  mount  the  platform  until  the  rear 
wheels  leave.  There  is  an  interval  of  about  one-half  car  length  be- 
tween cars  which  gives  an  interval  of  from  twenty  to  twenty-five 
seconds  in  which  all  of  the  operations  incidental  to  weighing  must 
be  performed.  During  this  interval  the  weigh-master  must  read 
the  initials,  number  and  light  weight  of  the  car  and  verify  the 
records  on  the  manifest  in  his  possession  and  in  addition  he  must 
weigh  the  car  and  make  record  on  the  manifest  of  the  total,  light 
and  net  weights. 

The  weight  must  be  taken  after  the  rear  wheels  have  mounted  and 
before  the  front  wheels  leave  the  platform.  The  car  is  free  to  move 
from  10  to  15  ft.  on  the  scale  platform  under  this  condition. 
This  represents  an  interval  of  time  of  from  one  and  one-quarter  to 
two  seconds  during  which  the  weight  may  be  taken.  Under  these 
conditions  all  cars  are  weighed  to  within  less  than  200  pounds  though 
many  of  them  exceed  175,000  pounds  in  total  weight.  The  average 
rate  of  weighing  is  slightly  less  than  three  cars  per  minute  while 
under  favorable  conditions  it  may  run  as  high  as  four  per  minute. 
This  rate  of  movement  makes  it  necessary  for  each  operation  to  be 
conducted  with  certainty  as  a  single  failure  will  interrupt  the  opera- 
tion of  the  entire  yard  until  the  car  involved  can  be  returned  to  the 
receiving  yard  for  re  weighing. 

The  illumination  at  night  must  be  the  equivalent  of  that  under 
daylight  conditions.  There  are  three  opportunities  for  the  weigh- 
master  to  read  the  initials  of  the  car,  etc. ;  on  the  front  end  of  the  car 
as  it  approaches  the  scales,  on  the  side  of  the  car  as  it  passes  over  the 
scales,  and  again  on  the  rear  end  of  the  car  as  it  leaves  the  scales. 


ILLUMINATING   ENGINEERING   PRACTICE 


40°  30°        20°       10°      0°       10°       20°        30 


Fig.  2. — Candle-power  curve  of  scale-lighting  unit  with  special  reflector;  200- watt  lamp; 

horizontal  plane. 


180°     170°      160°       150°          140° 


0°      10°      20°       30°         40° 


50 


Fig.  3. — Candle-power  curve  of  scale-lighting  unit  with  special  reflector;  200-watt  lamp; 

vertical  plane. 


MINICK:  LIGHTING  or  YARDS,  DOCKS,  ETC.  519 

A  special  reflector  has  been  designed  by  one  of  the  eastern  rail- 
roads for  this  service,  as  it  was  not  possible,  at  the  time  the  original 
installation  was  made,  to  purchase  on  the  open  market  a  reflector 
giving  the  desired  distribution.  This  reflector  is  of  the  angle  type 
with  the  lamp  pendant  and  the  axis  of  the  reflector  in  the  horizontal 
plane  or  at  right  angles  to  the  axis  of  the  lamp.  It  is  approximately 
13  in.  in  diameter  by  5%  in.  deep  and  was  designed  to  accommodate 
a  2  50- watt  vacuum- type  lamp.  The  2oo-watt  gas-filled  lamp  has 
lately  been  substituted  with  satisfactory  results  and  with  the  latter 
lamp  the  reflector  gives  maximum  ca'ndle-power  at  about  45  degrees 
in  the  horizontal  and  at  90  degrees  in  the  vertical  plane.  The  design 
is  such,  however,  that  the  larger  part  of  the  light  flux  in  the  vertical 
plane  lays  above  the  horizontal  or  between  the  angles  of  say,  70 
and  140  degrees.  A  few  of  the  recently  developed  types  of  porcelain 
enameled  reflectors  give  very  close  to  this  distribution  and  may  now 
be  purchased  on  the  open  market. 

Six  of  these  fixtures  are  mounted  along  the  front  of  the  scale 
house  facing  the  track  at  a  height  of  7.5  ft.  from  the  rail  to  the  bot- 
tom of  the  socket.  The  spacing  varies  slightly  with  local  conditions 
and  the  type  of  scales  used.  The  fixture  nearest  the  hump  is  ad- 
justed to  illuminate  the  front  end  of  the  car  as  it  approaches  the 
scales,  particularly  the  number  panel  which  invariably  appears  in 
the  upper  right-hand  corner  of  the  end  of  the  car,  the  observer  fac- 
ing the  car.  This  lettering  covers  an  area  approximately  16  in. 
high  by  30  in.  long,  with  the  bottom  from  9  to  10  ft.  above  the  rail. 
The  next  four  fixtures  are  adjusted  to  illuminate  the  lettering  on  the 
side  of  the  car,  which  covers  an  area  about  40  in.  high  by  10  or  12  ft. 
long,  though  in  many  instances  the  initials  extend  the  entire  length 
of  the  car.  The  bottom  of  this  lettering  is  about  4  ft.  above  the 
rails.  The  last  fixture  illuminates  the  rear  end  of  the  car  as  it  passes 
off  the  scales. 

Oil-fuel  head-lamps,  mounted  along  the  front  of  the  scale  house, 
were  originally  used  in  this  service.  They  were  very  unsatisfactory, 
as  the  chimneys  quickly  became  smoked  and,  even  under  the  most 
favorable  conditions,  the  intensity  on  the  side  of  the  car  was  low. 
Flame  arc  lamps  were  next  used  and,  although  the  intensity  of  the 
illumination  was  greatly  increased,  these  also  were  unsatisfactory 
principally  on  account  of  the  unsteadiness  of  the  arc  and  the  welding 
of  the  electrodes. 

Since  the  body  of  the  car  overhangs  the  trucks  at  each  end  by 
several  feet,  the  trailing  wheels  are  not  illuminated  and  it  is  difficult 


52O  ILLUMINATING   ENGINEERING   PRACTICE 

for  the  weigh-master  to  determine  quickly  when  the  car  has  left  the 
platform.  A  spot-lamp  is  placed  on  the  side  of  the  scale  house, 
several  feet  above  the  rail  and  near  the  end  of  the  platform,  with  the 
beam  trained  upon  the  ends  of  the  rails.  The  reflector  used  is  of 
the  concentrating  type  to  insure  high  intensity  of  illumination. 

The  scale  beam  is  mounted  within  a  bay  window  opposite  the 
center  of  the  scale  platform  with  a  clearance  of  about  3  ft. 
between  the  car  and  the  edge  of  the  window.  It  is  illuminated  by 
means  of  three  pendant  fixtures,  one  opposite  the  center  and  one 
opposite  each  end  of  the  beam.  These  are  mounted  7  ft.  from 
the  floor  to  the  bottom  of  the  reflector  and  in  a  line  12  in. 
back  from  and  parallel  to  the  scale  beam.  Special  reflectors  and 
25-watt  lamps  are  used  to  illuminate  only  the  scale  beam  and 
counterpoise.  These  reflectors  are  special  in  that  they  have 
unusually  long  skirts  to  prevent  direct  light  striking  the  glass  of  the 
window  and  being  reflected  in  such  manner  as  to  obscure  the  weigh- 
master's  vision.  Great  care  must  be  exercised  to  see  that  none  of 
the  polished  metal  parts  give  objectionable  reflection. 

As  each  car  moves  off  the  scales  and  down  the  grade  into  the  classi- 
fication yard  a  car  rider  mounts  it  and  regulates  its  speed  by  the 
hand  brakes  so  that  it  will  strike  the  cars  standing  in  the  yard  with 
only  sufficient  force  to  close  and  lock  the  coupler.  A  second  large 
lamp  mounted  on  a  pole,  usually  a  duplicate  of  the  equipment  at 
the  hump,  furnishes  sufficient  illumination  for  the  rider  to  mount  the 
car  safely. 

All  of  the  lamps  in  the  immediate  vicinity  of  the  scales  are  oper- 
ated from  multiple  circuits  with  control  switches  inside  the  scale 
house  within  easy  reach  of  the  weigh-master.  This  enables  him  to 
make  use  of  artificial  illumination  during  the  hours  of  dusk  and 
dawn  and  during  dark  periods^  of  the  day  when  the  natural  illumina- 
tion is  ample  for  all  other  yard  operations.  All  reflectors  outside 
the  scale  house,  particularly  those  in  the  immediate  vicinity  of  the 
scales,  must  have  skirts  not  only  to  shield  the  lamps  from  the  range 
of  vision  of  the  weigh-master  but  to  prevent  reflection  on  the  glass 
of  the  windows,  since  much  of  the  weighing  must  be  done  while  the 
windows  are  closed. 

The  scale  mechanism  requires  frequent  inspection,  particularly 
the  knife  edges,  knuckles,  etc.  Artificial  lighting  is  necessary  for 
this  purpose  as  all  of  the  mechanism  lies  within  the  vault  under- 
neath the  platform.  Many  structural  shapes  are  used  and  their  rein- 
forcements and  connections  offer  so  many  obstructions  to  general 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC. 


521 


illumination  that  this  service  is  necessarily  restricted  to  the  spot 
lighting  of  important  parts,  supplemented  by  a  limited  number  of 
bare  lamps  for  general  illumination.  Spot  lighting  is  secured  by  the 
use  of  porcelain  enamel  metal  angle  reflectors  of  the  concentrating 
type  fitted  with  25-watt  or  40- watt  lamps.  Portable  lamps  with 
extension  cord  connections  are  required  for  the  examination  of  the 
parts  lying  within  the  regions  of  shadow. 

Classification  Yard. — As  the  car  leaves  the  scales  it  passes  down 
an  incline  of  fairly  heavy  grade,  of  from  250  to  500  ft.  in  length, 
leading  to  the  " ladder  track"  connecting  with  the  yard  tracks. 


40 


40 


30°         20°       10°       0°       10°       20°         30° 

Fig.  4. — Candle-power  curve  ladder-track  unit,  maximum  and  minimum. 

The  yard  may  vary  in  width  from  eight  or  ten  to  as  many  as  twenty- 
five  or  more  tracks  so  that  the  ladder  track  will  contain  a  large 
number  of  switches  set  closely  together.  These  switches  are  oper- 
ated electrically  or  electro-pneumatically  from  a  tower  provided 
for  that  purpose.  Since  it  is  necessary  that  the  car  riders  see  all  of 
the  switches  clearly,  and  as  they  ride  on  the  rear  ends  of  the  cars, 
excellent  illumination  of  the  switches  must  be  provided. 

At  the  opposite  end  of  the  classification  yard  there  is  a  ladder  track 
which  connects  with  the  pull-out  yard.  The  type  of  lighting  units 
serving  the  ladder  track  may  usually  be  utilized  in  the  pull-in,  re- 


522 


ILLUMINATING   ENGINEERING    PRACTICE 


ceiving  and  pull-out  yards  to  advantage.  Two  principal  con- 
ditions must  be  satisfied.  With  a  mounting  height  of  35  ft.  the 
candle-power  values  in  the  3o-45-deg.  zone  must  be  relatively 
high  to  illuminate  properly  the  ladder  track  switches,  and  again  in 
the  6o-7o-deg.  zone  high  values  are  required  for  the  illumination 
of  the  tops  of  the  cars,  particularly  in  the  receiving  yard  which  may 
be  six  or  eight  tracks  wide.  At  30  degrees  the  candle-power  value 
should  be  not  less  than  about  600  nor  more  than  1300.  At  from 
60  to  65  degrees  it  is  fixed  very  closely  at  1000.  Fig.  4  shows  the 
maximum  and  minimum  values  of  candle-power  for  3  5 -ft.  mounting 
height. 

Traffic  rules  require  that  there  shall  be  safe  clearance  between  the 
poles  and  the  sides  of  cars  and  the  allowances  are  such  that  the  poles 


Height  in  Feet_pf  Unitsabove  Track* 

/ 

>/ 

x 

x 

s 

x 

x 

x 

X 

^ 

x 

X 

)   20  40  60  80  100  120  140  160  180  200  220  24 

Distance  in  Feet  between  Units 

Fig.  5. — Mounting  heights  and  spacings,  ladder-track  units,  pull-in-receiving  and 
pull-out  yards. 

must  be  set  at  least  10  ft.  from  the  center  of  the  track.  Pole 
steps  must  be  set  parallel  to  the  track  rather  than  at  right  angles  to 
it.  On  straight  track,  where  switches  and  other  local  conditions 
permit,  pole  spacings  should  not  exceed  100  ft.,  while  on  curved 
track  and  in  the  vicinity  of  switches,  they  should  be  much 
less  for  this  type  of  fixture.  Good  practice  requires  that  each 
switch  shall  have  at  least  one  lamp  not  further  than  30  to  40 
ft.  from  it.  Fig.  5  gives  maximum  spacing  for  mounting  heights 
up  to  80  ft.  Under  normal  conditions  spacings  should  preferably 
ably  be  about  75  per  cent,  of  the  values  given. 

The  common  conception  of  the  problem  of  classification  yard 
lighting  is  that  of  providing  a  fairly  even  distribution  of  light  over  a 
large  expanse  of  railway  tracks.  This,  however,  is  an  erroneous 
impression  as  the  absence  of  cars  means  that  no  work  is  being  per- 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC. 


523 


formed  and  light  is  then  unnecessary.  This  problem  has  on  several 
occasions,  been  likened  to  that  of  attempting  to  light  a  large  city, 
of  relatively  tall  buildings  and  narrow  streets  and  alleys,  entirely 
from  the  outskirts  of  the  city.  Owing  to  the  large  space  required 
for  clearance  purposes  between  cars  and  poles  or  other  obstructions, 
it  is  seldom  possible  to  secure  sufficient  space  for  pole  lines  through 
the  center  of  the  yard.  The  relatively  high  cost  and  the  difficulty 
of  securing  straight  poles  40  ft.  or  more  in  length  usually  makes  it 
necessary  to  confine  the  pole  line  construction  to  35-ft.  poles.  When 
set  in  5-ft.  holes  and  fitted  with  appropriate  pole  tops  for  supporting 


40°  30  20°       10°       0°       10°       20°        30°  40" 

Fig.  6. — Candle-power  curve,  classification  yard  unit;  maximum  and  minimum. 


the  lamps,  this  length  of  pole  gives  a  clear  height  of  the  lamp  above 
the  rail  of  from  32  to  35  ft. 

The  tops  of  the,  cars  must  be  well  illuminated.  The  riding  of 
moving  cars  is  a  more  or  less  hazardous  occupation  and  as  the  riders 
perform  their  principal  duties  on  the  tops  of  cars  it  is  essential  that 
they  be  given  sufficient  illumination  to  make  their  positions  secure. 
The  illumination  must  be  sufficient  to  enable  them  to  see  clearly  other 
cars  on  the  same  track  and  thus  judge  distances  and  so  regulate  the 
speed  of  the  cars  as  to  make  the  coupling  without  injury  to  the  load 
or  equipment.  Finally  the  illumination  between  the  cars  must  be 


524 


ILLUMINATING  ENGINEERING   PRACTICE 


sufficient  to  permit  each  rider  to  climb  off  a  car  and  cross  the  yard 
without  danger  of  accident. 

It  is  obvious  that  the  units  most  satisfactory  for  lighting  the  body 
of  the  yard,  under  the  conditions  described,  are  those  which  have 
very  flat  candle-power  distribution  curves.  With  an  effective 
mounting  height  of  only  about  20  ft.  above  the  tops  of  the  cars  it 


10  — 
0 


0    20    40    60   80  100  120 140  160 180  200  220  240  260  280  300  320  340  360  380  400 
Distance  in.  Feet  between  Units 

Fig.  7- — Mounting  heights  and  spacings  classification  yard  fixtures. 

is  necessary  that  candle-power  values  shall  be  very  high  immediately 
below  the  horizontal,  or  at  say  the  angle  of  80  to  85  degrees,  if  yards 
of  great  width  are  to  be  properly  illuminated.  Fig.  6  shows  the 
maximum  and  minimum  candle-power  values  required  for  a  mount- 
ing height  of  35  ft. 


Multiplying  Factor 
o  S  §  §  S  g 

/ 

/ 

/ 

/ 

/ 

— 

^^ 

^ 

10        20        30        40        50        60        70 
Height  in  Feet  of  Units  above  Tracks 

Fig.  8. — Multiplying  factors  yard  lighting  fixtures. 

While  it  is  desirable  to  mount  the  lamps  as  high  as  possible,  local 
conditions  will  largely  govern  this  feature.  As  previously  stated, 
poles  more  than  35  ft.  in  length  are  usually  difficult  to  secure.  On 
the  other  hand,  the  possibility  of  having  to  shift  the  pole  line  to  a 
new  location  at  some  future  time,  by  reason  of  changes  in  track 
arrangement,  etc.,  makes  it  particularly  desirable  that  short,  stout 
poles  be  used.  Lamp  spacings  will  vary  with  the  mounting  heights 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC.  525 

and  should  not  exceed  200  ft.  for  35  ft.  in  height.  It  is  good  practice 
to  stagger  the  poles  along  one  side  of  the  yard  with  reference  to  those 
on  the  opposite  side.  Fig.  7  shows  the  maximum  spacings  for  mount- 
ing heights  of  70  ft.  and  less.  It  is  common  practice  to  limit  the 
spacings  used  to  approximately  three-quarters  of  these  values. 

The  size  of  the  lamp  will  vary  with  the  spacing  and  mounting 
height.  The  candle-power  curves  shown  in  Fig.  4  and  Fig.  6  are 
based  upon  a  mounting  height  of  35  ft.  If  for  any  reason  it  is  found 
necessary  or  desirable  to  use  a  different  mounting  height,  in  any  of 
the  yards  to  which  reference  is  made,  the  size  of  the  lamp  must  be 
increased  or  decreased  sufficiently  to  give  candle-power  values 
which  may  be  determined  by  multiplying  the  values  shown  in  Fig. 
4  and  Fig.  6  by  the  factors  shown  in  Fig.  8  for  the  several  mounting 
heights. 

Pull-out  Yard. — The  pull-out  yard  lies  immediately  beyond  the 
classification  yard.  Here  full  trains  are  made  up  from  cars  removed 
from  the  classification  yard  and  prepared  for  movement  over  the 
main  line.  Like  the  pull-in  yard,  this  yard  consists  of  only  a  limited 
number  of  tracks,  each  long  enough  to  accommodate  a  full  train. 
Very  little  of  the  work  requires  men  to  pass  between  or  climb  over 
cars,  consequently  the  lighting  requirements  are  not  severe.  The 
type  of  lamp  used  in  the  pull-in  and  receiving  yards  and  along  the 
ladder  track  is  satisfactory  for  this  service  and  the  same  mounting 
heights  and  pole  spacings  will  apply. 

Service  and  Types  of  Illuminants. — Since  the  lamps  must  be 
placed  overhead,  poles  for  supporting  them  are  necessary.  Changes 
and  improvements  in  operating  conditions  and  methods  frequently 
require  that  the  tracks,  and  consequently  the  lamps,  be  changed  in 
location.  This  precludes  the  possibility  of  employing  anything 
approaching  permanent  construction,  such  as  underground  conduit 
systems,  steel  poles  or  towers,  catenary  construction  for  suspending 
a  large  number  of  small  lamps  over  the  body  of  the  yard,  etc.  The 
enormous  length  of  tansmission  line  required,  because  of  the  neces- 
sity for  locating  the  poles  along  the  outer  edge  of  the  yard  and  on 
other  unoccupied  ground  areas,  prohibits  the  use  of  multiple 
circuits.  Several  of  the  larger  yards  in  use  to-day  are  from  4  to 
6  miles  in  length  and  require  from  10  to  15  miles  of  pole  line 
construction.  Series  circuits  are  therefore  commonly  used. 

Practically  all  forms  of  arc  lamps  have  been  used  in  this  service 
with  varying  degrees  of  success.  Arc  lamps  are  not  entirely  satis- 
factory. They  require  frequent  trimming  and  trimming  in  an  active 


526  ILLUMINATING   ENGINEERING    PRACTICE 

freight  yard  is  a  more  or  less  dangerous  occupation.  Runways  for 
repair  and  supply  trucks  cannot  be  provided,  and  hence  the  trimmer 
must  make  his  rounds  on  foot  and  all  supplies  and  all  lamps  removed 
for  repairs  must  be  transported  by  hand.  These  items  must  be  given 
serious  consideration  and  that  system  of  lighting  should  be  selected 
which  reduces  to  a  minimum  the  difficulties  of  operation  and  mainte- 
nance, even  though  the  illumination  of  the  yard  be  interfered  with 
to  a  slight  extent. 

The  old  open  arc  lamp  was  probably  the  first  type  of  electric 
lamp  used  in  this  service.  It  was  undoubtedly  a  big  improvement 
over  the  trainman's  hand  lantern  then  in  common  use.  This  lamp 
soon  gave  way  to  the  direct-current  enclosed  arc  which  in  turn  was 
superseded  by  the  alternating-current  enclosed  arc  lamp.  All 
of  these  lamps  were  expensive  in  operation  as  compared  with  later 
types.  They  were  inefficient  and  did  not  give  proper  light  distri- 
bution for  the  service  required. 

The  luminous  or  magnetite  arc  lamp  approaches  most  closely  to 
the  ideal  from  the  standpoint  of  light  distribution.  It  gives  its 
maximum  candle-power  at  from  80  to  85  degrees,  thus  making  it 
possible  to  light  the  center  of  the  yard  fairly  well  even  though  it  be 
as  much  as  400  ft.  wide.  The  "flat  distribution"  also  permits 
the  use  of  comparatively  short  poles  which  is  a  big  advantage  from  a 
construction  standpoint.  Flame  arc  lamps  have  not  given  satis- 
faction for  two  principal  reasons:  first,  while  the  candle-power  in- 
tensities at  15  degrees  below  the  horizontal  are  sufficient  to  illuminate 
the  center  of  the  yard,  the  intensities  at  lower  angles  are  great  enough 
to  give  very  bright  spots  in  the  immediate  vicinity  of  the  lamp  poles, 
which  is  objectionable  to  the  car  riders;  and  second,  the  excessive 
flicker  of  the  arc  and  the  frequent  welding  of  electrodes  causes  annoy- 
ance and  inconvenience. 

Large  size  incandescent  lamps,  fitted  with  refracting  glassware, 
are  the  most  attractive  units  at  the  present  time.  By  their  use 
the  hazards  of  cleaning  and  trimming  are  reduced  to  a  minimum. 
The  shape  of  the  refractor  is  such  that  a  slight  shifting  of  the  lamp 
with  reference  to  the  refractor,  will  give  almost  any  shape 
of  candle-power  distribution  curve  desired.  Finally  the  energy 
consumption  is  nearly  as  low,  for  equal  yard  illumination,  as  for 
any  of  the  series  arc  systems  now  employed.  At  the  present  time, 
however,  the  sizes  of  street  series  incandescent  lamps  regularly 
manufactured  in  ampere  ranges  up  to  7.5  are  probably  not  large 
enough  to  be  competitive  with  the  luminous  arc  lamps  for  wide 


MINICK:  LIGHTING^  OF  YARDS,  DOCKS,  ETC.  527 

yards,  unless  pole  space  can  be  reserved  through  the  yard  for 
the  incandescent  lamps.  If  pole  space  can  be  reserved  for  this  pur- 
pose, the  distances  shown  in  Fig.  5  may  be  used  as  the  spacing 
between  pole  lines  through  the  yard.  The  6oo-c.p.  gas-filled  lamp 
with  refractor  unit  is  competitive,  both  in  power  consumption  and 
candle-power  distribution,  with  the  4.o-amp.  luminous  arc  lamp. 
The  latter  lamp,  however,  should  not  be  used  with  spacings  of  more 
than  200  ft.  The  6.6  amp.  luminous  arc,  which  may  be  used  with 
spacings  of  300  ft.  or  more,  is  a  much  more  powerful  unit  than 
any  of  the  incandescent  lamp  units  now  regularly  manufactured, 
unless  it  be  the  IQOO  candle-power  2o.o-amp.  series  lamp,  which  is 
not  a  desirable  lamp  on  account  of  the  necessity  for  using  a 
"  compensator." 

Projector  lamps  have  been  used  to  a  limited  extent  in  this  service. 
The  lamp  is  mounted  near  the  hump  and  the  beam  is  directed 
against  the  rear  end  of  each  car  as  it  passes  into  the  classification  yard. 
The  principal  objections  to  this  method  of  lighting  are:  the  extreme 
high  cost  of  operation,  the  necessity  for  the  yard  crew  to  avoid  fac- 
ing the  light  source  thus  increasing  the  difficulties  of  performing  their 
duties,  and  finally,  the  danger  of  accidentally  playing  the  beam  on 
passenger  trains  operating  on  the  adjacent  main  line,  thus  obscuring 
signals  and  possibly  interfering  with  the  vision  of  the  engineman  and 
fireman.  Mercury  vapor  lamps  of  high  candle-powers  and  flood- 
lighting units  have  also  been  used  to  a  limited  extent.  In  each  of 
these  installations  the  use,  of  steel  poles  or  towers,  75  ft.  or  more 
in  height,  is  necessary  though  the  number  of  lighting  units  is  mate- 
rially reduced.  The  use  of  towers  or  similar  structures  require  per- 
manent locations  which  are  not  always  to  be  had  at  reasonable 
expense. 

Transfer  Station. — Small  package  freight  must  be  classified  or 
sorted  for  destination  exactly  as  are  full  cars,  and  for  the  same  pur- 
pose. There  is  a  difference  in  the  method  of  operation,  however. 
The  character  of  the  freight  handled  requires  that  each  package  shall 
be  removed  from  the  way  collection  car  and  placed  in  another  car 
consigned  to,  or  to  some  point  near,  the  destination  of  that  package, 
where  another  classification  will  be  made.  Since  much  of  this 
freight  is  heavy,  trucks,  sometimes  two-wheel  hand  operated  and 
other  times  four-wheel  hand  or  motor  operated,  are  used  in  handling 
the  individual  packages.  This  necessitates  the  construction  of  heavy 
platforms  between  adjacent  tracks  at  approximately  the  elevation 
of  the  car  floor,  for  the  operation  of  the  trucks.  Much  of  this  freight 


528  ILLUMINATING   ENGINEERING   PRACTICE 

must  be  protected  from  the  weather  so  that  roofs  over  the  platforms 
are  required  and  frequently  sheds  are  provided  for  storing  freight 
which  has  been  unloaded  but  which  cannot  be  loaded  until  the  next 
or  a  succeeding  day. 

Usually  two  or  more  tracks,  sometimes  as  many  as  ten  or  fifteen, 
are  assigned  to  transfer  service.  The  platforms  will  vary  in  length 
to  accommodate  from  two  or  three  to  as  many  as  20  to  25  cars.  They 
must  be  about  15  ft.  wide  to  permit  the  operation  of  two  lines  of 
trucks  on  each  side  of  the  platform  if  necessary.  The  roof  is  sup- 
ported by  posts,  the  more  modern  types  of  construction  employing 
the  "umbrella"  type  of  roof,  supported  by  a  single  row  of  steel 
columns  along  the  center  of  the  platform.  The  tracks  are  so  ar- 
ranged that  the  edges  of  the  cars  are  close  to  the  edges  of  the  plat- 
form. Where  two  or  more  platforms  are  used  two  or  more  tracks 
are  placed  between  the  platforms. 

A  single  row  of  incandescent  lamps,  with  distributing  type  porcelain 
enamel  reflectors,  will  furnish  illumination  sufficient  for  the  opera- 
tion of  trucks  on  the  platform.  The  reflectors  should  shield  the 
filament  of  the  lamp  from  the  natural  range  of  vision.  With  a 
mounting  height  of  10  ft.  and  a  spacing  of  not  greatly  to  exceed 
20  ft.,  loo-watt  lamps  give  satisfactory  service. 

The  difficult  part  of  this  problem,  however,  comes  within  the  car. 
Here  the  address  and  lading  of  each  package  must  be  read  and  com- 
pared with  the  manifest.  No  satisfactory  method  of  illuminating 
the 'interior  of  the  car  has  yet  been  developed.  The  fixtures  used 
must  necessarily  be  of  the  portable  type  and  of  inexpensive  construc- 
tion, as  many  of  them  are  lost  through  being  left  in  the  cars.  They 
must  be  supported  from  overhead  as  the  trucks  must  have  access 
to  all  parts  of  the  car  floor.  At  the  present  time  portable  hand 
lamps  without  reflectors  are  used.  Plug  connections  for  this  ser- 
vice are  usually  supported  by  metal  conduits  from  overhead  or 
mounted  under  the  edges  of  the  platform. 

Docks  and  Terminal  Yards. — Export  freight  is  delivered  to  sea- 
coast  points.  Package  and  perishable  freight  is  delivered  to  large 
covered  piers  where  it  is  stored  until  boats  can  be  secured  for  load- 
ing. Car  load  freight  which  can  safely  be  exposed  to  the  weather,  is 
delivered  to  a  yard  in  which  it  is  unloaded  and  stored  on  the  ground 
to  await  water  shipment.  Upon  arrival  of  the  boat  it  is  reloaded 
on  cars  and  shifted  to  the  pier  or  dock  where  it  is  transferred  to 
the  boat. 

The  lighting  of  the  covered  pier  is  a  comparatively  simple  problem. 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC.  529 

*  Light  is  required  only  for  the  purposes  of  trucking  and  piling  or 
stacking  and  for  the  verification  of  the  manifest.  Usually  several 
rows  of  incandescent  lamps  fitted  with  distributing  type  porcelain 
enamel  reflectors  are  used  in  this  service.  The  candle-power  of  the 
lamp  will  of  course  vary  with  the  mounting  height  and  spacing.  The 
spacing,  however,  should  be  so  arranged  as  to  give  good  light 
distribution  without  shadows  on  the  truck-ways  and  the  lamps 
should  be  as  large  as  possible  consistent  with  these  and  other  local 
conditions.  An  illumination  intensity  of  from  0.75  to  1.25  foot- 
candles  along  the  truck- ways  is  sufficient.  "All  night  circuits" 
are  desirable  along  the  truck-ways  for  the  use  of  the  night 
watchman. 

The  lighting  of  the  storage  yard  represents  a  much  different  prob- 
lem. Here  freight  is  unloaded  by  locomotive  type  cranes  and  stored 
on  the"  ground  until  a  boat  arrives,  when  it  is  reloaded  and  moved 
to  the  pier  for  loading  on  board  ship.  The  tracks  are  arranged  in 
parrs,  one  on  which  the  cars  are  placed  to  be  unloaded  and  the  other 
for  the  use  of  the  crane.  The  crane  has  an  extreme  reach  of  from 
30  to  40  ft.  so  that  the  pairs  of  tracks  are  spaced  on  about  6o-ft. 
centers  to  permit  the  storage  of  materials  between  them.  This 
space,  which  has  a  clear  width  of  possible  48  to  50  ft.,  is  sometimes 
filled  completely  to  a  height  of  from  20  to  25  ft.  The  crane  may  be 
turned  360  degrees  if  necessary  and  when  lifting  loads  close  to  the 
crane  the  lifting  arm  stands  nearly  vertical,  the  extreme  height 
being  probably  50  ft. 

The  handling  of  freight  must  be  carried  on  during  night  hours,  and 
hence  artificial  illumination  is  necessary.  The  extreme  range  of 
operation  of  the  lifting  arm  of  the  crane  prevents  the  use  of  pole 
lines  with  lamps  suspended  from  the  poles.  Some  attempt  has  been 
made  to  furnish  lighting  service  by  the  use  of  portable  standards 
carrying  one  or  more  large  lamps  and  connected  by  flexible  cables 
and  plugs  to  a  system  of  underground  cables  installed  between  the 
unloading  tracks.  This  arrangement  has  not  been  satisfactory  or 
successful  for  several  reasons.  The  cost  of  installation  for  the  ser- 
vice performed  is  very  high  and  usually  not  warranted.  The  light- 
ing standards  must  be  short  enough  to  permit  free  range  of  move- 
ment of  the  crane  above  them  which  places  the  lamps  too  low  to 
be  of  value  in  unloading  gondola  cars  or  in  stacking  on  top  of  large 
piles.  The  cable  connections  offer  obstruction  to  walking  in  the 
vicinity  of  the  crane  and  they  are  frequently  broken  or  cut  by 
materials  falling  upon  them.  The  standards  must  be  light  enough 

34 


530  ILLUMINATING    ENGINEERING    PRACTICE 

for  convenient  handling  and  hence  the  weight  is  such  that  they  may 
be  overturned  readily  and  the  lamps  broken. 

An  attempt  has  also  been  made  to  mount  the  lighting  units  on 
the  crane  structure  itself  and  by  means  of  angle  reflectors  to  dis- 
tribute the-  light  in  useful  directions.  While  this  arrangement  has 
given  most  excellent  results  from  the  standpoint  of  illumination  a 
number  of  operating  difficulties  have  been  encountered  which  have 
not  been  entirely  overcome.  The  use  of  a  flexible  cable  for  supply- 
ing current  to  the  crane  from  an  underground  distributing  system 
is  undesirable  for  reasons  already  mentioned.  The  use  of  a  small 
steam-driven  generator,  mounted  on  the  crane  and  supplied  with 
steam  from  the  crane  boiler,  will  probably  not  give  satisfaction  as 
the  crane  boiler  is  designed  for  intermittent  duty  in  which  the  peak 
demand  for  steam  may  be  four  or  five  times  the  normal  rating  of 
the  boiler.  A  gasoline  or  oil  engine-driven  generator,  mounted  on  a 
small  truck  attached  to  the  crane,  has  been  used  in  this  service,  and 
where  lifting  magnets  are  used  for  the  handling  of  iron  and  steel 
products,  this  is  probably  the  most  satisfactory  arrangement  for 
securing  electrical  energy  in  sufficiently  large  quantities  to  meet  the 
demand.  The  vibration  of  the  crane  is  excessive,  especially  when 
releasing  loads  suspended  in  the  air  by  the  lifting  magnet,  and  lamp 
breakage  is  excessive  unless  shock  absorbers  are  used  in  mounting 
the  fixtures.  Several  devices  of  this  kind  are  being  developed  to 
relieve  the  situation. 

In  this  service  four  fixtures  are  mounted  on  top  of  the  cab,  two 
along  each  side.  An  additional  fixture  is  mounted  to  the  rear  and 
one  in  front  over  each  door.  The  lifting  arm  carries  two  fixtures,  one 
about  one-third  the  distance  up  from  the  bottom,  the  other  about 
one-third  the  distance  down  from  the  top.  The  seven  fixtures 
around  the  cab  are  adjusted  to  illuminate  the  complete  circle  sur- 
rounding the  crane  to  a  radius  of  about  80  ft.  This  plan  permits  the 
preparation  of  a  new  load  while  the  crane  is  engaged  in  stacking. 
The  two  fixtures  mounted  on  the  lifting  arm  are  arranged  to  illumi- 
nate the  load  while  suspended  in  the  air,  regardless  of  the  position  of 
the  lifting  arm,  and  also  while  it  is  being  stacked.  Five-hundred- 
watt  gas-filled  lamps  with  porcelain  enamel  angle  reflectors  of  the 
distributing  type  are  used  and  the  light  is  directed  away  from  the 
crane  so  as  not  to  interfere  with  the  vision  of  the  crane  operator. 

Heavy  materials  are  usually  handled  by  gantry,  or  bridge  type, 
cranes  from  pier  to  boat  or  boat  to  pier.  All  important  work  takes 
place  in  the  immediate  vicinity  of  the  crane  and  the  lighting  require- 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC.  531 

ments  in  general  are  similar  to  those  of  the  locomotive  crane.  Fix- 
tures are  mounted  on  the  crane  structure  and  appropriate  designs  of 
reflectors  are  employed  to  turn  the  light  in  useful  directions,  always 
keeping  in  mind  that  the  light  must  be  directed  away  from  the  crane 
operator.  The  lamps  used  vary  in  consumption  from  about  100  to 
500  watts  and  even  1000  watts,  depending  upon  the  character  of 
the  work  to  be  performed,  the  locations  of  the  fixtures,  mounting 
heights,  etc. 

PASSENGER  PLATFORMS 

Description. — Stations  are  provided  at  frequent  intervals  along 
the  main  or  running  tracks  for  the  accommodation  of  passengers  and 
the  handling  of  mail,  baggage  and  express  matter.  The  stations  at 
the  ends  of  divisions  and  at  other  important  points  where  crews  and 
locomotives  are  changed,  are  known  as  "Terminal  Stations."  All 
others  are  classed  as  "  Way  Stations."  Terminal  stations  are  usually 
large  and  serve  many  tracks  and  trains.  Some  idea  of  the  density  of 
traffic  at  a  large  terminal  may  be  gained  from  the  following  approxi- 
mate figures : 


Station 

Trains  per  day 

No.  tracks 

Broad  Street  Station  Phila                                  .  . 

t;oo 

16 

Pennsylvania  Station,  Pgh  

480 

19 

South.  Station   Boston                             

700 

28 

Pennsylvania  Station,  New  York  

525 

21 

Both  foot  and  truck  traffic  is  heavy  and  continuous,  requiring  the 
use  of  long,  wide  platforms  between  tracks  for  loading  and  unloading. 
Way  station  traffic  is  generally  light  and  intermittent. 

Terminal  Stations. — The  tracks  at  terminal  stations  are  usually 
arranged  in  pairs  with  platforms  between,  so  that  one  platform  will 
serve  two  trains.  These  platforms  are  usually  about  15  ft.  wide, 
though  the  tendency  of  late  has  been  to  increase  this  dimension 
slightly.  With  two  trains  at  the  same  platform,  it  may  be  necessary 
to  accommodate  as  many  as  1000  passengers  and  20  four-wheel 
trucks.  The  trucks  are  loaded  with  baggage,  mail  and  express 
matter,  each  piece  of  which  bears  an  address  or  number  tag.  The 
baggage  checks  are  made  of  either  cream,  red,  blue  or  green-colored 
cardboard  printed  in  black  ink.  Addresses  on  mail  bags  and  express 
matter  are  usually  written  with  indelible  pencil  on  white  or  yellow 


532  ILLUMINATING   ENGINEERING   PRACTICE 

paper.  These  must  be  identified  and  compared  with  the  way-bills 
which  may  have  corresponding  colors.  Colored  berth  and  seat 
checks  in  Pullman  service  must  also  be  identified.  In  the  older 
stations  the  platforms  are  protected  from  the  weather  by  a  single 
arched  roof  spanning  all  of  the  tracks  and  platforms.  In  the  more 
modern  stations  the  platforms  have  individual  roofs  each  supported 
by  a  single  row  of  columns  along  the  center  of  the  platform. 

Fairly  even,  though  not  necessarily  high,  illumination  is  required 
for  foot  and  truck  traffic.  Fairly  high  illumination  is  required  for 
the  verification  of  the  way-bills.  Finally,  light  is  required  on  the 
tops  of  the  cars  to  permit  icing,  watering,  etc.,  and  all  lamps  should 
be  shielded  to  enable  the  enginemen  to  judge  distances  and  stop 
his  train  at  the  proper  point.  Metal  reflectors,  having  an  angle  of 
cut-off  of  not  to  exceed  75  degrees,  are  commonly  used  in  train  sheds. 
The  mounting  height  should  be  not  less  than  22  ft.  nor  more  than 
25  ft.  above  the  rail.  The  spacing,  and  consequently  the  size  of  the 
lamp  will  vary  with  local  conditions,  particularly  the  location  of  the 
roof -supporting  members  to  which  the  fixture  must  be  attached. 
The  average  illumination  intensity  should  be  from  i.o  to  1.25  foot- 
candles.  Glass  reflectors  should  be  used  where  the  platforms  have 
individual  roofs. 

Way  Stations. — Each  way  station  is  provided  with  one  or  more 
platforms  for  the  loading  and  unloading  of  passengers,  mail,  etc. 
They  are  paved  with  wood,  brick  or  concrete  and  are  illuminated  at 
night  by  small  lamps  mounted  on  poles  along  the  edge  of  the  plat- 
form farthest  from  the  track  in  the  case  of  a  side  platform,  or  along 
the  center  of  the  platform  in  case  it  is  between  the  tracks.  The 
poles  are  spaced  at  intervals  of  approximately  one-half  car  length  to 
enable  the  enginemen,  by  counting  them,  to  judge  distances  and 
stop  his  train  at  the  proper  point.  At  points  200  ft.  at  each  side  of 
the  center  line  of  the  station  "approach  signs"  attached  to  lighting 
poles,  display  the  name  of  the  station.  The  bottom  of  this  sign  is 
approximately  8  ft.  above  the  platform  as  this  has  been  selected  as 
the  proper  height  for  convenient  reading,  either  from  within  the  car 
or  from  the  station  platform. 

Distributing  type  porcelain  enamel  metal  reflectors,  with  25-watt 
lamps  are  used  in  this  service.  At  the  more  important  stations 
40- watt  lamps  are  sometimes  used.  The  lamps  are  mounted  on 
metal  poles  about  9.5  ft.  above  the  platform  to  clear  loaded  baggage 
and  mail  trucks.  At  this  height  the  ordinary  types  of  reflectors 
shield  the  lamp  completely  from  the  view  of  the  enginemen.  A 


MINICK:  LIGHTING  OF  YARDS,  DOCKS,  ETC.  533 

special  reflector  has  been  designed  to  expose  a  small  portion  of  the 
incandescent  filament  so  that  the  engineman  may  count  the  poles 
by  the  flashes  of  bright  light  as  he  passes,  without  looking  in  that 
direction.  Where  shelters  are  used  transparent  numerals  are  some- 
times set  in  the  longitudinal  roof  trusses  opposite  the  lamps  to  indi- 
cate the  stopping  points  for  trains  of  various  lengths.  The  average 
illumination  of  way  station  platforms  need  not  exceed  i.o  foot-candle. 

CONCLUSIONS 

It  should  not  be  inferred  from  the  foregoing  that  only  a  limited 
number  of  sizes  and  types  of  lighting  units  can  be  used  to  advantage 
in  railway  service.  The  scope  of  railway  lighting  is  so  broad  that 
there  should  be,  and  there  probably  is,  a  distinct  field  for  each  of  the 
common  illuminants  of  the  present  day.  Few  railroads  have  es- 
tablished definite  standards  for  their  lighting  service.  There  are, 
however,  a  limited  number  of  general  conditions  which  are  beginning 
to  be  accepted  as  good  practice  in  railway  service  as  follows: 

1.  Incandescent  lamps  should  be  used  in  preference  to  arc  lamps  if  local  con- 
ditions will  permit. 

2.  The  candle-power  of  units  and  spacing  intervals  should  be  as  large  as 
possible  consistent  with  good  illumination. 

3.  Glass  reflectors  should  be  used  inside  office  buildings,  stations,  etc.,  alu- 
minized  metal  reflectors  in  shops  and  semi-exposed  places  and  porcelain  enamel 
reflectors  in  outside  service. 

4.  All  reflectors  should  shield  from  view  the  incandescent  filament  under 
normal  operating  conditions.     This  means,  generally,  an  angle  of  cut-off  of  from 
60  to  75  degrees. 

5.  All  fixtures  and  auxiliaries  should  be  as  strong  and  rugged  as  possible 
consistent  with  the  cost  of  installation. 

Bibliography 

H.  KIRSCHBERG  and  A.  C.  COTTON.  "Railway  Illuminating  Engineering 
Track  Scale  and  Yard  Lighting. "  Pittsburgh  Section  I.  E.  S.,  February  13, 1914. 

"Report  of  Committee  on  Outside  Construction  and  Yard  Lighting."  Pro- 
ceedings A.  R.  E.  E.,  1914. 

L.  C.  DOANE.  "Lighting  Railroad  Yards  with  Large  Incandescent  Lamps." 
Railway  Electrical  Engineer,  December,  1914. 

"Lighting  Classification  Yard,  New  York  Central  R.  R.,  Air  Line  Junction, 
Ohio."  Railway  Electrical  Engineer,  December,  1915. 

"Report  of  Committee  on  Illumination."  Proceedings  A.  R.  E.  E.,  1916. 
Unpublished  reports  of  the  Pennsylvania  Railroad. 


SIGN  LIGHTING 

BY    LEONARD    G.    SHEPARD 

I  have  made  no  attempt  to  find  a  record  of  the  first  sign.  It  is 
well  known  that  signs  have  been  in  common  use  for  centuries  and 
many  of  these  were  undoubtedly  more  or  less  illuminated. 

The  sign  of  this  age,  with  which  we  have  to  do  is  the  sign  made 
possible  by  the  modern  electric  lamps.  It  was  probably  about  1880 
that  signs  of  this  type  began  to  be  used. 

To  appreciate  the  present-day  development,  it  will  be  well  to 
know  something  of  the  earlier  electric  signs  and  their  construction. 

In  1883  a  temporary  sign  reading  "Welcome"  was  made  by  plac- 
ing the  old  style  wooden  base  sockets  on  a  wooden  background. 
The  wiring  between  sockets  was  done  on  the  back  of  the  sign.  The 
letters  were  2  ft.  in  height  and  were  formed  of  16  candle-power 
lamps,  spaced  about  6  in.  apart. 

In  1884,  an  electric  sign  reading  " Boston  Oyster  House"  was  de- 
signed for  more  permanent  use.  To  make  the  sockets  weatherproof 
they  were  filled  with  putty  and  the  wire  being  the  old  style  known 
as  underwriters  wire  was  wrapped  with  tape.  The  sign  body  and 
frame  were  made  entirely  of  wood  and  over  all  a  glass  case  was  built 
like  an  ordinary  show  case.  The  lamps  of  that  time  had  large 
plaster-of-Paris  bases  which  were  protected  in  this  sign  by  covering 
them  with  soft  rubber  bands  which  covered  also  the  outer  end  of 
the  sockets. 

A  double-faced  sign  made  about  this  time  reading  "Dime  Museum" 
is  said  to  have  been  about  12  ft.  long,  3  ft.  high  and  2  or  3 
ft.  thick  with  i8-in.  letters  made  up  of  electric  lamps.  Its  clumsy 
bulk  made  it  look  like  a  dog  house  hung  up  over  the  sidewalk. 

It  is  interesting  to  note  that  the  electric  flag  sign,  patriotically 
displayed  throughout  this  country  in  the  last  year  was  anticipated 
twenty-eight  years  ago  at  the  convention  in  Chicago  where  a  sign 
made  up  with  miniature  or  candelabra  lamps  was  flashed  on  during 
the  singing  of  the  "Star  Spangled  Banner." 

One  of  the  first  flashing  signs  was  made  for  the  World's  Fair  in 
1893.  The  letters  were  4  ft.  high  and  of  skeleton  construction 

535 


536  ILLUMINATING   ENGINEERING   PRACTICE 

attached  to  a  wire  mesh  backing.  The  mechanism  which  flashed 
on  one  letter  at  a  time  was  a  crude  affair  made  entirely  of  wood 
with  brass  strips  and  bronze  contact  brushes.  It  was  operated  by 
a  34-hp.  motor.  It  is  said  that  almost  as  much  light  came  from  the 
arcing  of  the  flasher  contacts  as  from  the  sign.  The  sign  was  con- 
sidered so  dangerous  that  a  man  was  kept  constantly  in  attendance. 
Possibly  the  largest  electric  sign  ever  made  was  constructed  in 
1899  during  the  reception  to  Admiral  Dewey  on  his  return  from 
Manila.  Letters  50  ft.  high  reading  "  Welcome  Dewey  "  were  placed 
on  the  Brooklyn  Bridge  and  could  be  read  from  Staten  Island  five 
miles  away.  There  was  no  background,  the  lamps  being  arranged 
on  streamers  to  form  the  letters.  No  sockets  were  used,  the  leading- 
in  wires  from  the  lamps  which  were  made  up  without  bases  being  so 
connected  that  five  lamps  were  put  in  series,  the  total  electromotive 
force  used  being  550  volt.  The  lamps,  of  which  over  8000  were  re- 
quired were  spaced  12  in.  apart  throughout.  This  sign  represented 
a  remarkable  example  of  series  wiring.  It  must  be  remembered 
that  the  failure  of  a  single  lamp  would  have  made  a  dark  section 
5  ft.  long  in  the  outline  of  the  letter. 

MODERN  SIGN  TYPES 

The  illuminated  sign  of  to-day  is  made  in  so  many  different  varie- 
ties and  is  used  in  such  varying  surroundings  that  it  will  be  neces- 
sary to  classify  and  explain  briefly  the  construction  of  the  several 
types  to  give  an  adequate  idea  of  the  state  to  which  the  industry 
has  developed. 

Roof  signs  (see  Figs,  i  and  2)  may  be  taken  as  including  all  the 
large,  more  or  less,  skeleton  types  installed  above  the  roof  level. 
In  these  signs  the  steel  supporting  structure  or  framework  is  usually 
the  most  important  item.  To  place  the  electrical  display  at  a 
proper  height  and  in  the  best  location  from  an  advertising  standpoint 
often  requires  a  considerable  structure  and  frequently  too,  it  is 
necessary  to  reinforce  the  building  and  to  carry  the  anchorage  mem- 
bers way  below  the  roof.  This  is  mentioned,  because  at  night  the 
framework  is  not  seen  and  no  idea  of  the  investment  involved  can 
be  obtained  without  a  full  appreciation  of  this  item. 

The  electrical  work  required  to  connect  the  sign  parts  to  the  near- 
est service  including  the  installation  of  the  flasher  and  fuse  blocks 
depends  more  upon  the  flashing  effects  than  upon  the  size  of  the  sign. 

The  sign  proper  or  sign  face  is  usually  quite  simple  in  construction 


Fig.   i. — Roof  sign. 


Fig.  2. — Roof  sign. 


(Facing  page~S36.) 


536  ILLUMINATING   ENGINEERING   PRACTICE 

attached  to  a  wire  mesh  backing.  The  mechanism  which  flashed 
on  one  letter  at  a  time  was  a  crude  affair  made  entirely  of  wood 
with  brass  strips  and  bronze  contact  brushes.  It  was  operated  by 
a  M-hp-  motor.  It  is  said  that  almost  as  much  light  came  from  the 
arcing  of  the  flasher  contacts  as  from  the  sign.  The  sign  was  con- 
sidered so  dangerous  that  a  man  was  kept  constantly  in  attendance. 
Possibly  the  largest  electric  sign  ever  made  was  constructed  in 
1899  during  the  reception  to  Admiral  Dewey  on  his  return  from 
Manila.  Letters  50  ft.  high  reading  "  Welcome  Dewey"  were  placed 
on  the  Brooklyn  Bridge  and  could  be  read  from  Staten  Island  five 
miles  away.  There  was  no  background,  the  lamps  being  arranged 
on  streamers  to  form  the  letters.  No  sockets  were  used,  the  leading- 
in  wires  from  the  lamps  which  were  made  up  without  bases  being  so 
connected  that  five  lamps  were  put  in  series,  the  total  electromotive 
force  used  being  550  volt.  The  lamps,  of  which  over  8000  were  re- 
quired were  spaced  12  in.  apart  throughout.  This  sign  represented 
a  remarkable  example  of  series  wiring.  It  must  be  remembered 
that  the  failure  of  a  single  lamp  would  have  made  a  dark  section 
5  ft.  long  in  the  outline  of  the  letter. 

MODERN  SIGN  TYPES 

The  illuminated  sign  of  to-day  is  made  in  so  many  different  varie- 
ties and  is  used  in  such  varying  surroundings  that  it  will  be  neces- 
sary to  classify  and  explain  briefly  the  construction  of  the  several 
types  to  give  an  adequate  idea  of  the  state  to  which  the  industry 
has  developed. 

Roof  signs  (see  Figs,  i  and  2)  may  be  taken  as  including  all  the 
large,  more  or  less,  skeleton  types  installed  above  the  roof  level. 
In  these  signs  the  steel  supporting  structure  or  framework  is  usually 
the  most  important  item.  To  place  the  electrical  display  at  a 
proper  height  and  in  the  best  location  from  an  advertising  standpoint 
often  requires  a  considerable  structure  and  frequently  too,  it  is 
necessary  to  reinforce  the  building  and  to  carry  the  anchorage  mem- 
bers way  below  the  roof.  This  is  mentioned,  because  at  night  the 
framework  is  not  seen  and  no  idea  of  the  investment  involved  can 
be  obtained  without  a  full  appreciation  of  this  item. 

The  electrical  work  required  to  connect  the  sign  parts  to  the  near- 
est service  including  the  installation  of  the  flasher  and  fuse  blocks 
depends  more  upon  the  flashing  effects  than  upon  the  size  of  the  sign. 

The  sign  proper  or  sign  face  is  usually  quite  simple  in  construction 


Fig.   i. — Roof  sign. 


Fig.  2. — Roof  sign. 


(Facing  Page~536.) 


Fig.  3. — Street  sign. 


Fig.  4. — Street  sign. 


Fig-  5- — Porcelain  enameled  steel  embossed  letter  sign. 


SHEPARD:  SIGN  LIGHTING  537 

consisting  of  light  galvanized  sheet  steel  cut  out  to  form  letters, 
figures  or  designs  and  backed  up  to  give  stiffness  and  to  enclose  the 
wiring.  No  internal  frame  is  necessary  as  it  is  customary  to  place 
a  lattice  work  of  light  channels  on  the  face  of  the  framework  to 
support  the  display.  The  galvanized  sheet  steel  serves  as  a  support 
for  the  sockets  and  as  a  background  for  the  painted  designs. 

The  face  of  the  letters  and  display  parts  is  most  commonly  flat 
or  flush  as  it  is  called.  Sometimes  where  the  parts  are  close  together 
a  flange  from  1.5  in.  to  4  in.  high  is  carried  around  the  edge  of  the 
face  to  give  contrast  and  contribute  to  a  clean-cut  design.  The 
flange  is  a  bad  collector  of  dust,  and  is  not  necessary  where  there  is 
plenty  of  spacing. 

Letters  with  a  flange  around  the  edge  of  the  face  or  stroke  are 
called  trough  letters.  There  is  a  special  bevel  trough  letter  made 
with  a  broad  flange  parallel  to  the  face,  on  the  outer  edge  of  the  bevel 
trough.  This  flange  is  painted  black  to  improve  the  daylight  effect 
as  it  makes  a  strong  contrast  against  a  light  sky.  An  example  of 
this  construction  may  be  seen  in  the  i5~ft.  letter  roof  sign  on  the 
Walkerville  factory  of  the  Ford  Company. 

Contrast  between  light  and  dark  colors  or  between  light  and 
shadow  is  very  necessary  to  a  sharp  clear  sign.  Frequently  a  flange 
or  trough  is  used  to  emphasize  an  important  line  or  to  separate  por- 
tions of  a  surface  which  flash  or  light  up  at  different  times. 

While  the  painted  surface  of  a  new  sign  may  be  relied  upon,  at 
least  in  clear  weather,  to  reflect  enough  light  to  bring  out  the  features 
of  a  design  under  ordinary  conditions,  the  lamp  filaments  must  be 
depended  upon  to  give  the  chief  outline.  The  sockets  therefore 
must  be  carefully  located  to  obtain  good  results.  The  distance  be- 
tween sockets  depends  upon  whether  the  sign  it  intended  principally 
for  distant  reading  or  for  a  maximum  effect  at  close  range.  In  the 
first  case  large  candle-power  lamps  can  be  spaced  well  apart  but  in 
the  second  the  sockets  must  be  close  together  to  avoid  a  crude  re- 
sult. About  4  in.  may  be  taken  as  a  good  standard  spacing  for 
close  work. 

While  many  excellent  signs  are  equipped  with  low  candle-power 
lamps  the  modern  tendency,  especially  in  the  large  cities/ is  toward 
greater  brilliancies.  Fifteen,  twenty  and  twenty-five  watt  tungsten 
lamps  are  frequently  used  although  the  popular  lamp  is  the  ten- 
watt  unit. 

The  beautiful  color  effects  are  obtained  by  the  use  of  colored 
lamps  either  natural  or  dipped  or  with  color  caps  or  hoods  of  colored 


538 


ILLUMINATING    ENGINEERING    PRACTICE 


glass  fitted  over  the  lamps  The  dipped  lamps  probably  allow  the 
greatest  variation  in  tint,  but  as  it  is  practically  impossible  to 
obtain  a  lacquer  that  will  adhere  permanently  to  the  bulb  in  all 
kinds  of  weather,  the  color  caps  or  hoods  are  more  satisfactory.  A 
good  illustration  of  the  difference  in  the  effect  of  the  cap  and  the 
hood  may  be  seen  in  the  many  flag  signs  installed  during  the  last 
year  or  two  (Fig.  6).  In  the  popular  4  ft.  flag  the  lamps  in  the 
union  are  placed  in  the  white  stars.  A  blue  cap  gives  the  proper 
blue  effect  for  that  portion  of  the  flag  while  the  direct  rays  from 
the  filament  behind  the  cap  light  up  the  white  stars.  On  the  red 


Fig.  3. — Electric  flag  sign. 

and  white  stripes  the  hoods  give  a  solid  red  or  white  effect  as  is 
desired. 

Certainly  any  discussion  of  modern  roof  signs  would  be  incomplete 
were  the  flashing  effects  omitted.  On  the  other  hand  a  book 
could  easily  be  written  in  explanation  of  the  many  variations  of  this 
phase  of  the  sign  industry. 

All  flashing  effects  are  produced  by  switching  on  certain  lamps  or 
groups  of  lamps  in  a  certain  sequence  and  with  due  regard  to  time 
periods.  The  mechanism,  usually  motor  driven,  which  makes  and 
breaks  the  many  different  electric  circuits,  some  as  fast  as  200  or 
300  times  a  minute,  some  with  currents  as  large  as  100  or  200  am- 
peres, is  a  very  important  part  of  the  installation.  As  might 
naturally  be  expected,  without  attention,  the  rapidly  vibrating 


SHEPARD:  SIGN  LIGHTING  539 

parts  will  often  work  out  of  adjustment  thereby  spoiling  the  effect 
of  the  entire  sign. 

In  general  the  skyrocket,  crawling  snake,  or  script  writing  effects 
are  the  most  expensive  in  both  flasher  and  wiring  while  the  con- 
tinuously moving  border  or  the  simple  on  and  off  effects  are  the 
easiest  to  obtain. 

Too  much  attention  cannot  be  given  to  the  care  or  maintenance 
of  an  electric  sign.  A  large  investment  may  be  robbed  of  the  greater 
part  of  its  earning  capacity  by  neglect.  A  sign  that  is  not  clean 
and  bright  and  in  which  the  lamps  are  not  all  active  or  one  in  which 
a  word  is  spelled  out  unevenly  or  with  one  letter  omitted  is  a  liability 
rather  than  an  asset.  Proper  provision  should  always  be  made  for 
maintenance  with  the  same  care  as  for  installation. 

The  discussion  of  the  roof  sign  has  included  many  items  which 
pertain  to  all  kinds  of  signs.  There  are  some  features  of  construc- 
tion, however,  which  are  very  different  in  signs  used  for  other 
purposes. 

The  sidewalks  or  street  signs  as  they  may  be  called  are  so  much 
nearer  to  the  observer  that  the  matter  of  detail  must  be  given  close 
attention. 

This  class  (see  Figs.  3  and  4)  usually  has  an  internal  frame  of 
steel,  sufficiently  rigid  to  prevent  the  buckling  of  the  face  under  fairly 
heavy  stress.  The  faces  or  sides  are  secured  to  the  frame  near  the 
edges  and  frequently  across  the  middle  especially  when  a  large 
number  of  sockets  weakens  the  face. 

The  varieties  of  ornamentation  are  innumerable.  The  most 
common  include  the  raised  mouldings  and  the  use  of  gold  leaf. 

On  these  flat  sign  bodies,  the  illuminated  letters  (see  Fig.  7)  are 
flush,  raised,  skeleton  letter,  trough  or  sunken,  but  in  each  case  the 
sockets  are  inserted  in  the  letter  stroke  to  make  the  reading  matter 
stand  out.  The  use  of  color  caps  and  hoods  is  common  as  with 
roof  signs. 

Porcelain  enameled  steel  has  been  found  very  satisfactory  as  a 
sign  material.  The  surface  is  an  excellent  reflector  and  can  be  very 
easily  cleaned.  The  letter  stroke  is  often  made  of  enamel  even 
when  the  body  of  the  sign  or  letter  is  painted  steel. 

In  one  special  form  of  construction  (Fig.  5)  each  letter  is  made  of 
an  embossed  porcelain  enameled  steel  plate.  The  form  of  the  plate 
is  such  as  to  give  strength  to  the  sign  and  allow  the  use  of  a  neat 
narrow  steel  frame  not  over  2  in.  in  width. 

In  another  familiar  form  of  street  sign  (Fig.  8)  a  border  of  lamps 


540 


ILLUMINATING   ENGINEERING   PRACTICE 


is  provided  around  the  reading  matter  or  design.  Where  the  lamps 
are  raised  perceptibly  above  the  panel  surface  by  the  use  of  a 
special  frame  or  otherwise  the  illumination  of  the  panel  is  usually 
quite  satisfactory,  but  where  the  socket  is  inserted  flush  with  the 
sign  face  very  poor  results  are  obtained.  All  the  slight  imperfec- 
tions in  the  surface  are  brought  out.  Again  the  angle  of  incidence 
is  so  large  that  very  little  light  is  reflected  toward  the  observer. 


Type  A 


Q 
O 
Q 
GTc 


TypeB 


TypeC 


Type  D  Type  E 

Fig.  7. — Type  A,  flush.     Type  B,  raised.     Type  C,  skeleton.     Type  D,  trough.     Type  E, 

sunken  grooved. 

Many  forms  of  transparencies  are  used  for  street  signs  (Fig.  9). 
As  a  class  they  are  of  little  value  from  the  standpoint  of  street 
illumination  but  they  are  often  very  pleasing  in  appearance  and 
have  their  field.  In  this  class  are  the  lens  sign  (Fig.  13)  the  per- 
forated or  cutout  letter,  the  canteen  and  the  ornamental  glass  types. 

As  a  class  almost  by  themselves  are  the  changeable  letter  signs 
(Fig.  10).  Many  of  these,  used  principally  for  theatres,  are  so  con- 
structed that  the  individual  letters  may  be  easily  removed  and  re- 
placed for  a  change  of  reading.  Even  in  the  transparencies  there 
are  changeable  letter  types.  For  general  advertising,  there  are 


FURNISHINGS 


- 


Fig.  8. — Panel  sign. 


Fig.  9. — Transparency. 


Fig.  10. — Changeable  letter  sign. 


(Facing  page  540.) 


Fig.  ii. — Motograph. 


Fig.   12. — Miniature  lamp  letter. 


SHEPARD:  SIGN  LIGHTING 


54i 


others  which  change  automatically  from  one  reading  to  another 
until  quite  a  story  may  be  told. 

A  late  form  of  changeable  letter  sign  which  combines  the  change- 
able feature  with  a  fascinating  moving  effect  is  the  motograph 
(Fig.  n).  In  this  sign,  the  letters  appear  on  the  right  and  move 
evenly  and  rapidly  across  the  face  as  though  attached  to  a  belt.  To 
produce  this  effect  a  large  number  of  lamps  arranged  in  horizontal 
and  vertical  rows  in  a  lamp  bank  on  the  sign  face  are  connected 
individually  with  contact  brushes  arranged  in  the  same  order  but 
very  close  together  in  a  brush  board  in  the  control  machine.  A 
perforated  ribbon  something  like  the  roll  in  a  piano  player  is  drawn 
continuously  across  the  brush  board.  Any  figure  such  as  a  letter 
perforated  in  the  ribbon  will  appear  on  the  lamp  bank.  The  brushes 
make  contact  through  the  perforations  with  a  fixed  metal  plate 


Type  A 

Fig.   13. — Type  A,  lens  sign. 


TypeB 

Type  B,  cut  out  glass  letter  sign. 


closing  the  electrical  circuit  and  lighting  up  the  proper  lamps  to 
form  the  letters. 

A  careful  observer  of  this  sign  will  note  that  the  vertical  strokes  of 
the  letters  do  not  appear  as  bright  as  the  horizontal  strokes.  The 
contacts  controlling  the  lamps  are  made  and  broken  so  quickly  that 
the  lamp  filaments  do  not  reach  their  full  brilliancy.  If  the  observer 
is  quite  near  the  sign  he  will  note  a  slight  blur  or  streak,  an  effect  like 
the  tail  of  a  comet  following  each  letter  across  the  sign,  because 
filaments  do  not  lose  brilliancy  fast  enough.  If  there  could  be  found 
a  lamp  in  which  the  filament  came  to  its  full  brilliancy  and  cooled  off 
more  rapidly  than  in  the  present  standard  tungsten  lamp  the  speed 
of  the  sign  could  be  materially  increased. 

Indoor  Signs  including  the  more  common  types  of  window  signs 
are  usually  transparencies  but  one  or  two  very  attractive  special 
types  have  been  developed  such  as  the  miniature  lamp  letter 
sign,  Fig.  12.  Each  letter  is  in  itself  a  lamp.  It  is  formed  by 
bending  a  glass  tube  into  the  form  of  the  letter  and  then  inserting 


542  ILLUMINATING   ENGINEERING   PRACTICE 

small  filaments  at  frequent  intervals.  These  filaments  are  con- 
nected in  series  thereby  enabling  each  lamp  letter  to  be  connected 
across  the  ordinary  service  wires. 

Flood  lighting  of  signs  is  a  new  development  of  the  art.  Under 
certain  conditions  the  effects  obtained  are  very  satisfactory. 

The  flood  of  light  illuminates  everything  in  its  field,  the  iron 
framework  as  well  as  the  letters.  For  this  reason  flood  lighting 
would  seem  to  be  better  adapted  to  the  illumination  of  large  solid 
areas  such  as  wall  signs,  buildings  or  bulletin  boards  than  skeleton 
types  of  signs. 

The  flood  lighting  reflectors  in  sign  work,  therefore,  supplant 
the  bulletin  board  reflectors  rather  than  the  lamp  letter  signs. 

ENGINEERING  FEATURES 

Except  in  the  case  of  transparencies  and  possibly  bulletin  board 
lighting  the  development  of  the  electric  sign  has  depended  upon  the 
development  of  the  sign  lamp. 

When  the  standard  lamp  available  was  the  16  c.p.  carbon  filament 
unit  consuming  as  much  energy  as  the  5o-watt  lamps  of  to-day  and 
the  cost  of  energy  was  many  times  as  high  as  now,  the  electric  sign 
of  large  size  was  out  of  the  question.  Again,  the  large  size  of  the 
available  lamp  bulbs  made  a  neat  well-proportioned  sign  impossible 
in  the  smaller  sizes.  The  development  of  the  8-c.p.  and  then  the 
4-c.p.  carbon  lamps  made  in  the  small  bulb  for  sign  use  gave  some 
encouragement  but  even  then  the  panel  or  border  lamp  signs  with 
their  comparatively  small  number  of  lamps  were  the  popular  types 
on  account  of  the  expense. 

All  of  the  early  lamps  were  used  at  the  regular  service  voltage  and 
consumed  considerable  energy.  Reducing  the  diameter  of  the 
filament  to  reduce  the  current  consumption  made  the  lamp  too  frail. 
It  was  then  found  that  by  cutting  the  4-c.p.,no-volt  filament  in  half, 
two  lamps  of  2  c.p.  each  could  be  made  to  be  used  in  series.  The 
appearance  of  this  new  lamp  about  1903  made  it  possible  to  form 
properly  letters  by  using  enough  lamps  and  without  too  much  expense 
although  the  signs  were  not  equal  to  the  brilliant  present-day 
standard. 

In  experimenting  with  the  55-volt  lamps  in  signs  previously  made 
for  no  volts  it  was  found  necessary  to  rewire  the  sockets  two  in 
series — an  expensive  process.  The  adoption  of  a  new  plan  in  sign 
wiring  called  the  series-multiple  system  solved  this  difficulty.  Two 


SHEPARD:  SIGN  LIGHTING  543 

equal  banks  of  lamps  as  in  the  two  sides  of  a  double-faced  sign  were 
connected  in  series. 

The  arrangement  called  series-multiple  wiring  meaning  a  series  of 
multiples  is  apt  to  prove  perplexing  to  the  wireman  especially  when 
there  are  several  circuits  on  the  same  sign  but  with  all  its  disad- 
vantages the  system  has  been  thoroughly  worth  while  for  those 
who  understood  it  as  its  use  means  a  large  saving  in  energy  cost. 

The  2-c.p.  55-volt  carbon  lamps  consumed  from  10  to  12  watts 
each.  When  the  tungsten  lamps  with  a  large  reduction  in  con- 
sumption were  produced  the  sign  industry  naturally  was  eager  to 
take  advantage  of  the  saving. 

To  put  out  a  lamp  with  a  filament  sufficiently  fine  to  hold  the  con- 
sumption below  10  watts  on  115  volts  was  apparently  impracticable 
especially  with  the  delicate  tungsten  filaments;  the  5-watt  lo-volt 
sign  lamp  which  was  announced  in  Jan.,  1909,  was  the  result.  The 
wiring  for  this  lamp  was  arranged  on  the  multiple  system  with  a 
transformer  when  alternating  current  was  used,  although  due  to  the 
half  ampere  current  consumption  per  lamp  the  heavy  currents  at  low 
voltage  meant  considerable  loss  in  voltage  drop  and,  therefore,  in  lamp 
brilliancy.  With  direct-current  special  wiring  was  imperative  and 
naturally  the  series-multiple  system  employed  with  the  57-volt 
carbon  lamps  was  arranged  to  include  10  or  n  groups  in  series. 
Where  the  sign  was  small  each  bank  had  only  a  few  lamps,  thus 
causing  too  great  a  stress  on  the  remaining  lamps  when  one  or  more 
burned  out.  Hence  for  small  signs  the  series  wiring  was  used  with 
10  or  ii  lamps  in  each  series.  While  the  series  system  is  the  most 
economical  from  the  standpoint  of  lamp  renewal  cost,  it  is  very  un- 
satisfactory in  large  signs  because  such  a  large  portion  of  the  sign  is 
placed  in  darkness  whenever  any  one  lamp  fails.  In  large  signs 
where  it  is  to  be  expected  that  one  or  more  lamps  will  burn  out  fre- 
quently the  sign  would  be  constantly  spoiled  in  appearance  if  the 
series  wiring  system  were  used,  and  hence  the  series-multiple  system 
is  employed  where  the  number  of  lamps  per  circuit  runs  over  100. 

With  the  series-multiple  system  using  5-watt  lo-volt  lamps  it  is 
possible  without  putting  over  15  amperes  on  a  circuit  to  include 
330  lamps  on  a  single  circuit  and  thereby  reduce  the  amount  of 
wiring. 

The  appearance  of  the  lo-watt  no- volt,  and  the  5-watt  55-volt 
sign  lamps  in  June,  1912,  and  of  the  7. 5-watt  no-volt  lamp  in  Sept., 
1915,  made  it  possible  to  eliminate  the  more  complex  series-multiple 
wiring  without  increasing  the  current  consumption.  At  this  date, 


544  ILLUMINATING   ENGINEERING   PRACTICE 

however,  the  5-watt  lo-volt  lamp  is  still  very  widely  used  on  account 
of  its  ruggedness. 

An  important  problem  related  to  sign  design  is  that  of  legibility. 
The  size,  shape,  spacing,  and  brilliancy  affect  the  readability  to  a 
marked  degree.  Many  signs  that  are  otherwise  well  made  are  ab- 
solutely unreadible  even  from  a  moderate  distance.  This  feature 
must  receive  attention. 

THE  ELECTRIC  SIGN  AS  A  LIGHT  SOURCE 

The  illumination  produced  in  its  vicinity  by  the  average,  fair-sized 
electric  sign  should  not  be  overlooked  as  it  is  a  point  in  its  favor.  In 
exterior  illumination  the  electric  sign  as  a  light  source  gives  ideal 
results.  The  lighting  of  a  street  with  large  numbers  of  small  candle- 
power  lamps  would  be  out  of  the  question  because  of  the  high  cost 
of  such  a  method.  However,  when  merchants  along  any  good 
business  street  make  use  of  a  quantity  of  properly  designed  signs, 
that  street  needs  no  other  illumination,  and  is  the  best  and  the  most 
satisfactorily  lighted  street  in  the  city.  The  signs  need  not  neces- 
sarily project  even  over  the  sidewalk.  The  sidewalks  can  be  clear 
of  lamp  posts,  while  the  illumination  is  pleasing  and  adequate. 

It  frequently  happens  in  a  poorly  lighted  district  that  the  electric 
sign  on  the  corner  drug  store  stands  out  as  the  only  bright  spot.' 
Its  cheering  influence  is  unconsciously  appreciated  by  all. 

THE  ELECTRIC  SIGN  INDUSTRY 

It  is  estimated  that  there  are  approximately  15,000  electric  signs 
in  New  York  City  and  10,000  in  Chicago.  In  many  small  cities 
there  are  more  signs  per  capita  than  in  either  of  these. 

If  we  consider  only  the  cities  and  towns  in  the  United  States  with 
a  population  of  over  5000  and  assume  that  the  number  of  signs  per 
capita  is  80  per  cent,  of  the  New  York  average  there  would  be  a  total 
of  240,000  signs  in  this  country. 

In  an  ordinary  installation,  the  principal  items  of  cost  are  the 
sign  proper  or  the  sign  body;  the  hanging  and  wiring;  the  lamps, 
color  caps  and  flasher  and  the  permits. 

The  average  total  cost  might  be  taken  as  $65.  The  capital  in- 
vested in  electric  signs  would  then  be  about  $15,600,000. 

It  is  probable  that  this  is  considerably  underestimated  as  many 
roof  signs  cost  between  $5000  and  $10,000  each.  The  lamp  invest- 
ment alone  is  about  $4,000,000. 

The  average  power  consumption  per  sign  in  Chicago  is  760  watts. 


$357>°°o  in  electrical  supplies. 


SHEPARD:  SIGN  LIGHTING  545 

The  rated  sign  load  for  the  country  on  this  basis  would  be  182,000 
kw.  and  at  5  cents  per  kw.-hour  with  a  four-hour  service  period  per 
day  the  energy  used  would  bring  a  return  of  $i  i  ,000,000  per  year  to 
the  central  stations. 

It  is  estimated  that  the  sign  business  has  not  been  developed  to 
20  per  cent,  of  what  it  should  be.  If  properly  developed  the  esti- 
mates given  would  be  multiplied  by  5. 

To  realize  what  the  electric  sign  business  means  to  other  industries 
consider  one  class  of  signs,  the  roof  signs,  and  note  the  material  and 
labor  involved. 

One  thousand  roof  signs  averaging  40  ft.  high  by  50  ft.  long  would 
require : 

7500  tons  of  steel  @  $200  =  $1,500,000  installed. 

1,000,000  sign  lamps.  .  .$160,000 

3,000,000  ft.  of  wire 40,000 

1,000,000  sockets 50,000 

10,000  Ibs.  of  tape 2,000 

500,000  ft.  of  conduit 355ooo 

10,000  gal.  of  paint 70,000 

besides  30,000  flashers,  20,000  time  switches,  5000  meters,  2000 
transformers,  many  cutouts,  fuse  plugs,  color  caps  and  probably 
about  600,000  board  ft.  of  platform  lumber. 

The  labor  cost  for  designers;  sheet  metal  workers,  painters, 
electricians,  structural  iron  workers,  teamsters  and  roofers  costs 
about  $675,000.  The  railroads  get  about  2,200,000  ton-miles  of 
freight.  One  sign  manufacturer  alone  pays  $36,000  a  year  for 
freight. 

The  figures  above  apply  to  only  a  portion  of  one  class  of  signs. 
Another  class,  the  porcelain  enamel  steel  signs,  would  keep  an 
immense  enameling  factory  busy,  while  the  many  varieties  of  glass 
signs  mean  a  large  glass  business.  One  sign  company  alone  employs 
upward  of  fifty  traveling  men  continually  throughout  the  year. 
Much  more  data  could  be  presented  to  show  the  extent  to  which 
the  application  of  artificial  light  to  the  illuminated  sign  has  been 
carried.  Sufficient  has  probably  been  given  to  indicate  in  a  general 
way  the  extent  to  which  the  art  has  been  developed. 

'  ORDINANCES 

Having  in  mind  the  size  of  the  electric  sign  business  one  realizes 
that  those  who  advocate  abolishing  signs  altogether  are  hardly  apt 
to  succeed,  at  least  without  a  long  fight  and  a  hard  one. 

35 


546  ILLUMINATING   ENGINEERING   PRACTICE 

For  the  good  of  the  industry  itself,  however,  as  well  as  to  meet 
the  radicals  part  way,  certain  consideration  should  be  given  to 
the  size,  type  and  location  of  signs.  Proper  ordinances  should  be 
enacted  and  enforced  to  prevent  the  installation  of  a  huge  ungainly 
and  possibly  dangerous  street  sign.  Municipalities  are  considering 
this  subject  with  more  and  more  care.  Probably  the  majority  of 
towns  and  cities  of  over  5000  population  now  have  sign  ordinances. 
Unfortunately  many  of  the  ordinances  seem  to  have  been  introduced 
with  the  object  of  discouraging  the  industry  rather  than  of  controlling 
it.  It  is  important  to  use  a  proper  hanging  rig  with  a  street  sign 
including  sufficiently  heavy  chains  and  cables  and  adequate  wall 
attachments.  A  wise  manufacturer  will  make  these  parts  unques- 
tionably strong,  and  a  wide-awake  city  inspector  will  see  that  they 
are  properly  installed. 

THE  EFFECT  OF  THE  ILLUMINATED  SIGN  ON  A  COMMUNITY 

From  what  has  been  said  in  reference  to  street  illumination 
alone,  a  strong  argument  can  be  made  for  the  illuminated  sign. 
This  point  is  relatively  unimportant,  however,  as  the  big  value  of 
illuminated  signs  to  any  community  lies  in  their  wonderful  bright- 
ening, boosting  and  cheering  influence.  Compare  any  two  towns 
with  and  without  these  signs.  The  one  is  alive  while  the  other  is 
dead.  The  merchant,  the  real  estate  man,  the  politician,  everyone 
realizes  what  it  means  to  a  town  to  have  the  reputation  of  being  alive. 
Certainly  it  means  enough  to  warrant  a  place  in  any  list  of  industries 
of  the  country. 


1 

o 

I 

tj 

.  £  o 

<gw    cy 


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s.  ^  ^ 

The  celebr  o,  ^  O 

-   additional  light         §f  c   3 

Prior  •  ^  s   O 


procedur  JJ* 

a. 


8*8 


BUILDING  EXTERIOR,  EXPOSITION  AND  PAGEANT 

LIGHTING 

BY  W.  D'A.  RYAN 

Since  the  days  of  the  cave  man,  fire  or  light  has  been  a  factor  in  all 
festivities.  The  victories  of  the  barbarians  were  usually  celebrated 
at  night  around  the  blazing  camp  fires.  The  war  dance  of  the 
American  Indian  would  not  have  been  complete  without  fire. 

Decorative  lighting  for  commercial  purposes  originated  with  the 
Chinese.  They  employed  paper  lanterns  in  festooning,  and  gas 
jets  in  accentuating  the  architectural  lines  of  their  shops  and  build- 
ings. The  celebrations  of  all  ages,  when  carried  into  the  night,  have 
called  for  additional  lighting,  both  for  utilitarian  and  spectacular 
purposes.  Prior  to  the  advent  of  electricity  such  lighting  was  ac- 
complished mainly  by  gas  jets  and  fireworks. 

Systems  of  illumination  falling  within  the  classification  forming  the 
title  of  this  lecture  differ  from  utilitarian  lighting  systems  in  that  ef- 
ficiency of  light  production  assumes  a  secondary  role.  The  import- 
ant point  to  keep  in  mind  in  planning  lighting  schemes  of  this  char- 
acter is  the  necessity  for  conforming  to  the  architectural  ideas  or  at 
least  to  accomplish  the  lighting  results  without  conflicting  with  the 
architecture.  To  do  this  successfully  requires  cooperation  between 
the  architect  or  decorator  and  the  illuminating  engineer  to  a  much 
greater  degree  than  is  common  practice  to-day.  The  lighting 
features  should  receive  study  from  the  time  of  the  conception  of  the 
architectural  details  and  be  developed  along  with  the  structural  plans. 
Of  course  many  creditable  lighting  installations  have  been  made  in 
buildings  which  were  completed  before  the  lighting  was  considered, 
but  such  procedure  is  a  mistake  when  it  is  possible  to  develop  the 
illumination  plans  in  parallel  with  the  architectural  plans. 

All  architecture  is  created  primarily  for  observation  in  daylight; 
therefore,  if  we  are  to  retain  the  architectural  details  of  a  structure 
when  viewed  by  artificial  light  we  must  approximate  daylight 
conditions.  Daylight  consists  of  a  strong  directional  light  from  the 
sun  supplemented  by  diffused  light  from  the  clouds;  the  former  creates 
sharp  shadows,  and  the  latter  relieves  them  to  a  greater  or  less  extent. 

547 


548  ILLUMINATING    ENGINEERING   PRACTICE 

A  total  absence  of  sunlight  (unidirectional  light)  would  create  a 
shapeless  monotonous  mass  because  there  would  be  no  shadows, 
while  a  total  absence  of  reflected  light  (diffused  light)  would  create 
extreme  high  lights  and  harsh  shadows.  A  combination  of  these 
two  kinds  of  light  is,  therefore,  necessary  to  give  proper  perspective 
to  architectural  forms,  and  if  we  are  to  be  successful  in  displaying 
the  work  of  artists  and  architects  by  artificial  light  we  must  approxi- 
mate the  distribution  found  in  nature,  that  is,  a  strong  directional 
light  coming  from  above  if  possible  and  supplemented  by  relief 
lighting. 

For  a  good  many  years  the  only  method  employed  in  lighting 
a  building  for  emphasis  was  outline  lighting,  that  is,  locating  rows 
of  incandescent  lamps  around  the  cornices,  windows,  etc.  This 
method  is  still  used  for  amusement  parks,  motion  picture  theatres, 
etc.,  but  is  too  closely  akin  to  the  bizarre  for  our  modern  monumen- 
tal and  commercial  buildings.  In  such  a  scheme  of  lighting  the 
building  merely  serves  as  a  background  upon  which  to  display  lamps 
and  when  the  lamps  are  lighted  only  a  skeleton  of  light  appears  and 
the  building  is  obscured  by  the  glare.  Its  other  disadvantages  are 
the  diminution  of  artistic  effectiveness  at  close  range,  similarity  of 
effects  from  different  view  points,  the  suppression  or  complete  ob- 
literation of  architectural  features,  the  economic  necessity  of  exten- 
sive untreated  surfaces  and  severe  eye  and  nerve  strain  resulting 
from  the  glare. 

Very  little  can  be  stated  in  the  way  of  concrete  rules  covering 
spectacular  lighting.  Such  lighting  depends  for  its  success  on  an 
appeal  to  the  senses  in  which  respect  it  is  similar  to  paintings  and 
music.  Light,  color  and  motion  are  essential  features  in  any  spec- 
tacular display  and  by  arranging  them  in  interesting  combinations 
many  varied  and  beautiful  effects  may  be  obtained.  Steam,  vapor 
and  water  afford  excellent  reflecting  media  upon  which  to  play  light. 
Color  always  adds  interest  and  is  readily  obtained  by  passing 
white  light  through  light-absorbing  gelatine  sheets  or  colored  glass. 
The  illuminating  engineer  must  use  his  own  ingenuity  in  devising 
spectacular  lighting  features. 

EXPOSITION  LIGHTING 

In  our  study  of  the  lighting  treatment  of  expositions,  it  is  well 
to  review  briefly  the  methods  and  results  of  former  expositions. 
From  such  a  study  we  learn  that  after  the  introduction  of  electricity 


would  create  a 
be  no  shadows, 
it)  would  create 
.nation  of  these 


:ul  in  disp; 
VG  must  aj 
:rong  direc 
icnted  bv 


w 


;net.hod  employed  in  lighting 

locating  rows 

.tc.     This 

e  theatres, 

;ern  monumen- 

dvantag 
ange,  simila 


jtrain  re 


a  ts  success  c>; 


O 
»  cu 


;.. 


tich  to  pi; 

ined  by 


ngenuity 


of  expositions,  it 

ler  expo 

titroduction  of  ele 


RYAN:  BUILDING  EXTERIOR  549 

for  illumination  purposes  at  the  Louisville,  Ky.,  Exposition  in 
1883,  outline  lighting  has  characterized  all  expositions  prior  to  the 
Panama-Pacific  International  Exposition  in  1915.  The  exposition 
at  Louisville  was  notable  for  two  reasons,  first  because  a  method  of 
bringing  the  lighting  up  from  zero  to  full  candle-power  was  in- 
troduced, and  secondly  because  it  was  the  last  exposition  in  this 
country  where  gas  was  utilized  in  the  plans  for  the  main  portion  of 
the  lighting. 

In  the  Court  of  Honor  at  the  Columbian  Exposition,  Chicago, 
1893,  outline  lighting  with  incandescent  lamps  was  used  extensively 
while  arc  lamps  and  ornamental  posts  were  employed  for  the  lighting 
of  the  grounds.  An  electric  fountain  and  the  Edison  Tower  forming 
part  of  the  exhibit  of  the  General  Electric  Company  and  made  up  of 
10,000  i6-c.p.  carbon-filament  lamps  were  the  spectacular  features. 

The  first  complete  method  of  decorative  and  grounds  lighting 
by  incandescent  lamps  was  at  the  Mississippi  and  International 
Exposition,  held  at  Omaha,  in  1898.  The  illumination  received  the 
highest  award  at  this  exposition. 

At  the  Fourth  Universal  Exposition  held  at  Paris  in  1900  was 
found  a  mixture  of  all  types  of  illuminants;  incandescent  lamps 
predominating  with  a  total  of  76,720  used  for  decorative  purposes. 
The  Electricity  Building  and  the  Eiffel  Tower  were  the  principal 
features.  The  crest  of  the  Electricity  Building  was  covered  by  the 
" Great  Star"  placed  behind  the  figure  of  Marqueste,  the  genius  of 
electricity.  The  star  had  68  points  made  up  of  gilded  tubing  and 
was  lighted  from  the  rear  by  six  6o-amp.  projectors.  Electric  foun- 
tains illuminated  by  arc  and  incandescent  lamps  equipped  with 
color  screens  were  used.  Other  lighting  features  were  the  Palace  of 
Optics  with  14,000  i6-c.p.  and  32-c.p.  lamps,  the  Palace  of  Light  with 
12,000  lamps,  Monumental  Gate  with  1385  incandescent  lamps  in 
colored  globes  and  the  Trocadero  Palace  festooned  with  thousands 
of  gas  jets. 

At  the  Pan  American  Exposition,  held  at  Buffalo  in  1901,  in- 
candescent outline  lighting  probably  reached  its  maximum  effect- 
iveness. Incandescent  lamps  of  low  candle-power  were  used  ex- 
clusively and  the  number  of  lamps  was  gradually  increased  so  as  to 
form  a  climax  at  the  Tower. 

At  the  Louisiana  Purchase  Exposition  of  St.  Louis,  1904,  because 
of  its  size,  the  ideas  carried  out  at  Buffalo  could  not  be  adopted, 
so  it  was  necessary  to  spread  out  the  lighting  to  a  certain  extent 
and  accentuate  certain  prominent  features.  All  of  the  main  build- 


550  ILLUMINATING   ENGINEERING   PRACTICE 

ings  were  lavishly  festooned  with  incandescent  lamps.  Each  lighting 
unit  along  the  colonnade  was  provided  with  three  lamps,  white, 
emerald  and  amethyst,  each  color  on  a  separate  circuit.  Water 
rheostats  were  used  for  dimming  each  color  and  to  bring  the  general 
illumination  gradually  up  to  full  candle-power.  Arc  lamps  were 
employed  throughout  the  grounds  and  Nernst  lamps  were  used  in 
the  Exposition  buildings.  The  monumental  feature  of  the  Exposition 
was  the  Cascades,  illuminated  by  incandescent  lamps. 

At  the  Lewis  and  Clark  Exposition  at  Portland,  Oregon,  in  1905 
incandescent  lamps  were  used  exclusively.  One  of  the  features  of  the 
Exposition  was  the  illumination  of  the  lake.  Around  the  lake 
5o-c.p.  lamps  were  placed  in  marine  receptacles  on  the  bottom  of  the 
lake  at  intervals  of  three  feet. 

At  Jamestown  in  1907  the  decorative  lighting  of  the  buildings 
was  done  with  8-c.p.  lamps  placed  on  approximately  i2-in.  centers. 
A  water  rheostat  of  25oo-kw.  capacity  was  used  to  " build  up" 
the  lighting.  Four  minutes  were  allowed  for  the  lamps  to  reach 
their  normal  candle-power.  One  of  the  illuminating  features  was  the 
Administration  Building  with  the  projector  beams  forming  a  fan 
behind  the  dome.  Another  pleasing  feature  was  Flirtation  Walk 
where  flashing  lamps  were  placed  in  trees  and  shrubbery  to  give  a 
firefly  effect. 

One  of  the  best  examples  of  outline  lighting  as  it  should  be  em- 
ployed for  exposition  buildings  was  shown  at  the  Alaska- Yukon 
Pacific  Exposition  in  Seattle  in  1909.  A  total  of  250,000  lamps  was 
used  there  for  all  purposes.  For  decorative  purposes  8-c.p.  all 
frosted  carbon  lamps  were  employed.  The  grounds  were  lighted 
by  200  special  electroliers,  each  containing  39  2o-c.p.  lamps. 

Gas  was  found  admirably  suited  to  outline  the  buildings  at  the 
coronation  in  London,  1910.  The  effect  produced,  because  of  the 
continual  flicker  of  the  light,  was  interesting  and  lively. 

The  illumination  of  the  Panama-Pacific  international  Exposition 
represented  the  latest  developments  in  exposition  lighting  and 
marked  an  epoch  in  the  science  of  lighting  and  the  art  of  illumination. 
Previous  expositions  had  depended  upon  outline  lighting  for  night 
effects  and  this  method  had  probably  reached  the  limit  of  its  possi- 
bilities at  the  Pan-American  Exposition  at  Buffalo.  The  outline 
method,  therefore,  was  set  aside  and  a  system  of  screened  or  masked 
flood  and  relief  lighting  was  employed. 

During  the  period  elapsing  between  the  Louisiana  Exposition 
and  the  Panama-Pacific  International  Exposition  wonderful  advances 


O 

g* 

W    <u 


<3    rt  ^ 

8s* 


O         o 

O 

3  ' 
I 


ps.     Each  lighting 
;h  three  lamps,  white. 
a  a  separate  circuit,     \\ 

rheost  >r  and  to  bring  the  general 

all  candle-power.     Arc  lamps  were 

emplo\  ind  Nernst  lamps  were  used  in 

theExi  lonumental  feature  of  the  Exposition 

'incandescent  lamps. 

Portland,  Oregon,  in  1905 

^      .     One  of  the  features  of  the 

2-   •  •  e  lake.     Around  the  lake 

»n  the  bottom  of  the 

I* 

o 

.o  cv  -ting  of  the  buildings 

*   j§  in.  centers. 

jjf  •&  g  build  up" 

uT^  J  i»e  lamps  to  reach 

2  ».  U     imi  nating  features  was  the 

«  2.  cj ••,.••    beams  forming  a  fan 

•  •  •  .  -s} •  .§  cu : : . .     e       -  Flirtation  Walk 

§»'§*§  urubbery  to  give  a 

o  «    O 

:  ^Sighting  as  it  should  be  effi- 
gy snown  at  the  Alaska-Yukon 
a       total  of  250,000  lamps  was 
•>es.     Forj-^lecorative  purposes  8-c.p.  all 
SC  >unds  were  ]i 

*f  />  2o-c.p.  lamps. 

N"  hidings  ; 

^  :-d,  because  • 

and  lively, 
international  E  . 
exposition   lighting  and 
and  the  art  of  illumin 

iline  lighting  for  night 
Cached  the  limit  of  its 
!i  at  Buffalo.  ..The  outline 
'em  of  screened  or  me 

Dur  the  Louisiana  Exposition 

and  tl  ' on  wonderful  advances 


t 

sumr 


0 


RYAN:  BUILDING  EXTERIOR  551 

had  been  made  in  the  efficiencies  of  all  types  of  lighting  units. 
Thus  it  was  possible  to  illuminate,  in  the  main  groups  of  buildings 
and  grounds,  approximately  8,000,000  sq.  ft.  of  horizontal  and  ver- 
tical surfaces  with  intensities  ranging  from  o.i  to  0.25  foot-candle 
in  the  incidental  gardens  and  roadways,  from  0.25  to  3  foot-candles 
on  the  building  facades  and  adjacent  lawns  and  gardens,  and  from 
5  to  15  foot-candles  on  the  towers,  flags  and  sculptural  groups.  The 
lighting  load  on  the  main  group  of  buildings,  including  the  window 
lighting  and  the  scintillator,  was  approximately  5000  kw.  The 
total  connected  load  for  all  purposes,  including  the  "Zone"  foreign 
and  state  sections  and  exhibitors,  for  lighting,  incidental  heating, 
motor,  and  other  service  was  13,954  kw.  with  a  maximum  peak  of 
8200  kw.  and  an  average  peak  of  7880  kw. 

While  the  lighting  of  the  Exposition  was  primarily  electric,  all 
modern  light  sources  of  intrinsic  merit  were  utilized,  and  a  number 
of  excellent  gas  features  were  introduced.  About  four  miles  of 
streets  in  the  foreign  and  state  sections  were  illuminated  with  high 
pressure  "gas  arc  lamps"  equipped  with  2o-in.  opal  globes  mounted 
with  their  centers  about  16  ft.  above  the  roadways  on  ornamental 
poles  spaced  approximately  100  ft.  apart,  staggered.  The  same 
type  of  lamp  was  used  for  emergency  lighting  on  the  kiosks  through- 
out the  grounds.  Five-mantle  "enclosed  gas  arc  lamps,"  in  the 
"Zone"  section  and  the  same  type  of  lamp,  in  smaller  sizes,  fur- 
nished emergency  lighting  at  the  gates  and  important  exits  from  the 
main  group  of  buildings.  Gas  flambeaux  were  introduced  in  the 
effects  in  the  "Court  of  Abundance"  and  the  "North  Approach." 
The  total  gas  flow  for  the  purposes  mentioned  was  approximately 
15,000  cu.  ft.  per  hour. 

Furnishing  wonderful  contrast  to  the  soft  illumination  of  the  pal- 
aces, was  the  "Zone"  or  amusement  section,  with  all  the  glare  of  the 
bizarre,  giving  the  visitor  an  opportunity  to  contrast  the  illumination 
of  the  future  with  the  light  of  the  past.  As  we  passed  from  the 
"Zone"  with  its  blaze  of  light,  we  entered  a  pleasing  field  of  entice- 
ment. We  were  first  impressed  with  the  beautiful  colors  of  the  her- 
aldic shields,  on  which  were  written  the  early  history  of  the  Pacific 
Ocean  and  California.  Behind  these  banners  were  luminous  arc 
lamps  in  clusters  of  two,  three,  five,  seven  and  nine,  ranging  in  height 
from  25  to  55  ft.  We  looked  from  the  semi-shadow  upon  beautiful 
vistas  and  the  Guerin  colors,  which  fascinating  in  daytime,  were  even 
more  entrancing  by  night.  The  lawns  and  shrubbery  surrounding 
the  buildings  and  trees  with  their  wonderful  shadows  appeared  in 


552  ILLUMINATING   ENGINEERING   PRACTICE 

magnificent  relief  against  the  soft  background  of  the  palaces.  The 
"Tower  of  Jewels"  with  its  102,000  "Novagems,"  which  suggested 
the  official  title  "Jewel  City"  standing  mysteriously  against  the 
starry  blue-black  of  the  night,  might  be  said  to  have  surpassed  the 
dreams  of  Aladdin. 

The  Courts  of  Flowers  and  Palms  each  received  treatment  in  keep- 
ing with  its  oriental  architecture.  The  towers  were  flood  lighted  by 
arc  standards  and  projectors  and  the  shadows  thus  created  were 
illuminated  with  colored  light  at  the  various  levels. 

In  the  Court  of  Flowers  the  incandescent  standard  and  lantern 
served  to  give  a  subdued  illumination  throughout  the  court.  The 
balustrade  standard  in  the  Court  of  Palms  was  equipped  with  a  20-in. 
glass  sphere  and  the  Colonnade  was  lighted  by  large  staff  hanging 
basket  fixtures. 

As  we  passed  through  the  approach  to  the  "Court  of  Abundance" 
from  the  east,  with  its  masked  shell  standards  strongly  illuminating 
the  cornice  lines  and  gradually  fading  to  twilight  in  the  foreground, 
and  entered  the  Court,  we  were  impressed  with  the  feeling  of  mys- 
tery analogous  to  the  prime  conception  of  the  architect's  wonderful 
creation.  Soft  radiant  energy  was  everywhere;  lights  and  shadows 
abounded,  fire  hissed  from  the  mouths  of  serpents  into  the  flaming 
gas  caldrons  and  sent  its  flickering  rays  over  the  composite  Spanish- 
Gothic-Oriental  grandeur.  Mysterious  vapors  rose  from  steam- 
electric  caldrons  and  also  from  the  beautiful  fountain  group  sym- 
bolizing the  earth  in  formation.  The  cloister  lanterns  and  snow- 
crystal  standards  gave  a  warm  amber  glow  to  the  whole  Court,  and 
the  organ  tower  was  illuminated  in  the  same  tone  by  colored  projec- 
tor rays. 

Passing  through  the  "Venetian  Court"  we  entered  the  "Court  of 
the  Universe,"  where  the  illumination  reached  a  climax  in  dignity 
thoroughly  in  keeping  with  the  grandeur  of  the  Court.  Here  an  area 
of  nearly  half  a  million  square  feet  of  horizontal  and  vertical  surface 
was  illuminated  by  two  fountains  rising  95  ft.  above  the  level  of  the 
sunken  gardens,  one  symbolizing  the  rising  sun  and  the  other  the 
setting  sun. 

The  shaft  and  ball  surrounding  each  fountain  was  glazed  in  heavy 
opal  glass,  which  was  coated  on  the  outside  in  imitation  of  travertine 
marble,  so  that  by  day  they  did  not  in  any  sense  suggest  the  idea  of 
light  sources.  High  efficiency  incandescent  electric  lamps  installed 
in  these  two  columns  gave  a  combined  initial  (bare  lamp)  candle- 
power  of  approximately  500,000  and  yet  the  intrinsic  brilliancy  was 


COURT  OF  ABUNDANCE 

Showing  the  Organ  Tower  and  the  fiery  serpent  flambeaux.    The  orange  colored  cloister 

lanterns,  the  flaring  gas  and  ruby  steam  caldrons,  and  the  torches  on  the  tower 

combined  to  produce  a  feeling  of  mystery  in  this  Court  at  night. 


55 


•iground  of  the  palaces.    The 
"       •  which  suggested 

against  the 


•-•.rpassed  the 
di  c 

ved  treatment  ink 
The  towers  were  flood  lighted' 
e  shadows  thus  created  Y. 
i  the  va 

<ard  and  Ian4 
i.it  the  court. 
ppedwitha  2o-in. 
;e  staff  hanging 

Court  of  Abundance'7 

illuminating 

the  foreground, 

3DWACWUaA  1O  mJOO         ,e  feeling 

•loteiofo  boioloo  agjtBio  ariT    .xuBddmBft  Jnsqiaa  ^i9&  oili  bim  iswoT  afigiO  9tti 
no  aerioio*  9iit  bnB  ,anotbiB3  niB^Ja  y;cfin  bnB  8Bg  gnhfift 


.irigin  JB  JiuoD  aifft  ni  ^6*a^m  lo  snifeai  B  aouboTq  o^  bariicfraoD 

I 

.-e  composite  Spanish- 
5   rose  from  steam- 
iful  fountain  group  s 

h  in  formation.     The  cloister  lanterns  air 
-  gave  a  warm  amber  glow  to  the  whole  Court, 
the  organ  t< 

Court  of 

i  iimax  in  dignity 
:i  the  grandeur  of  t.    Here  an  area 

and  vertical  su 
ft.  above  the  lev; 
n  and  the  other 

•n  was  glazed  in  he 
in  imitation  of  tr,' 

mn  a  of 

"  scent  electric  lamps  : 

ic  brillia 


RYAN:  BUILDING  EXTERIOR  553 

so  low  that  the  fountains  were  free  from  disagreeable  glare  and  the 
great  colonnades  were  bathed  in  a  soft  radiance.  For  relief  lighting 
three  incandescent  lamps  were  placed  in  specially  designed  cup 
reflectors  located  in  the  central  flute  to  the  rear  of  each  column. 
This  brought  out  the  Pompeiian  red  walls  and  the  cerulean  blue  ceil- 
ings with  their  golden  stars,  and  at  the  same  time  the  sources  were  so 
thoroughly  concealed  that  their  location  could  not  be  detected  from 
any  point  in  the  court. 

The  perimeter  of  the  sunken  gardens  was  marked  by  balustrade 
standards  of  unique  design  consisting  alternately  of  Atlante  and 
Caryatides  supporting  an  urn  in  which  were  placed  incandescent 
lamps  of  relatively  low  candle-power.  The  function  of  these  lamps 
was  purely  decorative. 

The  great  arches  were  carried  by  concealed  lamps,  red  on  one  side 
and  pale  yellow  on  the  other,  thereby  preserving  the  curvature  and 
the  relief  of  the  surface  decorations.  The  balustrade  of  this  court, 
70  ft.  above  the  "Sunken  Gardens/'  was  surmounted  by  90  sera- 
phic figures  with  jeweled  heads.  These  were  cross  lighted  by  180 
incandescent  projector  lamps,  the  demarcation  of  the  beams  being 
blended  out  by  the  light  of  the  fountains  of  the  "Rising  Sun"  and 
"Setting  Sun." 

Passing  through  the  "Venetian  Court"  to  the  west,  we  entered 
the  "Court  of  Four  Seasons"  classically  grand.  We  were  then  in  a 
field  of  illumination  in  perfect  harmony  with  the  surroundings,  sug- 
gesting peace  and  quiet.  The  high-current  luminous  arc  lamps 
mounted  in  pairs  on  25-ft.  standards  masked  by  Greek  banners  were 
wonderfully  pleasing  in  this  setting.  The  white  light  on  the  columns 
caused  them  to  stand  out  in  semi-silhouette  against  the  warmly  il- 
luminated niches  with  their  cascades  of  falling  water,  and  the  placid 
central  pool  reflected  in  marvelous  beauty,  scenes  of  enchantment. 

Having  reviewed  in  order  the  illuminations  mysterious,  grand  and 
peaceful,  we  emerged  from  the  west  court  upon  lighting  classical  and 
sublime,  the  magnificient  "Palace  of  Fine  Arts"  bathed  in  what 
might  be  called  "Triple  moonlight,"  casting  reflections  in  the  lagoon 
impossible  to  describe.  The  effect  was  produced  by  projectors 
located  on  the  roofs  of  the  "Palace  of  Food  Products"  and  "Palace 
of  Education"  supplemented  by  concealed  lamps  in  the  rear  cornice 
soffits  of  the  colonnade. 

Having  passed  through  the  central,  east  and  west  axes  of  the 
Exposition,  there  were  many  more  marvels  to  be  seen.  If  one  had 
wished  to  study  the  art  of  illumination  he  could  have  visited  the 


554  ILLUMINATING   ENGINEERING   PRACTICE 

Exposition  every  evening  throughout  the  year  and  still  have  found 
detail  studies  of  interest.  For  instance,  he  could  have  seen  artificial 
illumination  in  competition  with  daylight.  On  certain  occasions  the 
projectors  flood  lighted  the  towers  before  the  sun  went  down.  If 
some  were  fortunate  enough  to  have  been  present  in  the  northwest 
section  of  the  "Court  of  the  Universe"  and  watched  the  marvelous 
effect  of  the  "Tower  of  Jewels"  as  the  daylight  vanished  and  the 
artificial  illumination  rose  above  the  deepening  shadows  of  the  night, 
they  saw  the  prismatic  colors  of  the  jewels  intensify  and  the  "  Tower  " 
itself  become  a  vision  of  beauty  never  to  be  forgotten. 

At  night  the  "South  Garden"  could  very  properly  have  been 
called  the  "Fairyland  of  the  Exposition."  When  light  was  first 
turned  on,  the  five  great  towers  were  bathed  in  ruby  tones  and  they 
appeared  with  the  iridescence  of  red  hot  metal.  This  gradually 
faded  to  delicate  rose  as  the  floodlight  from  the  arc  projectors  con- 
verted the  exterior  of  the  towers  into  soft  Italian  marble.  The 
combination  of  the  light  from  the  projector  arc  lamps  (white)  and 
that  from  the  concealed  incandescent  lamps  (ruby)  produced  shad- 
ows of  a  wonderful  quality.  Each  flag  along  the  parapet  walls  had 
its  individual  projector  which  converted  it  into  a  veritable  sheet  of 
flame.  As  a  primary  line  of  color  the  heraldic  shields  and  cartouche 
lamp  standards  produced  a  wonderful  effect  against  the  travertine 
walls  bathed  in  soft  radiance  from  the  luminous  arc  lamps,  which  also 
brought  out  the  color  of  the  flowers  and  lawns  and  created  pleasing 
shadows  in  the  palms  and  other  tropical  foliage.  This  was  supported 
by  a  secondary  effect  in  the  decorative  incandescent  electric  stand- 
ards along  the  "Avenue  of  Palms"  and  throughout  the  gardens. 
A  finishing  touch  was  added  by  the  effect  of  "Life  within  "  created  by 
the  warm  orange  light  emanating  from  all  the  Exposition  windows 
supported  by*  rose  red  light  in  the  towers,  minarets,  and  pylon 
lanterns. 

To  the  west  the  enormous  glass  dome  of  the  "Palace  of  Horti- 
culture" was  converted  into  an  astronomical  sphere  with  its  revolv- 
ing spots,  rings  and  comets  appearing  and  disappearing  at  the  horizon 
and  changing  colors  as  they  swung  through  their  orbits.  The  action 
was  not  mechanical,  but  astronomical. 

To  the  east  the  "Festival  Hall"  was  flood  lighted  by  luminous 
arc  lamps  and  accentuated  by  orange  and  rose  light  from  the  corner 
pavilions,  windows,  and  lanterns  surmounting  the  dome.  All  of 
this  view  was  reflected  in  the  adjacent  lagoon  and  possessed  a  dis- 
tinctive charm  which  will  long  remain  in  the  memory. 


SOUTH  PORTAL,  PALACE  OF  INDUSTRIES 

Showing  35-foot  Cartouche  Standards.     This  illustrates 
excellent  depth  of  detail  with  normal  shadows. 


554  RING    PRACTICE 

liout  the  year  and  still  have  fo 
stance,  he  could  have  seen  artificial 
iaylight.     On  certain  occasions  the 
towers  befon  n  went  down.     If 

to  have  bee  in  the  nortK 

ed  the  marvelous 

/els"  as. the  daylight  vanished  and  the 
the  deepening  shadows  of  the  night, 
he  jewels  intensify  and  the  "IV 
t  y  never  to  be  forgotten. 
h  Garden"  could  very  properly  have  "been 
-lion."     When  light  was  first 
!.n  ruby  tones  and  they 
This  gradually 
ire  projectors  con- 
marble.     The 
ite)  and 
ed  shad- 

wi 'io  aoAJAq  ,jAT«oq  Hiuoeet  .wall*  had 

.aW*bnBl8  eriDuolfifiD  ioo^StghiWckfifcle  sheet  of 
^wobjjria  Lranoti  rftiw  liuteb  to  rffqsf)  Jabl^xa  ;  n&  cartouche 
•uced  a  wonderful  effect  against  the  travertine 
is  bathed  in  soft  radiance  f  ous  arc  lamps,  which 

the  color  of  the  /  and  created  pleasing 

alms  and  other  tropical  foliage.     This  was  suppor 
by  a  secondary  effect  in  the  decorative  incandescent  electric  sta. 
ards  along  the  "Avenue  of  Palms"  and  throughout  the  g 
rushing  touch  was  added  by  the 

. 

Ion 

• 

.lie  " Palace  of  Horti- 
phere  with  its  revolv- 
.appearing  at  the  hor i 
ugh  their  orbits.     The  action 

*°  •  •  as  flood  lighted  by  lumir- 

arc  lamps  ar,  ,nd  rose  light  from  the  corner 

.lions,  windc  mounting  the  dome.     All 

view  was  reflected  in  the  adjacent  lagoon  and  possessed  a 

tinctive  charm  which  will  long  remain  in  the  memory. 


RYAN:  BUILDING  EXTERIOR  555 

Purely  spectacular  effects  were  confined  to  the  scintillator  at  the 
entrance  of  the  yacht  harbor.  This  consisted  of  forty-eight  36-in. 
projectors  having  a  combined  projected  candle-power  of  over 
2,600,000,000.  This  battery  was  manned  by  a  detachment  of 
U.  S.  Marines. 

A  modern  express  locomotive  with  8i-in.  drivers  was  used  to 
furnish  steam  for  the  various  fireless  fireworks  effects  known  as  "  fairy 
feathers,"  "sunburst,"  "chromatic  wheels,"  "plumes  of  paradise," 
"  Devil's  fan,"  etc.  The  locomotive  was  so  arranged  that  the  wheels 
could  be  driven  at  a  speed  of  fifty  or  sixty  miles  per  hour  under 
brake,  thereby  giving  forth  great  volumes  of  steam  and  smoke 
which,  when  illuminated  with  various  colors,  produced  a  wonderful 
spectacle. 

The  aurora  borealis  created  by  the  projector  beams  reached  from 
the  Golden  Gate  to  Sausalito  and  extended  for  miles  in  every  direc- 
tion. The  production  of  " Scotch  plaids"  in  the  sky  and  the  "birth 
of  color,"  the  weird  "ghost  dance,"  "fighting  serpents,"  the  "spook's 
parade,"  and  many  other  effects  were  fascinating. 

Additional  features  consisted  of  ground  mines,  salvos  of  shells 
producing  flags  of  all  nations,  grotesque  figures  and  artificial  clouds 
for  the  purpose  of  creating  midnight  sunsets. 

Over  300  scintillator  effects  had  been  worked  out  and  this  feature 
of  the  illumination  was  subject  to  wide  variation.  Atmospheric 
conditions  had  a  great  influence  upon  the  general  lighting  effects; 
for  instance,  on  still  nights  the  reflections  in  the  lagoons  reached  a 
climax,  particularly  the  "Palace  of  Fine  Arts"  as  viewed  from 
"Administration  Avenue;"  the  facades  of  the  "Palace  of  Educa- 
tion" and  "Palace  of  Food  Products"  as  seen  in  the  waters  through 
the  colonnade  of  the  "Palace  of  Fine  Arts;"  the  "Palace  of  Horti- 
culture" and  "Festival  Hall"  from  their  respective  lagoons  in  the 
"South  Garden;"  the  colonnades  and  the  Novagems  on  the  heads 
of  the  seraphic  figures  and  the  "Tower  of  Jewels"  as  reflected  in  the 
water  mirror  located  in  the  north  arm  of  the  "  Court  of  the  Universe." 

On  windy  nights  the  flags  and  jewels  were  seen  at  their  best.  On 
foggy  nights  there  were  produced  over  the  Exposition  wonderful 
beam  effects  impossible  at  other  times. 

When  the  wind  was  blowing  from  the  land  the  scintillator  display 
was  different  from  nights  when  the  wind  was  blowing  from  the  bay. 
A  further  variety  was  introduced  in  the  action  of  the  smoke  and 
steam  on  calm  nights. 

On  the  evening  of  St.  Patrick's  day  all  the  projectors  were  screened 


556  ILLUMINATING    ENGINEERING    PRACTICE 

with  green,  and  not  only  the  towers  but  every  flag  in  the  Exposition 
took  on  a  new  aspect. 

Orange  in  various  shades  was  the  prevailing  color  for  the  evening 
of  "Orange  Day,"  and  on  the  ninth  anniversary  of  the  burning  of 
San  Francisco,  the  Exposition  was  bathed  in  red,  with  a  strikingly 
realistic  demonstration  of  the  burning  of  the  "Tower  of  Jewels." 

Never  before  was  there  such  flexibility  in  lighting  on  so  large  a 
scale,  making  it  possible  at  very  small  expense  and  on  short  notice 
to  introduce  modifications  in  the  illuminating  effects.  This  was 
made  feasible  by  use  of  the  great  number  of  projectors,  which  on 
ordinary  occasions  projected  white  light,  but  by  the  introduction  of 
screens  the  coloring  could  be  completely  changed. 

Briefly,  the  lighting  equipment  consisted,  primarily  of  direct, 
masked,  concealed  and  projector  lamps,  representing  an  harmonious 
blending  of  luminous  arc,  projector,  incandescent  electric  and  gas 
lamps. 

The  high  current  luminous  arc  lamp  was  selected  for  general 
flood  lighting  of  the  facades,  lawns,  and  shrubbery  on  account  of  its 
high  efficiency,  and  relatively  low  maintenance  cost  where  great 
quantities  of  white  light  was  required. 

Projectors  were  used  for  illuminating  the  towers  and  minarets, 
flags  and  other  features  where  concentration  was  necessary. 

High  efficiency  electric  incandescent  lamps  m  all  ratings  from  10 
to  1500  watts  were  employed  generally  throughout  the  Exposition, 
especially  where  space  was  limited,  warm  tones  were  required  and 
flexibility  was  of  fundamental  importance. 

High-pressure  gas  lamps  played  an  important  part  in  street  light- 
ing in  the  Foreign  and  State  sections;  as  did  also  low-pressure  gas 
lamps  for  emergency  purposes  and  gas  flambeaux  for  special  effects. 


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GENERAL  DESCRIPTION  OF  EXHIBITION 


The  exhibition  was  collected  in  rooms  adjacent  to  the  Lecture  Hall  of  the 
Engineering  Building  of  the  University.  It  covered  about  8000  square  feet  of 
floor  area. 

Exhibits  were  supplied  by  manufacturers  in  the  lighting  field,  electric  and  gas 
lighting  companies,  research  and  development  laboratories,  the  Navy  Depart- 
ment and  others.  It  was  the  intention  in  designing  and  collecting  these  exhibits, 
to  minimize  the  commercial  element  and  to  emphasize  the  educational  value. 
It  was  the  purpose  not  only  to  have  apparatus,  equipments  and  illustrations 
available,  but  to  afford  those  taking  the  lecture  course  every  opportunity  to 
familiarize  themselves  with  the  material  exhibited  and  with  its  use  and  signifi- 
cance. The  scope  will  be  indicated  by  the  following  partial  list  of  exhibits: 


ILLUMINANTS 
Set  of  mazda  lamps 
Set  of  miniature  mazda  lamps 
Historical  collection  of  incandescent 

electric  lamps 
Historical   collection  of    gas   "arc" 

lamps 
Comparison  of  gas  lamps  of   1906 

with  those  of  1916 
Collection    of    metallic    flame    and 

flaming  arc  lamps 

ILLUMINANT  ACCESSORIES 
Window  lighting  reflectors 
Reflectors  for  interior  illumination 
Reflectors  for  exterior  illumination 
Collection  showing  the  development 

of  diffusing  glassware 
Decorative  lighting  glassware 
Fixtures — electric  and  gas. 

LIGHTING  UNITS  FOR  PARTICULAR 

PURPOSES 

Artificial  daylight  equipments 
Mine  lamps 
Street  lighting  units 
Car  lighting  equipments 
Flood  lights  and  searchlights. 

PHOTOMETRIC  APPARATUS 
Bar  photometers 
Integrating  spheres 
Portable  photometers 
Light  niters 
Photometer  heads 
Electrical  instruments 
Physical  photometers 
Spectrophotometers 
Flicker  photometers 
Photoelectric  cell 
Color  vision  correction  screens 
Acuity  apparatus. 

OPTICAL  APPARATUS 
Spectrum  projector 
Comparison  spectroscope. 


OPHTHALMOLOGICAL  APPARATUS 

Visual  acuity  devices 
Ophthalmoscopes 
Visual  sensitometer 
Pupillary  diameter  wedges 
Apparatus  showing  retinal  inertia 
Threshold  photometer 
Illuminated  test  cards 
Color  test  apparatus. 

COLOR  APPARATUS 

Spectrophotometer 

Colorimeter 

Color  mixture  wheel 

Color  booths 

Color  triangle 

Variable  neutral  tint  scheme. 

MANUFACTURING  PROCESSES 

Cabinet  illustrating  the  manufacture 

of  mazda  lamps 
Display   illustrating    characteristics 

of  gas  lamps. 

LIGHTING  PRACTICE 

Statistics  showing  importance  of  illu- 
mination in  mining  operations 

Model  of  dark  room  lighting 

Model  of  street  lighting 

Display  showing  present  methods  of 
interior  illumination 

Model  showing  art  gallery  lighting 

Relation  of  illumination  to  safety 
and  output. 

SPECIALTIES 

Exhibit  of  Bureau  of  Standards  illus- 
trating the  work  of  the  Bureau 

Exhibit  of  Navy  Department,  illus- 
trating signaling  by  lighting 

Exhibit  of  fluorescence  due  to  ultra- 
violet light 

Diagrams  of  luminous  efficiencies. 


557 


Fig.   i. — View  of  exhibit  room  No.  I,  looking  west. 


Fig.  2. — View  of  exhibit  room  No.  i,  looking  east. 
558 


Fig.  3. — Exhibit  of  Benjamin  Electric  Company. 


Fig.  4. — Exhibit  of  Bosch  &  Lomb. 
559 


Fig.  5.— Exhibit  of  Bureau  of  Standards. 


Fig.  6. — Exhibit  of  Central  Electric  Company. 


Fig.  7. — Research  laboratory  exhibit  of  Eastman  Kodak  Company. 


Fig.  8. — Electrical  Testing  Laboratories,  interior  lighting  model. 


Fig.  9. — Electrical  Testing  Laboratories,  interior  lighting  model. 


Fig.  10. — Electrical  Testing  Laboratories,  interior  lighting  model. 
562 


Fig.   ii. — Electrical  Testing  Laboratories,  interior  lighting  model. 


Fig.   12. — Electrical  Testing  Laboratories,  street  lighting  model. 
563 


Fig.  13. — Electrical  Testing  Laboratories,  exhibit  of  street  lighting  units. 


Fig.  14. — Electrical  Testing  Laboratories,  exhibit  of  lighting  accessories. 

564 


Fig.  15. — Exhibit  of  Nela  Engineering  Department,  General  Electric  Company. 


Fig.   16. — Exhibit  of  Xela  Engineering  Department,  General  Electric  Company. 

565 


Fig.   17. — Exhibit  of  Edison  Lamp  Works,  General  Electric  Company. 


Fig.  1 8. — Exhibit  of  Consulting  Engineering  Laboratory,  General  Electric  Company. 

S66 


Fig.  19. — General  Electric  flood  lighting  projectors. 


Fig.  20. — Exhibit  of  General  Gas  Light  Company. 
567 


Fig.  21. — Exhibit  of  Leeds  &  Northrup  Company. 


Fig.  22. — Exhibit  of  Macbeth-Evans  Company. 
568 


Fig.  23. — Exhibit  of  National  X-ray  Reflector  Company. 


Fig.  24. — Exhibit  of  the  Philadelphia  Electric  Company. 
569 


Fig.  25. — Exhibit  of  Simon  Ventilighter  Company. 


Fig.  26. — Portable  lamp  exhibit  of  Frank  H.  Stewart  Electric  Company. 
570 


Fig.  27.— Light  signals  exhibited  by.  U.  S.  Navy. 


Fig.  28. — Optical  instruments  exhibited  by  Wall  &  Ochs. 
S7i 


Fig.  29. — Gas  lamps  exhibit,  Welsbach  Company. 


Fig.  30. — Electric  lamp  exhibit  of  Westinghouse  Company. 
572 


INDEX 


Abbreviations,  photometric  units,  34         Candle-power  curve  explanation,  4 


Absorption-of-light  method  of  calcu- 
lation, 15 

Accessories,  bibliography,  210 
car  lighting,  509 
glass    structural     characteristics, 

1 86 

applications,  199 
light  absorption  table,  426 
lighting,  183 
mirrored,  203 
prismatic,  200 

street  illumination,  424,  479 
Acetylene  flame,  brightness,  47 
Arc,  carbon,  brightness,  47 
intrinsic  brilliancy,  216 
lights,  brightness,  47 
mercury,  brightness,  47 
Art  museum  lighting,  264 
Auditorium  illumination,  326 
Automobile  headlights,  220 
bibliography,  250 
regulations  various  states,  222 

Banks,  illumination,  372 

Bar  photometer,  109 

Bibliography    (various    subjects,   see 

name  of  subject). 
Brightness,  artificial  sources,  47 

conversion  table,  39 

definition,  31 

measurements,  123 

units,  25 

conversion  table,  39 
Brilliancy  projection  sources,  216 
Burners,  gas,  characteristics,  168 

Calculations,  absorption  -  of  -  light 
method,  15 

illumination,  i 
Candle,  brightness,  47 

definition,  30 


definition,  30 

diagram,  flux  summation,  12 

distribution,  cylindrical  source,  8 

space  representation,  10 

spherical  source,  8 
per  sq.  in.  method  to  determine,  27 
Space  distribution,  i 
various  definitions,  33 
Carbon  filaments  (see  filaments). 

lamps  (see  lamps,  also  filaments). 
Cars,  electric  lighting,  495 

axle  driven  system,  496 

headend  system,  495 

straight  storage  system,  496 
gas  lighting,  493 
illumination  design,  497 

driving,  504 

fixtures,  510 

glass  ware,  509 

intensities,  497 

interurban,  508 

parlor,  507 

passenger  coaches,  497 

postal,  507 

private,  507 

reflectors,  509 

sleeping,  505 

smoking,  506 

street,  508 

Churches,  illumination,  296,  326 
ritualistic  illumination,  301 
Color,  brightness  definition,  270 
in  lighting,  267 
lighting  media,  287 
measurements,  271 
mixture  applications,  291 
photometry,  271 
saturation,  269 
science  practice,  268 
terminology,  269 
Colorimetry,  272 


573 


574 


INDEX 


Contracts,  street  illumination,  484 
Cost    data,    various  illumination  sys- 
tems (see  name  of  system). 
Curve  characteristic,  definition,  33 
performance,  definition,  33 

Daylight,  artificial,  290 

production,  282 
illumination,  19 
measurements,  124 
Decoration  by  illumination,  253 
Dining  room  lighting,  263 
Dirt  cause  of  depreciation,  48 

table,  49 
Distribution   circuits  failing    lighting, 

355 

street  lighting,  472 

curves,  definitions,  33 

Docks,  illumination,  528 

bibliography,  533 

Drafting  room  illumination,  319 

Exposure,  definition,  30 

Factories,   illumination,  bibliography, 

361 

cost  data,  358 
distribution  circuits,  355 
intensities,  table,  350 
lamps,  available,  343 
maintenance,  357 
requirements,  344 
typical  plans,  351 
values       for        manufacturing 

spaces,  342 
production,  relations  to  lighting, 

337 

Filaments,  carbon,  brightness,  47 
intrinsic  brilliancy,  216 
tungsten,  brightness,  47 
Filters,  light,  102 
Fixtures,  car  lighting,  510 
design,  207 

gas  lamps,  residence  lighting,  180 
Flame,  acetylene,  brightness,  47 
candle,  brightness,  47 
intrinsic  brilliancy,  216 
kerosene  brightness,  47 
Flicker  photometer,  100 


Flood  lighting,  239 

architectural  results,  261 
bibliography,  251 
calculations,  92 
Flux,  luminous,  definition,  29 

summation,     candle-power     dia-. 

gram,  12 

graphical  construction,  14 
Freight  yards  illumination,  514 

Gas  burners,  characteristics,  168 

lamps  (see  lamps), 
fixtures  (see  fixtures). 

lighting,  cars  (see  car  lighting). 

mantles  (see  mantles). 
Gasolene  lamps  (see  lamps). 
Glare  avoidance,  57 

characteristics  headlights,  224 

definition,  55 

elimination,  65 

street  illumination,  90,  448,  478 
Glass  accessories  (see  also  accessories), 
structural  characteristics,  186 

colored,  transmission  coefficients, 
200 

light  losses,  195 

transmission  coefficients,  193 
Glassware,  bibliography,  210 

car  lighting,  509 

light  losses  analysis,  195 

uses,  199 
Globes  (see  also  glassware). 

light  absorption,  table,  426 
Gymnasium  illumination,  322 

Harcourt  lamp,  characteristics,  105 
Headlighting,  bibliography,  250 
Headlights,  automobile,  220 

glare  characteristics,  224 

railway  equipment,  229 
Hue,  definition,  269 

Ignition,  gas  lamps,  173 
Illumination,  aesthetic  effects,  61 
application  of  color,  289 
architectural  aspects,  253 
artistic,  253 
aspects,  398 


INDEX 


575 


Illumination,  auditorium,  326 
banks  (see  banks), 
calculations,  i 

absorption-of -light  method,  15 

daylight,  19 

flood  lighting,  92 

point-by-point,  50 

zone-flux  methods,  51 
cars  (see  car  lighting), 
churches,  297,  326 
colored  light,  bibliography,  294 

surfaces,  280 
daylight  (see  daylight), 
decorative,  253 
definition,   30 
depreciation  due  to  dirt,  48 

table,  49 
design  effect  of  accessories,  42 

examples,  73 

location  of  units,  71 

process,  64 

selection  of  source,  64 
docks  (see  docks), 
effect  of  ceiling  bright,  24 

on  production,  337 
expositions,  547 

history,  549 
exterior,  building  fronts,  440 

calculation,  84 

choice  of  lamps,  95 

classification,  81 

color  effects,  96 

principles,  77 

public  monuments,  91 

strict  classification,  86,  89 
factory  (see  factory). 

classification,  347 

costs,  340 

legislation,  341 

requirements,  344 

typical  plans,  351 
fixtures  (see  fixtures), 
flood  lighting,  239 
fundamental  characteristics,  309 
gas  developments,  165 

fixtures,  1 80 
hygiene,  53 

indoor  vs.  outdoor,  432 
industrial  establishments,  337 


Illumination,  intensities  industrial  serv- 
ices table,  350 

interior  design,  calculations,  37 
principles,  37 

large  rooms,  329 

library,  323 

measurements,  116,  122 

compensated  test  plates,  112 

nomenclature,  29 

office  (see  office). 

outside  works,  513 

pageants,  547 

physical  aspects,  397 

psychological  aspects,  396 

public  monuments,  91 

quantity  for  eye  efficiency,  59 

railway  cars  (see  cars). 

residences  (see  residences). 

schools,  311 

static  tester,  121 

store  (see  store). 

street  (see  streets). 

sunlight,  46 

theaters,  330 

units,  i,  29 

values,  manufacturing  spaces,  342 
street  lighting,  89 

various  classes  of  service,  61 

window  (see  window). 

yards  (see  yards). 
Illuminometer,  Macbeth,  116 
Intensities  illumination,    various   ser- 
vices (see  name  of  service). 
Intensity,  luminous,  definition,  30 

Kerosene,  flame,  brilliancy,  47 

Lambert,  definition,  25,  31 
Lamp,  definition,  32 
Lamps,  accessories,  definition,  34 
acetylene,  efficiency,  41 
alcohol,  efficiency,  41 
arc  development,  146 

enclosed      carbon — engineering 

date,  154 

flame  engineering  data,  154 
illumination  characteristics,  149 
luminous,  154 
bibliography,  162 


576 


INDEX 


Lamps,  carbon  (see  filaments), 
care,  160] 

comparison,  definition,  32 
electric  classification,  132 
developments,  131 
factory  lighting,  343 
filament,  development,  133 
gas,  filled,  134 
incandescent  developments,  133 

operation,  136 

Mazda,  engineering  data,  138 
street  lighting  data,  139 
train  lighting  data,  138 
miniature  developments,  143 
specific  output,  units,  34 
factory  illumination,  343 
freight  yard  illumination,  525 
gas,  distant  control,  176 
efficiency,  168 

electro-magnetic  values,  178 
fixtures,  1 80 
ignition,  173 
photography,  182 
pilot  consumption,  176 
special  application,  181 
gasoline  efficiency,  41 
Harcourt  characteristics,  105 
kerosene,  efficiency,  41 
Moore    carbon    dioxide    develop- 
ment, 145 

old-fashioned  simulation,  285 
selection,  161 

signal,    illumination    characteris- 
tics, 245 
standard  electric,  characteristics, 

106 

tests,  basis,  34 
definition,  32 
tube,  carbon  dioxide,  283 
development,  145 
mercury    vapor,    development, 

157 

quartz  tube,  158 
X-ray  development,  146 
tungsten  (see  filaments). 

lumens  output  table,  40 
Libraries,  illumination,  307,  323 
Light,  absorption  by  accessories,  195 
table,  426 


Light,  colored,  bibliography,  294 

distribution  calculation,  385 

filters,  102 

losses,  glassware,  195 

projection  applications,  213 

sources,  brightness,  47 
brilliancy,  216 

transmission    coefficient,    colored 

glass,  200 
Lighthouses,  bibliography,  252 

projector  applications,  242 
Lighting  accessories,  183 

(see  illumination). 
Lumen,  definition,  30 
Lux,  definition,  30 

Mantles,  intrinsic  brilliancy,  216 
gas,  brightness,  47 

physical  character,  166 
Mazda  lamps  (see  lamps,  electric). 
Mirror,  accessories,  203 
Moore  lamp  (see  lamps,  tube). 
Motion  picture  projectors,  246 
Museum,  art,  lighting,  264 

Office  illumination,  363,  366 
bibliography,  390 
cost,  368 
design,  371 
types,  369 

Photography,  gas  lamps,  182 
Photometer,  bar,  109 
flicker,  100 

converted  from  Zummer-Brod- 

hun,  100 
physical,  99 
Sharp-Millar,  117 
tests,  definition,  33 
Photometry,  abbreviations,  34 
gas-filled  lamps,  125 
integrating  sphere,  no 
bulky  lamps,  112 
lamps,  quick  handling,  113 
standard,  103 
shades  and  reflectors,  126 
liquid  filters,  102 
practice,  99 

projection  apparatus,  129 
standard  lamps,  103 


INDEX 


577 


Pintsch  gas  car  lighting,  494 

Point  source,  study,  1 1 

Posts  for  street  lamps,  480 

Projection,  general  principles,    biblio- 
graphy, 250 
transparencies,  bibliography,  252 

Projector  applications,  transparencies, 
246 

Projectors,  photometry,  129 

Quartz  tube  lamps  (see  lamps,  tube). 

Radiation,  luminous,  definition,  31 
Railway  headlights,  229 
bibliography,  250 
Rating,  illuminants,  34 
Reduction  factor,  definition,  33 
Reflection,  buildings,  477 

coefficient,  definition,  32,  191 
table,  193 

measurements,  127 

pavements,  449,  477 

walls,  ceilings,  floors,  17 
Reflectors  (see  also  accessories). 

aluminium,  properties,  197 

bibliography,  210 

car  lighting,  509 

design,  196 

effects  on  illumination  design,  42 

enameled  properties,  198 

gas,  184 

glass  manufacture,  189 

light  absorption  table,  426 
losses,  analysis,  195 
projection,  214 

metal,  184 

manufacture,  188 

optical  properties,  191 

parabolic  characteristics,  217 

photometric  properties,  197 

reflection  coefficients,  193 

uses,  185 

utilization  factors,  table,  52-56 
Residences,  basement  illumination,  412 

bath  room  illumination,  411 

bedroom  illumination,  411 

den  illumination,  409 

dining  room  illumination,  405 

hall  illumination,  410 


Residences,  illumination,  395 
artistic  aspects,  398 
gas  lamp  fixtures,  180 
physical  aspects,  396 
psychological  aspects,  396 
practical  applications,  400 
kitchen  illumination,  409 
library  illumination,  407 
living  room  illumination,  402 
music  room  illumination,  408 
porches  illumination,  412 
sunparlor  illumination,  409 

Schoolrooms,  illumination  values,  315 
Schools,  illumination,  307,  311 
Searchlight,  equipments,  235 
Searchlighting,  bibliography,  251 
Shade,  definition,  271 
Shadows,  effects  on  eye,  58 
Sharp-Millar  photometer,  117 
Shop  room,  illumination,  319 
Signal  lamps  (see  lamps). 

lights,  bibliography,  252 
Signaling,  projector  applications,  245 
Signs,  electric,  effect  on   community, 

546 

engineering  features,  542 
industry,  544 
modern  types,  536 
ordinances,  545 
lighting,  535 
Sky,  brightness,  46 
Sky-light,  artificial,  289 

glass,  light  losses,  analysis,  195 
illumination  calculations,  21 
Spectrophotometry,  272 
Standard,   luminous,    primary   defini- 
tion, 32 

luminous,    representative    defini- 
tion, 32 

reference,  definition,  32 
working  definition,  32 
Stations,  freight  illumination,  527 

passenger  illumination,  531 
Steradian,  definition,  3 
Stores,  direct  lighting  equipment,  378 
gas  lighting,  380 
illumination,  363,  373 
systems,  377 


578 


INDEX 


Stores,  show  case  lighting,  381 
window  lighting,  382 

Street  illumination  accessories,  424, 479 
arc  lamps,  467 
bibliography,  459 
calculations,  428 
city  requirements,  461 
contracts  practice,  486 

requirements,  485 
contractual  relations,  484 
cost  reduction  effect,  491 
design,  435,  456,  469,  474 
effect  of  pavements,  477 
electric  circuits,  472 
flux  on  street,  430 
glare  reduction,  90,  448,  478 
graded  results,  482 
history,  415 
illuminants,  characteristics,  423, 

465 

recent  history,  420 
incandescent  lamps,  468 
lamp  locations,  451 
large  units,  466 
measure  of  service,  488 
pavement  reflection,  449 
posts  and  mountings,  480 
public  policy,  492 
purposes,  415 
scope,  417 
small  units,  467 
state  control,  490 
street  classification,  465 
tests,  453 

typical  intensities,  438 
values  of  illumination,  89 
variability  along  street,  441 


Street  illumination,  visual  characteris- 
tics, 430 
Streets,  classification  for  illumination, 

465 
Sunlight,  artificial,  289 

Tint,  definition,  271 
Theaters,  illumination,  330 
Transmission  coefficients  (see  name  of 

material) . 

light  through  glass,  192 
measurements,  127 

Tungsten   lamps   (see    filaments,  also 
lamps). 

Units  illumination,  i 

photometric,  table,  34 
Utilization  factor,  definition,  18 

factors,  table,  52-56 

Valves,  electro  magnetic,  178 
Vehicle  head  lights,  220 
Visibility,  definition,  30 
Vision  phenomena,  463 

Window  illumination,  363,  382 

bibliography,  390 
light  distribution,  calculation,  385 
source,  calculation,  21 

X-ray  lamps  (see  lamps). 

Yard  illumination,  bibliography,  533 
freight  classification  illumination, 

521 

illuminants,  525 
illumination,  514 


SEVENTH 


OVERDUE. 


-At)^i^4989— 


oo / u / 


GENERAL  LIBRARY    U.C.  BERKELEY 


B000330B1M 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


